The precise number of Okenia taxa inhabiting the Mediterranean Sea, as well as their general taxonomy, varies according to different specialists. So far, eight valid species have been reported from the area: Okenia aspersa (Alder & Hancock, 1845), Okenia cupella (Vogel & Schultz, 1970), Okenia elegans (Leuckart, 1828), Okenia hispanica Valdés & Ortea, 1995, Okenia impexa Er. Marcus, 1957, Okenia leachii (Alder & Hancock, 1854), Okenia mediterranea (Ihering, 1886), and Okenia zoobotryon (Smallwood, 1910). Of these, only three (O. elegans, O. hispanica, and O. mediterranea) have their type localities in the Mediterranean Sea, whereas the others were described from different biogeographic areas and later included in the Mediterranean biota. We carried out a review on Mediterranean Okenia species through an integrative approach, based on a wide literature search and a morphological and molecular analysis of available type material and samples collected recently. The present study confirmed the presence of O. aspersa, O. elegans, O. hispanica, and O. mediterranea in the Mediterranean Sea, although leaving remaining questions about some of those taxa. The distribution of O. cupella, O. impexa, and O. zoobotryon is limited to the western Atlantic, and of O. leachii to the eastern Atlantic. All specimens previously identified as O. cupella, O. impexa, and O. zoobotryon by different authors in the Mediterranean Sea were repeatedly misidentified. Thus, we describe Okenia problematica sp. nov. and Okenia longiductis sp. nov., from the “Mediterranean” Okenia cupella/impexa and O. zoobotryon. We also consider here Okenia pusilla Sordi, 1974 a nomen dubium and include a redescription of the holotype of O. cupella. A molecular phylogeny, including all the sequenced Okenia species, was performed in order to evaluate the evolutionary relationships of the newly described species with the other congeneric taxa.
Citation: Pola M, Paz-Sedano S, Macali A, Minchin D, Marchini A, Vitale F, et al. (2019) What is really out there? Review of the genus Okenia Menke, 1830 (Nudibranchia: Goniodorididae) in the Mediterranean Sea with description of two new species. PLoS ONE 14(5): e0215037. https://doi.org/10.1371/journal.pone.0215037
Editor: Geerat J. Vermeij, University of California, UNITED STATES
Received: February 5, 2019; Accepted: March 25, 2019; Published: May 1, 2019
Copyright: © 2019 Pola et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Data Availability: The electronic edition of this article conforms to the requirements of the amended International Code of Zoological Nomenclature, and hence the new names contained herein are available under that Code from the electronic edition of this article. This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN. The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix “http://zoobank.org/”. The LSID for this publication is: 5F42073C-02B6-458E-99CE-F4814D959E3C. The electronic edition of this work was published in a journal with an ISSN, and has been archived and is available from the following digital repositories: PubMed Central and LOCKSS.
Funding: MP was partially funded by the research project PR2018-039 funded by University of Cadiz (Spain), FC was partially funded by ADViSE PG/2018/0494374 and SPS was partially funded by DNAqua-Net CA15219. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interest exist.
The genus Okenia Menke, 1830 (Gastropoda: Nudibranchia: Goniodorididae) includes around 50 valid species worldwide and is composed by small to medium-sized sea slugs, whose distribution spans from cold, temperate, and tropical waters of the Pacific Ocean to the north and south Atlantic Ocean, including the Mediterranean Sea, and bathymetric range goes from the intertidal to the 160 meters depths of Okenia vancouverensis (O´Donoghue, 1921) [1–6]. Little is known about the phylogenetic relationships between Okenia and related genera. Gosliner  synonymized Hopkinsia MacFarland, 1905, Hopkinsiella Baba, 1938, and Sakishimaia Hamatani, 2001 with Okenia based on morphological characters, while Pola et al.  and Paz-Sedano et al.  confirmed the monophyly of the genus Okenia and the synonymy of Hopkinsia and Hopkinsiella proposed by Gosliner  based on preliminary molecular data. An even more intricate situation concerns Okenia alpha taxonomy, since many new species of Okenia have been recently described [2–5, 7–13], some of these lack complete morphological descriptions [5, 12–13], and many new species still require description . The validity of species identification has varied historically according to the views of different authors [2, 15–19], with cryptic or pseudocryptic species still being discovered at the beginning of the twenty-first century . This makes the known distribution of previously described Okenia species uncertain, thereby creating confusion of true geographic ranges [5, 13, 18]. As a result, the overall knowledge of Okenia is incomplete, and reviews focusing on selected biogeographic areas and determining the true identity and spread of Okenia taxa worldwide need to be undertaken through both genetic and anatomical studies.
The precise number of Okenia species inhabiting the Mediterranean Sea has also varied according to different specialists. While the Mediterranean malacofauna is generally considered to be one of the best studied worldwide [20–21], the origin of the modern Mediterranean molluscan assemblage is complex [20–23], which often lead to the discovery of cryptic diversity even in well-known groups [24–29]. This is compounded further by potentially similar species introduced by anthropogenic activities from the Atlantic and Indo-Pacific bioregions [30–31].
Here we carry out a review on the Mediterranean Okenia species using an integrative approach, based on the information existing in the literature, supplemented by morphological and molecular analysis of samples newly collected and type material from the Mediterranean Sea and outside. A molecular phylogeny including sequenced Okenia species was also undertaken to elucidate the evolutionary relationships of the newly collected specimens with the other congeneric taxa.
Material and methods
Published data and source of newly collected specimens
Indexed and grey literature were examined for published Mediterranean records of taxa belonging to the genus Okenia, especially those accounts concerning faunistic, taxonomic, and biogeographic studies of Mollusca. Bibliographic data were critically analysed and taxonomically updated to the latest nomenclature available, following the World Register of Marine Species . At the same time, a GenBank search was carried out to check for barcodes of Okenia material from the Mediterranean Sea. Once these two preliminary steps were achieved, Okenia specimens were collected from marinas by hand or from several Mediterranean localities by SCUBA diving, and preserved in 96–100% ethanol. All examined material was deposited either at the Museo Nacional de Ciencias Naturales (MNCN, Madrid, Spain) or Stazione Zoologica Anton Dohrn (SZN, Naples, Italy). In addition, we borrowed two specimens of Okenia angelensis Lance, 1966 and the holotype of Okenia cupella (Vogel & Schultz, 1970) from the California Academy of Science (CASIZ, San Francisco, California) and the Smithsonian National Museum of Natural History (USNM, Washington D. C., United States) respectively, in order to add them to our dataset and thus allow for morphological and molecular comparisons with our collections.
The external morphology of specimens was examined from photographs of living Okenia specimens and from laboratory observations. Internal organs were removed following a dorsal incision and drawn using a Nikon SMZ-1500 dissecting microscope with an attached camera lucida. Special attention was paid to the buccal mass and the reproductive system. Each buccal mass was removed and dissolved in 10% sodium hydroxide to remove surrounding tissue. The labial cuticle and radula were then rinsed in water. These structures and the penis were initially examined under the light microscope, then photographed using the software cellSense, and subsequently dried (apart from the radula) by critical point using hexamethyldisilazane. All these parts were finally mounted and sputter coated for examination under a Hitachi S3000N scanning electron microscope (SEM).
DNA extraction, amplification and sequencing
DNA was extracted from foot tissue and performed using the DNeasy Blood and Tissue Kit (Qiagen) according to the manufacturer’s instructions or by proteinase K-digestion followed by a standard phenol-chloroform protocol . Partial sequences of cytochrome c oxidase I (COI), 16S ribosomal RNA (16S rRNA), and histone H3 (H3) were amplified using LCO1490 and HCO2198 universal primers for COI , 16S ar-L and 16S br-H for 16S rRNA , and H3AD5’3’ and H3BD5’3’ for H3 . For the DNA extracted using the DNeasy Blood and Tissue Kit, the master mix for the PCR was prepared in the following order: nuclease-free water up to 25 μl volume reaction, 2.5 μl of Qiagen buffer, 2.5 μl of dNTP (2 mM), 5 μl of ‘Q-solution’ (Qiagen), 1.5–3.5 μl magnesium chloride (25 mM), 0.5 μl of each forward and reverse primer (10 mM), 1 μl of DNA polymerase (250 units), and 2.5 μl of DNA. Amplifications were performed with an initial denaturation for 5 min at 95 °C, followed by 35 cycles of 30 s at 95 °C, 30–45 s annealing at 49 °C for COI, 52 °C for 16S rRNA, and 50 °C for H3, and 45 s at 72 °C with a final extension of 5 min at 72 °C. For the DNA extracted using the phenol-chloroform protocol, the master mix for the PCR was prepared in the following order: nuclease-free water up to 15 μl volume reaction, 3 μl of Promega buffer, 0.3 μl of dNTP (10 mM), 1 μl magnesium chloride (25 mM), 0.3 μl of each forward and reverse primer (10 mM), 0.1 μl of Promega HotStart DNA polymerase (2 units), and 1.5 μl of DNA. Amplifications were performed for COI with an initial denaturation for 2 min at 94 °C, followed by 35 cycles of 60 s at 94 °C, 60 s annealing at 48 °C and 90 s at 72 °C, with a final extension of 10 min at 72 °C. For 16S rRNA amplifications were performed with an initial denaturation for 2 min at 94 °C, followed by 35 cycles of 45 s at 94 °C, 60 s annealing at 51 °C and 90 s at 72 °C, with a final extension of 10 min at 72 °C. Finally, H3 amplifications were performed with an initial denaturation for 2 min at 94 °C, followed by 40 cycles of 30 s at 94 °C, 30 s annealing at 54 °C and 60 s at 72 °C, with a final extension of 10 min at 72 °C. Successful PCR products were purified and sequenced by Macrogen, Inc.
Sequences were assembled and edited using Bioedit v7.2.5  and aligned using MEGA6 . Protein-coding sequences were translated into amino acids for confirmation of alignment using the genetic code invertebrate mitochondrial DNA for COI and universal code for H3. All sequences were blasted in GenBank to check for contamination. The most variable regions from the 16S rRNA alignment were removed by using both the default settings and the standard options for stringent and less stringent selection in Gblocks . Excluding “indel-rich” regions, the tree was in general the same. Therefore, final analyses were performed with all bases included. Sequences of COI, 16S rRNA, and H3 were trimmed to 658, 470, and 328 base pairs, respectively. The evolutionary models were selected using jModelTest-2.1.7  under the Bayesian information criteria . For COI and H3 the evolutionary model was determined separately for the first, second, and third codon position. Evolutionary models for COI were TIM2+I+G, TPM3uf+I, and TrN+I+G for the first, second, and third codon position, respectively. The TPM1uf+I+G evolutionary model was selected for 16S rRNA gene. For H3 gene, TIM2+I was selected for first codon position, JC for second codon position, and TVM+G for third codon position. Bayesian Inference (BI) and Maximum Likelihood (ML) analyses were conducted for individual genes as well as for the concatenate of a minimum of two (COI+16S rRNA) (H3+COI) to three genes (H3+COI+16S rRNA). BI analysis was performed using the software package MrBayes v3.1.2b  for ten million generations with two independent runs and sampling frequency of 1000. ML analysis was performed using the software package RAxML v7.04 . To determine the nodal support in ML a 50000 bootstrap analysis was implemented. Only nodes supported by bootstraps values ≥ 75  and posterior probabilities ≥ 0.96 were considered statistically significant . The trees obtained were visualized in FigTree v1.3.1  and edited in Adobe Photoshop CC 2014.
Species delimitation analyses
In order to compare the genetic distances amongst specimens of Okenia included in this study, we calculated the pairwise uncorrected p-distances for COI and H3 using PAUP*4.0b 10.0 . All codon positions were considered for the analysis. Analyses of species delimitation—Bayesian Poisson Tree Process (bPTP)  and Automatic Barcode Gap Discovery (ABGD) —were conducted on the COI ingroup sequences. bPTP analysis was done using the bPTP webtool (https://species.h-its.org), running 200000 MCMC generations, Thinning = 100 and Burn-in = 0.1. ABGD analysis was run using Kimura (K80) evolutive model, a relative gap width (X) = 1, a divergence of intraspecific diversity between 0.0001 and 0.1 and Nb bins = 20. The matrix was loaded into the online ABGD webtool (http://wwwabi.snv.jussieu.fr/public/abgd/abgdweb.html).
The electronic edition of this article conforms to the requirements of the amended International Code of Zoological Nomenclature, and hence the new names contained herein are available under that Code from the electronic edition of this article. This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN. The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix “http://zoobank.org/”. The LSID for this publication is urn:lsid:zoobank.org:pub:5F42073C-02B6-458E-99CE-F4814D959E3C. The electronic edition of this work was published in a journal with an ISSN, and has been archived and is available from the following digital repositories: PubMed Central and LOCKSS.
Published data of Okenia taxa living in the Mediterranean Sea to date
The literature analysis revealed records of several valid taxa, some of which were considered to be misidentifications, and some belonging to taxa now considered as junior synonyms (Table 1; Fig 1). To date, the Okenia taxa considered as valid and recorded from the Mediterranean Sea can be divided into two groups according to their external colour patterns. This is based on those with striking colours (4 taxa), and those with whitish/brownish tonalities (4 taxa) (Table 1).
Locality numbers as in Table 1. Type localities of species described from the Mediterranean Sea marked with a red dot. When the exact locality was unknown, a dot was placed in the middle of the sighting area accompanied by a question mark.
The group with striking colours is the “easiest” to determine and consists of three taxa originally described from the Mediterranean Sea: Okenia elegans (Leuckart, 1828) (type species of Okenia Menke, 1830), Okenia hispanica Valdés & Ortea, 1995, and Okenia mediterranea (Ihering, 1886), and three junior synonyms of O. elegans, namely Idalia dautzenbergi Vayssière, 1919, Euplocamus cirriger Philippi, 1839, and Euplocamus laciniosus Philippi, 1841, described from the Mediterranean Sea (Table 1; Fig 1). The other species of this group, Okenia leachii (Alder & Hancock, 1854), was originally described from Torbay, Great Britain (Atlantic Ocean). Our literature research revealed that the presence of O. leachii in the Mediterranean Sea is doubtful and should be rejected. It was first listed as occurring in the Mediterranean Sea by Ihering , but such a statement was then either ignored [63, 107] or questioned  by subsequent authors and was finally dismissed since it was not based on preserved material. Then, a single specimen from the western Ligurian Sea was collected and published as an alleged “new record from the Mediterranean Sea” by Cattaneo-Vietti  (Table 1; Fig 1), but this was not supported by either a photograph or preserved material, and is now considered doubtful by Cattaneo-Vietti (pers. comm.) himself. The only photograph of O. leachii published in the Mediterranean literature, or in Mediterranean Sea slug websites, is in Trainito & Doneddu , which shows a specimen from Eilean Siar (Great Britain, Atlantic Ocean) (B. Picton, pers. comm.). This exclusion reduces the number of valid Okenia species with striking colours recorded from the Mediterranean to three.
The group with external colouration of white or brownish tonality, with the exception of Okenia aspersa (Alder & Hancock, 1845), has had a troublesome taxonomic and biogeographic history. Two species, Okenia cupella (Vogel & Schultz, 1970) and Okenia impexa Er. Marcus, 1957, were recorded for the Mediterranean basin but not originally described from the area. As explained in Note 3 of Table 1, these two species are valid at their original type localities. However, the Mediterranean records are contradictory and records ascribed to those two taxa in the Mediterranean simply belong to a single entity (Table 1 and notes therein). Two separate taxa (Okenia pusilla Sordi, 1974 and Okenia impexa banyulensis Schmekel, 1979), both described from the Mediterranean Sea, are now considered a junior synonym and an invalid introduction, respectively (Table 1 and notes 4, 5). Finally, Okenia zoobotryon (Smallwood, 1910), a species associated with the supposedly worldwide distributed bryozoan Amathia verticillata (delle Chiaje, 1822), was also recently recorded as a non-indigenous species within the Mediterranean Sea based on external morphology only [54, 74, 101] (Table 1; Fig 1). However, more or less simultaneously, Pola  reviewed the worldwide taxonomy and distribution of Okenia zoobotryon, showing that it is confined to its type locality region, ranging from Bermuda to Cuba (Western Atlantic Ocean). Indeed, these latter Mediterranean records could be explained by human-mediated introductions; however, further examination of Mediterranean specimens is required, since external morphology may be highly misleading in this taxon .
Recentely collected Okenia taxa living in the Mediterranean Sea and data mining from GenBank
Our field collections provided four different taxa. Two of them were clearly identified as Okenia elegans (Leuckart, 1828) and Okenia mediterranea (Ihering, 1886). However, despite their external similarities, the remaining taxa could not be clearly ascribed either to Okenia zoobotryon or Okenia cupella/impexa. For this reason, we listed our specimens as Okenia sp. 1 and Okenia sp. 2 until the molecular and morphological studies were undertaken below (Table 2). Those specimens morphologically examined are listed in the systematic section, whilst our recently sequenced specimens are listed in Table 3.
Species, number of specimens (N), vouchers, locality and coordinates, collector, and collection date.
Specimens sequenced during the present study marked with an asterisk.
GenBank revealed sequences available for three taxa recorded from the Mediterranean Sea: Okenia aspersa (Alder & Hancock, 1845) (COI, 16S, and H3), Okenia zoobotryon (Smallwood, 1910) (COI and H3), and Okenia sp. A (COI and 16S). Okenia aspersa was originally described from Cullercoats (England) in the North Sea , and the sequenced specimen come from Cape Ferret on the Atlantic coast of France, while Okenia zoobotryon was originally described from Bermuda [99–100], and the sequences come from its type locality. The specimen identified as Okenia sp. A was collected from Sabaudia Lake (Italy) and already partially sequenced in Paz-Sedano et al. . In this study, we added the H3 sequence and revealed its true identity. Moreover, several additional sequences of species belonging to the genus Okenia and the family Goniodorididae were retrieved from GenBank to compare our results and the evolutionary relationships of the newly barcoded taxa, as well as species to be used as outgroups (see Table 3).
The phylogenetic tree based on concatenated genes sequences including all species of Okenia Menke, 1830 and other taxa belonging to Goniodorididae H. Adams & A. Adams, 1854 is shown in Fig 2. All taxa belonging to Goniodorididae (highlighted with an orange circle) are clustered in a well-supported clade (BI = 1, ML = 85) (Fig 2). The resulted tree showed a polytomy including the two Goniodoris species included in the analysis and Okenia species, suggesting that their phylogenetic relationship is still unresolved (BI = 1, ML = 93) (Fig 2).
Orange circle indicates Goniodorididae taxa. Pink branches represent Okenia taxa. Okenia species living in the Mediterranean Sea highlighted in blue. New species identified in the present study highlighted in yellow. Different colours highlighted in bPTP and ABGD species delimitation analyses represent potential different taxa.
With respect to the species of the genus Okenia, our analyses have not been able to clarify all the existing relationships among the species included. However, we identified four main clades: i) the first one clusters Okenia felis Gosliner, 2010 and Okenia picoensis Paz-Sedano, Ortigosa & Pola, 2017 (BI = 1, ML = 92) (Fig 2); ii) the second includes Okenia zoobotryon (Smallwood, 1910), Okenia harastii Pola, Roldán & Padilla, 2014, and Okenia angelensis Lance, 1966 (BI = 0.99, ML = 73), with O. zoobotryon and O. angelensis as sister species (BI = 1, ML = 98) (Fig 2); iii) the third clade includes Okenia amoenula, Okenia mediterranea, Okenia aspersa, and Okenia elegans (BI = 0.99, ML = 85), with O. amoenula being sister species of O. mediterranea (Fig 2), and O. aspersa sister species of O. elegans (BI = 0.98) (Fig 2); iv) the last supported clade includes Okenia brunneomaculata Gosliner, 2004, Okenia pellucida Burn, 1967, Okenia vena Rudman, 2004, and Okenia sp. 1, with a BI value of 0.99 (Fig 2).
Okenia sp. 1 from Lago di Sabaudia, Mar Piccolo, Porto Ercole, and Naples (Mediterranean Sea) and Okenia sp. 2 from Gallipoli (Mediterranean Sea) are respectively grouped in single clades (Fig 2). Among the Okenia taxa retrieved from Genbank, Okenia sp. A (MNCN 15/70411) deserves specific mention. That specimen, originating from Lago di Sabaudia (Italy) and previously barcoded in a paper including some of us as co-authors (SPS and MP) , proved to be conspecific with Okenia sp. 1 (Fig 2). Our bPTP and ABGD species delimitation analyses clearly support that Okenia sp. 1 and Okenia sp. 2 belong to single entities each (Fig 2), with a COI uncorrected p-distance between them of 17.4–17.9% (Table 4). Interestingly, despite their external similarities, Okenia sp. 1 did not prove to be conspecific with either the topotypical O. zoobotryon or its sister species Okenia angelensis Lance, 1966. In addition, despite the fact that no material of Okenia impexa was available to us (see below for further discussion on this taxon) and the COI sequence of Okenia cupella was not obtained, the H3 analysis showed that Okenia sp. 2 is not conspecific with the holotype of Okenia cupella, with an uncorrected p-distance between these two taxa of 11.6% (Table 4). Indeed, it should be noted that all the Mediterranean taxa analysed here, except Okenia sp. 1 and Okenia sp. 2, were represented by single specimens due to their rarity; this somehow makes the results from the species delimitation analysis (ABGD and bPTP) less powerful then usual. However, on the other hand, both morphological comparisons and molecular results point toward the fact that Okenia sp. 1 and Okenia sp. 2 are undescribed species; thus, they are formally described below in the Systematics section. At the same time, as to allow for comparisons, also the holotype of O. cupella is redescribed here.
Order Nudibranchia Cuvier, 1817
Family Goniodorididae H. Adams & A. Adams, 1854
Genus Okenia Menke, 1830
Type species Idalia elegans Leuckart, 1828 by monotypy
For a detailed synonymy and diagnosis of the genus see Rudman .
Okenia longiductis sp. nov.
(formerly Okenia sp. 1)
A. Specimen from Lago di Sabaudia (Italy). Photograph by A. Macali. B. Specimen from La Grande-Motte (France). Photograph by D. Minchin. C. Egg-masses on Amathia verticillata from La Grande-Motte (France). Photograph by D. Minchin. D. Specimen from Mar Piccolo, Taranto (Italy). Photograph by G. Colucci. Size (alcohol-preserved specimens) ~9 mm maximum length.
Internal anatomy. A. Buccal bulb. B. Female portion of the reproductive system. C. Male portion of the reproductive system. D. Reproductive system extended. Abbreviations: am, ampulla; bc, bursa copulatrix; bp, buccal pump; fgm, female gland mass; hd, hermaphroditic duct; oe, oesophagus; og, oral glands; pr, prostate; ra, radular sac; rs, receptaculum seminis; sgl, salivary gland; ud, uterine duct; va, vagina; vd, vas deferens. Scale bars: 1 mm.
Scanning electron micrographs (SEM) and light microscope photographs (LMP). A. SEM. Detail of jaw elements (SZN-MOL0019). B. LMP. Detail of cuticle elements surrounding lips (SZN-MOL0006). C. SEM. Frontal view of entire radula (MNCN15.05/88169). D. SEM. Frontal view of radula. Detail of rachis, internal, and external teeth (MNCN15.05/88172). E. SEM. Detail of internal teeth (MNCN15.05/88172). F. SEM. Detail of external teeth (MNCN15.05/200036). Scale bars: A, 10 μm; B, 10 μm; C, 300 μm; D, 50 μm; E, 30 μm; F, 30 μm.
Light microscope photographs (LMP). A. Distal part of the penis, with penial spines (MNCN15.05/200040). B. Vas deferent with penial spines (SZN-MOL0005). Scale bars: 1mm.
Okenia zoobotryon sensu Trainito & Doneddu : 23 (Fig).
Okenia cf. zoobotryon sensu Ballesteros et al. : 8.
Okenia zoobotryon sensu Lipej et al. : 134–135 (Figs).
Holotype: Lago di Sabaudia (Italy), 0–1 m (Table 2), 3 mm preserved (dissected) (Table 3) (MNCN15.05/200035). Paratypes (P): Lago di Sabaudia (Italy), 0–1 m (Table 2; Fig 3A): P1–3 mm preserved (dissected) (Table 3) (SZN-MOL0003); P2–3.5 mm preserved (dissected) (Table 3) (SZN-MOL0004); P3–3 mm preserved (dissected) (Table 3) (MNCN15.05/70411); P4–4 mm preserved (dissected) (Table 3; Fig 5F) (MNCN15.05/200036). Mar Piccolo (Italy), 3–5 m (Table 2; Fig 3D): P5–9 mm preserved (dissected) (Table 3; Fig 6B) (SZN-MOL0005); P6–7 mm preserved (dissected) (Table 3) (MNCN15.05/200037). Porto Ercole (Italy), 0–1 m (Table 2): P7–7.8 mm alive, 5 mm preserved (dissected) (Table 3; Fig 5B) (SZN-MOL0006); P8–5.8 mm alive (Table 3) (MNCN15.05/200038); P9–5.4 mm alive (Table 3) (SZN-MOL0007); P10–6.2 mm alive (Table 3) (MNCN15.05/200039). Naples (Italy), 0–1 m (Table 2): P11–7.8 mm alive (dissected) (Table 3; Fig 6A) (MNCN15.05/200040); P12–7 mm alive (Table 3) (SZN-MOL0008); P13–6.1 mm alive (Table 3) (MNCN15.05/200041); P14–5.4 mm alive (Table 3) (SZN-MOL0009); P15–8.9 mm alive (Table 3) (MNCN15.05/200042).
Lago di Sabaudia (Italy), 0–1 m (Table 2; Fig 3A): 5 mm preserved (dissected) (SZN-MOL0011); 5 mm preserved (dissected) (SZN-MOL0010); 4 mm preserved (dissected) (MNCN15.05/88164); 7 mm preserved (MNCN15.05/88165). Mar Piccolo (Italy), 3–5 m (Table 2; Fig 3D): 7 mm preserved (dissected) (MNCN15.05/88166); 6 mm preserved (SZN-MOL0012); 8 mm preserved (Table 3) (SZN-MOL0013); 6 mm preserved (SZN-MOL0014); 6 mm preserved (MNCN15.05/88167); 6 mm preserved (MNCN15.05/88168). Porto Ercole (Italy), 0–1 m (Table 2): 7 mm alive (Table 3) (SZN-MOL0015); 6.8 mm alive (dissected) (Table 3; Fig 5C) (MNCN15.05/88169); 6.1 mm alive (dissected) (Table 3) (MNCN15.05/88170). Naples (Italy), 0–1 m (Table 2): 6.8 mm alive (Table 3; Fig 5A) (SZN-MOL0019); 5.8 mm alive (Table 3) (SZN-MOL0017); 8 mm alive (Table 3) (SZN-MOL0018); 6.2 mm alive (Table 3) (MNCN15.05/88171). La Grande-Motte (France), 0–0.5 m (Table 2; Fig 3B): 3 mm preserved (dissected) (SZN-MOL0016); 6 mm preserved (dissected) (Fig 5D) (MNCN15.05/88172); 3 mm preserved (MNCN15.05/88173).
External morphology (Fig 3).
Preserved specimens up to 9 mm maximum length. Elongated body ending in long and pointed posterior end of foot. Well-developed notal border with variable range of lateral and dorsal papillae, always symmetrically distributed on each body side. Lateral papillae: 5–8 on each body side; 1 additional papilla may be present in most posterior part of notum. Distribution as follows: 2 located in front of rhinophores, 1 behind gill, 3–6 between rhinophores and gill. Lateral papillae elongated, relatively short and thin. Dorsal papillae: 5–11. Distribution as follows: at least 3 papillae always present in front of gills and 1 behind rhinophores; remaining papillae (1–7) dispersed. Shape of dorsal papillae may vary, being similar to laterals or small bumps in mantle. Rhinophores long and slender, bearing 4–6 lamellae each. Gill composed of 7–8 tripinnate branches surrounding anus. Two large and well-developed oral tentacles, one each side of mouth. Foot elongate, rounded at anterior part, without visible propodial tentacles. Reproductive opening on right lateral side of body, located in first third of body. Entire body covered by conspicuous spicules.
Colour pattern (Fig 3).
Translucent white background, slightly grey due to transparency of internal organs; body with scattered white, dark brown, light brown, and cream spots. Dark brown spots more concentrated around base of rhinophores, outer face of gill branches and mouth; in remaining parts, intensity of brown spots variable among different specimens, sometimes giving a general brown tone. Papillae with white spots and few random brown spots. Translucent posterior end of foot with scattered white and brown dots, similar to body. Translucent foot with few dispersed dark brown spots.
Buccal bulb thick and muscular (Fig 4A). Large number of rounded oral glands (og) surround anterior opening of bulb (Fig 4A). Buccal pump (bp) large and expanding dorsally and backwards (Fig 4A). Radular sac (ra) short, descending ventrally (Fig 4A). Thin oesophagus (oe) inserting into buccal bulb behind buccal pump (Fig 4A). One rounded salivary gland (sgl) on both sides of oesophagus (Fig 4A). Labial cuticle surrounding lips and expanding inside buccal pump; thin and weak part located inside buccal bulb with honeycomb-shaped jaw elements (Fig 5A); harder elements surrounding lips (Fig 5B). Radular formula of all dissected specimens 26–30×188.8.131.52.1 (Fig 5C). Shape of teeth similar in all localities. Inner lateral tooth with single large and robust cusp and robust and wide base (Fig 5D–5E). Cusp large, robust, and pointed with internal masticatory margin usually bearing 10–13 fine, pointed denticles; centrals being longer than those located at the ends (Fig 5D–5E). Posterior end of cusp well developed, ending in a sort of prominent wing (Fig 5D–5E). Outer lateral tooth much smaller, with large base and 2 relatively thin and pointed cusps, upper one wider than lower one (Fig 5F).
Large reproductive system located in anterior third of body. Hermaphroditic duct (hd) thin and long beginning in ovotestis, located inside digestive-hermaphroditic gland, then expanding into a kidney-shaped ampulla (am) (Fig 4B–4D). Postampullatory duct narrow, connecting ampulla to female gland mass (fgm), dividing into oviduct and swollen prostatic portion (pr) of vas deferens (vd) (Fig 4C–4D). Prostate becomes narrow again, continues as an extraordinarily long and coiled duct, especially in its most distal part, where there are many closed loops (Fig 4B–4D). Vas deferens long, widening approximately in middle part and continuing narrowing towards thin ejaculatory duct, ending in penis (Fig 4C–4D). Penis with penial spines (Fig 6). Spines elongate and covering much of vas deferens (Fig 6). Thickness of vagina (va) similar to ejaculatory end of vas deferens (Fig 4D). Vagina considerably long connecting with big and elongated bursa copulatrix (bc) (Fig 4B and 4D). Base of the bursa copulatrix connected to the receptaculum seminis (rs) by a very long, thin, and finely coiled duct (Fig 4B and 4D). Receptaculum seminis slightly smaller and rounder than the bursa copulatrix (Fig 4B and 4D). Thin uterine duct (ud) entering female gland and emerging at base of receptaculum seminis (Fig 4B and 4D). Female gland very well developed (Fig 4B–4D).
Mar Piccolo (Taranto), Naples, Lago di Sabaudia (Latina), Porto Ercole (Grosseto) (Italy), and La Grande-Motte (Hérault) (France) (present study). Literature records of O. zoobotryon from the Mediterranean Sea [54, 74, 101] (Fig 1; Table 1) also belong to this taxon. In fact, despite we were not able to study morphologically or molecularly those specimens as they were only mentioned in species lists or have disappeared, present evidences strongly point towards repetitive misidentification in the Mediterranean Sea, and thus we listed them here in the synonymy of the newly described species.
We always found this species living in the infralittoral zone (up to 5 m depth) on the arborescent bryozoan Amathia verticillata (delle Chiaje, 1822) (Fig 3B and 3D). The same depths and feeding association hold for previous records belonging to Okenia zoobotryon [54, 74, 101]. White and ring-shaped egg-masses were present on this bryozoan (Fig 3C).
Okenia longiductis sp. nov. resembles Okenia zoobotryon (Smallwood, 1910) and Okenia angelensis Lance, 1966 due to their similar general body shape and external colour pattern. Okenia zoobotryon is a taxon originally described from Bermuda [99–100] and recently reported from some Mediterranean localities—Pialassa Baiona, Ravenna (Italy), 23.09.2012: Trainito & Doneddu , F. Ioni, pers. comm.; Cala Maset, Sant Feliu de Guíxols (Spain): Ballesteros et al. ; entire coastline of Slovenia: Lipej et al. . Okenia angelensis is only known to occur in a wide area of the Eastern Pacific [108–110]. However, our morphological analyses revealed clear external and internal differences between O. longiductis sp. nov. and these other taxa. A detailed comparison between these three taxa is shown in Table 5. Moreover, our molecular results, including those from species delimitation analyses, support that the three taxa are valid and distinct species (Fig 2), with a COI uncorrected p-distance of 17.2–17.8% between O. longiductis sp. nov. and O. angelensis and of 17.9–18.3% between O. longiductis sp. nov. and O. zoobotryon (Table 4).
Okenia problematica sp. nov.
(formerly Okenia sp. 2)
A. Holotype (MNCN15.05/200034); B. Paratype (SZN-MOL0001). Photographs by F. Vitale. Size (alcohol-preserved specimens) ~2.5 mm maximum length.
A. Buccal bulb. B. Reproductive system. Abbreviations: am, ampulla; bc, bursa copulatrix; bp, buccal pump; fgm, female gland mass; hd, hermaphroditic duct; oe, oesophagus; ra, radular sac; rs, receptaculum seminis; sgl, salivary gland; ud, uterine duct; va, vagina; vd, vas deferens. Scale bars: 1 mm.
A. SEM. Frontal view of radula. Detail of rachis, internal, and external teeth (MNCN15.05/200034). B. SEM. Detail of internal and external lateral teeth (MNCN15.05/200034). C. SEM. Detail of internal teeth (SZN-MOL0002). D. SEM. Detail of external tooth (SZN-MOL0002). E. LMF. Detail of penial spines (MNCN15.05/200034). F. SEM. Detail of penial spines (SZN-MOL0002). Scale bars: A, 50 μm; B, 30 μm; C, 20 μm; D, 10 μm; E, 10 μm; F, 20 μm.
Okenia impexa Marcus, 1957 sensu Templado : 250.
Okenia cupella sensu Templado et al. : 100, 197.
Okenia cupella (Vogel y Schultz, 1970) sensu García-Gómez et al. : 464 (Fig).
Holotype: Gallipoli (Italy), 30 m (Table 2): 2 mm preserved (dissected) (Table 3; Figs 7A, 8, 9A–9B and 9E) (MNCN15.05/200034). Paratype: Gallipoli (Italy), 30 m (Table 2): (Fig 7B), 1.5 mm preserved (dissected) (Table 3) (SZN-MOL0001).
Aiguafreda (Spain), 10 m (Table 2): 2.5 mm preserved (dissected) (Fig 9C–9D and 9F) (SZN-MOL0002); 2 mm preserved (dissected) (MNCN15.05/88162). Cala Joncols (Spain), 11 m (Table 2): 2 mm preserved (MNCN15.05/88163).
External morphology (Fig 7).
Preserved specimens up to 2.5 mm maximum length. Elongated body ending in long and pointed posterior end of foot. Well-developed notal border with 8 lateral papillae symmetrically distributed on each body side. Distribution of papillae as follows: 1 located in front of rhinophores, 1 at same level of rhinophores, 4 between rhinophores and gill, 2 behind gill, both arising from same stalk. Shape and size of papillae slightly variable; anteriormost 2 papillae on each side long and finger-like, followed by 1 shorter and subsequently increasing in size. All papillae behind rhinophores with rounded widening at tip, more or less evident, being more marked in papillae behind gill. Single papilla in mid-dorsal line similar in size and shape to lateral ones, also rounded tipped. Rhinophores long and slender, bearing 6–9 lamellae each. Gill composed of 4 unipinnate branches surrounding anus. Two anteriormost branches sharing stalk. Two short oral tentacles, one each side of mouth. Foot long and slender, with 2 small but elongated propodial tentacles in anterior part. Reproductive opening on right lateral side of body, usually at short distance from rhinophore. Entire body covered by conspicuous spicules.
Colour pattern (Fig 7).
White translucent background; body and gill covered with many and concentrated brown spots; white-yellowish spots scattered randomly. Rhinophores and papillae translucent with cream-white and scattered light brown spots. White translucent tail with mostly white and scattered brown dots. White translucent foot without spots.
Buccal bulb thick and muscular (Fig 8A). Labial glands absent. Buccal pump (bp) large and expanding dorsally and backwards (Fig 8A). Radular sac (ra) short descending ventrally (Fig 8A). Thin oesophagus (oe) inserting into buccal bulb behind buccal pump (Fig 8A). One small and rounded salivary gland (sgl) on both sides of oesophagus (Fig 8A). Labial cuticle surrounding lips and expanding inside buccal pump. Small jaw elements present, but lost in all samples during manipulation before SEM. Radular formula of Gallipoli (Italy) specimens 12×184.108.40.206.1. Spanish specimens with similar formula (10–12×220.127.116.11.1). Shape of teeth similar in both localities (Fig 9B–9C). Inner lateral tooth large and hook-shaped with strong base (Fig 9B–9C). Upper cusp large, robust, and pointed with internal masticatory margin usually bearing 7–9 thin long denticles, decreasing in size progressively from tip to lower part of tooth (Fig 9B–9C). Posterior end of cusp well developed, ending in a sort of prominent wing (Fig 9A–9C). Lower cusp smaller, curved, and pointed (Fig 9A–9C). Outer lateral tooth even smaller, with large base and 2 relatively thin and pointed cusps, upper one longer than lower one (Fig 9D).
Reproductive system located in anterior third of body. Hermaphroditic duct (hd) elongated and beginning in ovotestis, located inside digestive-hermaphroditic gland, then expanding into a large and kidney-shaped ampulla (am) (Fig 8B). Postampullatory duct short, connecting ampulla to female gland (fgm), dividing into oviduct and prostatic portion of vas deferens (vd). Prostate not morphologically differentiated. Vas deferens widens in middle part and continues slightly narrowing towards ejaculatory duct, ending in penis. Penis with thin and relatively long and hooked penial spines (Fig 9E–9F). Thickness of vagina (va) similar to ejaculatory end of vas deferens (Fig 8B). Vagina relatively long connecting with rounded bursa copulatrix (bc) (Fig 8B). Receptaculum seminis (rs) pear-shaped arising in middle of vagina, supported by short duct (Fig 8B). Thin uterine duct (ud) entering female gland and emerging in base of receptaculum seminis (Fig 8B).
Gallipoli (Lecce, Italy), Aiguafreda (Barcelona, Spain), and Cala Joncols (Girona, Spain) (present study). Literature records of Okenia cupella and Okenia impexa from the Mediterranean Sea [18, 52, 54, 57, 85, 95, 98] (Fig 1; Table 1), except that of Okenia pusilla Sordi, 1974 from Ischia Island  (see below in Remarks), also belong to this taxon. In fact, despite attemps to examine previously collected samples, we were not able to study morphologically or molecularly those specimens as they were only mentioned in species lists or have disappeared from collections. However, we present evidence strongly pointing towards repetitive misidentification in the Mediterranean Sea, and thus we listed them here in the synonymy of the newly described species.
We always found Okenia problematica sp. nov. at depths below 10 m. The specimens collected in Gallipoli (Italy) were found on an artificial reef located on a sandy bottom, amidst unidentified hydrozoans and encrusting bryozoans. No environmental data were collected for the Spanish specimens (Aiguafreda and Cala Joncols). Templado , Valdés & Ortea , García-Gómez et al. , and Ballesteros et al.  mostly reported its cryptic presence in rocky infralittoral bottoms (~5–10 m) with the bryozoan Margaretta cereoides (Ellis & Solander, 1786). However, Schmekel , Schmekel & Portmann , Valdés & Ortea , and Ballesteros et al.  also reported its presence at 5–15 m depth on infralittoral algae such as Halimeda Lamouroux or Corallinaceae Lamouroux taxa and Codium vermilara (Olivi) Delle Chiaje.
Okenia problematica sp. nov. has had a troublesome taxonomic history. In fact, it is very likely to have been already recorded in the Mediterranean Sea both as Okenia cupella (Vogel & Schultz, 1970) and as Okenia impexa Er. Marcus, 1957. Moreover, two names related to O. cupella and O. impexa were introduced based on Mediterranean material, namely Okenia pusilla Sordi, 1974 and Okenia impexa banyulensis Schmekel, 1979 (Table 1). Such a nomenclatural chaos presumably comes from the extreme similarities of this group of species with a similar external colour pattern and characteristic tip of the papillae, as well as the absence of detailed descriptions of holotypes and/or topotypical specimens. Okenia cupella was originally described from the western Atlantic, in Virginia . Despite its great similarities in external appearance and radular features with other Okenia species, this taxon has been widely recorded from both sides of the Atlantic Ocean [12, 51], including the western Mediterranean Sea (Table 1; Fig 1). The situation is similar for O. impexa, originally described from Brazil , recorded from both sides of the Atlantic Ocean [111–112] and also from the western Mediterranean Sea (Table 1; Fig 1). However, we found that our specimens from the Mediterranean supposedly belonging to either taxa, actually belong to the same entity. To clarify the taxonomic position of our specimens we morphologically and molecularly studied the holotype of O. cupella (see redescription below and Table 3), whereas for O. impexa we had to rely on the original description  and the topotypical specimens figured by Sales et al. , as the type material only consists of slides of syntypes . Our morphological analyses based on published and new data revealed several major differences between O. problematica sp. nov. and O. impexa and O. cupella (Table 6). Moreover, the molecular results for the H3 gene analysis confirm the observed morphological dissimilarities, supporting the hypothesis that O. problematica sp. nov. and O. cupella are different species, with a H3 uncorrected p-distance of 11.6%, value higher than other H3 uncorrected p-distances within and between species (Table 4).
The identity of the material that provided the basis for the names O. pusilla and O. impexa banyulensis is also uncertain. Okenia pusilla was originally described based on the external anatomy and radular features of a single specimen deposited at CIBM—Centro Interuniversitario di Biologia Marina ed Ecologia Applicata “G. Bacci” (Livorno, Italy). Sordi  described this species as having four brown gill branches, with the two anteriormost sharing the same stalk, eight papillae on each side of the body, yet lacking dorsal papillae, a character highlighted in the original description in order to differentiate between O. pusilla and O. impexa—see also the holotype drawn in Sordi  and photographed in Cattaneo-Vietti et al. . However, our specimens always bore dorsal papillae, despite being of similar size with respect to the holotype of O. pusilla. Furthermore, the poor description of the radular features did not enable us to compare our material with that described by Sordi . Unfortunately, it was not possible to retrieve the type material for a direct comparison, as this material has been presumably lost (S. De Ranieri, pers. comm.). For these reasons, O. pusilla cannot be ascribed with confidence to any known Okenia species, including O. problematica sp. nov. For the sake of nomenclatural stability, we here state that O. pusilla should be recognized as a nomen dubium (ICZN : glossary).
Schmekel  described Okenia impexa banyulensis based on two main differences when compared with the original description of O. impexa: i) the shape of the lateral tooth, with two cusps in the Mediterranean specimens vs three cusps in the western Atlantic specimens; ii) the shape of papillae, with Mediterranean specimens having very short, pointed, digitiform tubercles and lateral posterior appendages of the notum border apically swollen and rounded vs club-shaped tubercles and claviform though pointed lateral posterior appendages in western Atlantic specimens. Indeed, specimens recorded by Schmekel  would appear to be conspecific with O. problematica sp. nov. Again, our efforts to locate the material analyzed by Schmekel  failed, as it is presumably lost. In fact, it was not found in the Natural History Museum Basel (Switzerland) nor the Natural History Museum of Munich (Germany) (Michael Schrödl, pers. comm.), where it was originally deposited . Furthermore, Schmekel  wrote “If further observations in the western Atlantic confirm the constancy of the above mentioned geographical differences, I name the eastern Atlantic subspecies Okenia impexa banyulensis”. Thus, having been published after 1960 and proposed conditionally, O. impexa banyulensis is not available under ICZN  rules (art. 15.1) (see also Table 1).
Okenia cupella (Vogel & Schultz, 1970)
Internal anatomy. A. Buccal bulb. B. Reproductive system. Abbreviations: am, ampulla; bc, bursa copulatrix; bp, buccal pump; fgm, female gland mass; hd, hermaphroditic duct; oe, oesophagus; ra, radular sac; rs, receptaculum seminis; sgl, salivary gland; ud, uterine duct; va, vagina; vd, vas deferens. Scale bars: 1 mm.
Preserved specimen not reaching 1 mm maximum length (Fig 10). Original description of external morphology is complete and well detailed (see Vogel & Schultz ; Figs 1–3), and therefore we avoided including it here.
Buccal bulb thick and muscular (Fig 11A). Buccal pump (bp) large and rounded, expanding dorsally and backwards (Fig 11A). Radular sac (ra) descending ventrally (Fig 11A). Thin oesophagus (oe) inserting into buccal bulb behind buccal pump, this union being surrounded by nervous system. Small and slightly elongated salivary glands (sgl) on either side of oesophagus (Fig 11A). Labial cuticle surrounding lips and expanding inside buccal pump. Radular formula observed under light microscope: 10–11×18.104.22.168.1 (Fig 12A), thus fitting its original description: radula bearing 10 rows of teeth pointed posteriorly, and inner lateral teeth with masticatory margin with 9 denticles . Due to tiny size and transparency of radula, we were not able to extend it properly in order to prepare and examine it in detail with scanning electron microscopy.
Reproductive system located in anterior part of body. Thin and elongate hermaphroditic duct (hd) beginning in ovotestis, located inside digestive-hermaphrodite gland, then expanding into big, thick, and rounded ampulla (am), being almost half of entire reproductive system (Fig 11B). Postampullatory duct thin, connecting ampulla to female gland (fgm), dividing into thin oviduct and slightly wider prostatic portion of vas deferens (vd) (Fig 11B). Distal end of prostatic part continues as a long and wide ejaculatory duct, ending in penis. Penis with large, wide, and hooked penial spines (Fig 12B). Vagina (va) shorter and thinner than vas deferens, connecting with rounded bursa copulatrix (bc). Receptaculum seminis (rs) elongate arising near to this union. Receptaculum seminis slightly smaller than bursa copulatrix. Long and once rolled uterine duct (ud) entering female gland and emerging in base of receptaculum seminis.
The taxonomy and biogeography of Okenia species in the Mediterranean Sea has been mainly based on external morphology, with the sole exception of very few works which studied the internal anatomy of selected specimens [17, 18, 57], and the majority of papers dealing with local biota relied only on external resemblances for identification (Table 1 and references therein). However, despite the presence of diagnostic characters even in external morphology (e.g. general colour and shape of papillae), identifications of Okenia taxa have proved to be a challenging task worldwide, resulting in multiple misidentifications that have confused the geographical distribution of species within this genus. This indeed asked for the necessity of an in-depth morphological and molecular review of Mediterranean Okenia species.
Based on our preliminary literature research, eight valid species were usually reported from the Mediterranean Sea, namely Okenia aspersa (Alder & Hancock, 1845), Okenia cupella (Vogel & Schultz, 1970), Okenia elegans (Leuckart, 1828), Okenia hispanica Valdés & Ortea, 1995, Okenia impexa Er. Marcus, 1957, Okenia leachii (Alder & Hancock, 1854), Okenia mediterranea (Ihering, 1886), and Okenia zoobotryon (Smallwood, 1910). However, not all of them can be considered to be actually occurring in the area (Fig 13). This result should not be considered surprising as many molluscan groups of the Mediterranean Sea have not been fully subjected to a focused review based on literature and modern genetic methods, and Mediterranean checklists are often still being compiled. In addition, deletion of species reported in local, national, or Mediterranean checklists is an ongoing and time-consuming process necessary towards a homogenization of the general knowledge of the Mediterranean malacofauna [31, 114–120].
Okenia elegans. Miramare (Trieste, Italy) [65–66]. Photo: D. Poloniato. Okenia hispanica. Holotype. Estepona (Malaga, Spain) [52, 74]. Photo: D. Moreno/Fauna Ibérica (MNCN-CSIC). Okenia leachii. Eilean Siar (Great Britain) . Photo: B. Picton. Okenia mediterranea. Santa Maria al Bagno (Lecce, Italy) (Table 2: MNCN15.05/88174). Photo: F. Vitale. Okenia aspersa. Noli (Savona, Italy) [19, 91]. Photo: F. Betti. Okenia problematica sp. nov. (= Okenia cupella/impexa sensu AA [18, 52, 54, 57, 85, 95, 98]). Gallipoli (Italy) (Table 4: MNCN15.05/200034). Photo: F. Vitale. Okenia longiductis sp. nov. (= Okenia zoobotryon sensu AA [54, 74, 101]). Lago di Sabaudia (Italy) (Table 2). Photo: A. Macali.
Interestingly, two of the six remaining species, O. cupella/impexa [18, 52, 54, 57, 85, 95, 98] and O. zoobotryon [54, 74, 101], were constantly misidentified in the past literature. We name here the specimens previously identified as those species for the Mediterranean Sea as Okenia problematica sp. nov. and Okenia longiductis sp. nov., respectively. Our result agrees with the current molluscan literature for the Mediterranean Sea. Indeed, the validity of many taxa described or recorded from the Mediterranean Sea over centuries has yet to be confirmed by molecular means or by barcoding of material . The study of selected phylogenetic clades, through integrative approaches, has already brought to unexpected results, including the discovery or description of several new species even within widely studied groups [24, 27–29, 121–122]. At the same time, our result also confuted the possible occurrence of O. zoobotryon in the Mediterranean Sea, thus contributing to shed light on local bioinvasions.
Unfortunately, the elusive character of Okenia taxa has prevented us to answer additional questions that still remain unresolved. We were unable to obtain fresh material of O. aspersa, and we relied on a specimen previously barcoded from nearby its Atlantic type locality. Several authors have already highlighted differences between Mediterranean and eastern Atlantic specimens [2, 16–18, 85]. Consequently, should cryptic diversity be eventually found within this taxon, no name is available for Mediterranean specimens, taking also into account that Doris quadricornis Montagu, 1815 [= Okenia quadricornis (Montagu, 1815)] was suppressed in 1974 under ICZN Opinion 1014 , and thus it will again result in a species new to science. Another Mediterranean species worth a mention is O. hispanica. Despite being described more than 20 years ago, it is still only known from its holotype [18, 52, 74, 123] (Fig 13). The presence of Mediterranean endemisms, with a narrow distribution restricted to the Alborán Sea, is already a well-known phenomenon [124–125], although debated in the recent literature . Taking into account that no freshly collected specimens were available for this study, and that the holotype was originally fixed in formaldehyde, further field work is necessary to determine if O. hispanica is a truly Mediterranean endemism or the knowledge of its distribution is still incomplete; new collections are also required to evaluate its phylogenetic relationships with congeneric taxa. Finally, the external and internal anatomy of the O. mediterranea specimen studied here did fit well with that previously described by Cervera et al. . However, Schmekel  and Cervera et al.  already highlighted that O. mediterranea is made up of two different morphs highly variable in their external anatomy and colour pattern. Unfortunately, the original description lacks detail of its internal anatomy, and therefore a topotype is needed to evaluate putative taxonomic differences between these colour morphs.
In summary, the present study significantly clarifies the biodiversity of the genus Okenia in the Mediterranean Sea, by i) recapitulating all the known records of species within this genus, and highlighting the questionable ones; ii) restricting the taxonomically validated records to O. aspersa, O. elegans, O. hispanica, and O. mediterranea, although leaving doubts on some of those taxa; iii) adding O. problematica sp. nov and O. longiductis sp. nov to the present Mediterranean biota; iv) resolving the status of some intricated taxonomic problems, and proposing Okenia pusilla as a “nomen dubium”. Finally, it restricts the presence of O. cupella, O. impexa, and O. zoobotryon to the western Atlantic, and of O. leachii to the eastern Atlantic. A comparative table highlighting main external diagnostic characters and radular formula of Okenia species living in the Mediterranean is reported in Table 7.
Manuel Ballesteros (Spain), Marina Poddubetskaia (France), and Enrico Ricchitelli (Italy) provided specimens. Juan Lucas Cervera Currado, Karla Araujo, Maria del Rosario Martín Hervás, and additional members of the nudilab in Cadiz (UCA, Spain) helped in sequencing. Enrique Rodríguez (UAM, Spain) helped with SEM photos. Stefano De Ranieri (Italy), Michael Schrödl (Germany), Luiz Ricardo Lopes de Simone (Brazil), and Ellen Strong (Washington) provided information on type materials. Terrence Gosliner and Dimitri Smirnoff (CAS, San Francisco) provided sequences of Okenia angelensis, and the former also helped to improve a first version of this manuscript. Filippo Ioni (Italy) provided data on the “Okenia zoobotryon” specimen figured in Trainito & Doneddu (2014). Federico Betti (Italy), Diego Moreno (Spain), Bernard Picton (Great Britain), and Diego Poloniato (Italy) provided photos of Okenia specimens. Pasqualina Fiorentino (Italy) provided literature.
- 1. OBIS (Ocean Biogeographic Information System). National Benthic Interfauna Database. 1998. www.iobis.org/mapper/?taxon_id=650791.
- 2. Gosliner TM. Phylogenetic systematics of Okenia, Sakishimaia, Hopkinsiella and Hopkinsia (Nudibranchia: Goniodorididae) with descriptions of new species from the tropical Indo-Pacific. Proc Calif Acad Sci. 2004;5: 125–161.
- 3. Gosliner TM, Bertsch H. Systematics of Okenia from the Pacific Coast of North America (Nudibranchia: Goniodorididae) with Descriptions of Three New Species. Proc Calif Acad Sci. 2004;55(22): 414–430.
- 4. Rudman WB. Further species of the opisthobranch genus Okenia (Nudibranchia: Goniodorididae) from the Indo-West Pacific. Zootaxa. 2004;695: 1–70.
- 5. Edmunds M. Opisthobranchiate Mollusca from Ghana: Goniodorididae. J Conchol. 2009;40(1): 37–54.
- 6. Bouchet P, Rocroi J, Hausdorf B, Kaim A, Kano Y, Nützel A, et al. Classification, Nomenclator and Typification of Gastropod and Monoplacophoran Families. Malacologia. 2017;61(1–2): 1–526.
- 7. Pola M, Roldán P, Padilla S. Molecular data on the genus Okenia (Nudibranchia: Goniodorididae) reveal a new cryptic species from New South Wales (Australia). J Mar Biol Assoc U.K. 2014;94: 587–598.
- 8. Paz-Sedano S, Ortigosa D, Pola M. A new Okenia Menke, 1830 from the Azores Island, Portugal. Spixiana. 2017;40(1): 13–22.
- 9. Camacho-García YE, Gosliner MT. A New Species of Okenia (Gastropoda: Nudibranchia: Goniodorididae) from the Pacific Coast of Costa Rica. PCA. 2000;55(33): 431–438.
- 10. Rudman , 2007 Rudman WB. Two new species of Okenia (Gastropoda: Nudibranchia: Goniodorididae) eastern Australia and Tanzania. Zootaxa. 2007;1657: 57–67
- 11. Gosliner TM. Two New Species of Nudibranch Mollusks From the Coast of California. Proc Calif Acad Sci. 2010; 61(16): 623–631.
- 12. Ortea J, Espinosa J. Nueva especie del género Okenia Menke, 1830 (Mollusca: Nudibranchia) de Cuba. Avicennia. 2000;12/13: 84–86.
- 13. Ortea J, Moro L, Bacallado JJ, Caballer M. Nuevas especies y primeras citas de babosas marinas (Mollusca: Opisthobranchia) en las islas Canarias y en otros archipiélagos de la Macaronesia. Vieraea. 2014;42: 47–77.
- 14. Gosliner TM, Valdés A, Behrens DM. Nudibranch & Sea Slug Identification. Indo-Pacific. 2sd ed. Jacksonville: New World Publications, Inc; 2018.
- 15. Alder J, Hancock A. Monograph of the British nudibranchiate Mollusca: with figures of all the species. London: Ray Society; 1845.
- 16. Thompson TE, Brown GH. Biology of opisthobranch molluscs. Volume II. London: Ray Society; 1984.
- 17. Cervera JL, López-González PJ, García-Gómez JC. Taxonomic and geographical range data on two species of Okenia (Gastropoda: Nudibranchia: Doridacea) from the Eastern Atlantic. Veliger. 1991;34: 56–66.
- 18. Valdés A, Ortea J. Revised Taxonomy of Some Species of the Genus Okenia Menke, 1830 (Mollusca: Nudibranchia) from the Atlantic Ocean, with the Description of a New Species. Veliger. 1995;38(3): 223–23.
- 19. Betti F, Cattaneo-Vietti R, Bava S. First records from the Ligurian Sea of the cold water species Okenia aspersa and Doto koenneckeri (Gastropoda: Nudibranchia). Mar Biodivers Rec. 2015;8: e105.
- 20. Oliverio M. The Mediterranean molluscs: the best known malacofauna of the world: so far. Biogeographia. 2003;24: 195–208.
- 21. Sabelli B, Taviani M. Chapter 16. The making of the Mediterranean molluscan biodiversity. In: Goffredo S, Dubinsky Z, editors. The Mediterranean Sea: its History and Present Challenges. Dordrecht: Springer; 2014. pp. 285–396.
- 22. Bianchi CN, Morri C. Marine Biodiversity of the Mediterranean Sea: Situation, Problems and Prospects for Future Research. Mar Pollut Bull. 2000;40(5): 367–376.
- 23. Coll M, Piroddi C, Steenbeek J, Kaschner K, Lasram FBR, Aguzzi J, et al. The biodiversity of the Mediterranean Sea: estimates, patterns, and threats. PLoS ONE. 2010;5(8): e11842. pmid:20689844
- 24. Crocetta F, Bonomolo G, Albano PG, Barco A, Houart R, Oliverio M. The status of the northeastern Atlantic and Mediterranean small mussel drills of the Ocinebrina aciculata complex (Mollusca: Gastropoda: Muricidae), with the description of a new species. Sci Mar 2012;76(1): 177–189.
- 25. Barco A, Houart R, Bonomolo G, Crocetta F, Oliverio M. Molecular data reveal cryptic lineages within the northeastern Atlantic and Mediterranean small mussel drills of the Ocinebrina edwardsii complex (Mollusca: Gastropoda: Muricidae). Zool J Linnean Soc. 2013;169: 389–407.
- 26. Aissaoui C, Puillandre N, Bouchet P, Fassio G, Modica MV, Oliverio M. Cryptic diversity in Mediterranean gastropods of the genus Aplus (Neogastropoda: Buccinidae). Sci Mar 2016;80(4): 521–533.
- 27. Aissaoui C, Galindo LA, Puillandre N, Bouchet P. The nassariids from the Gulf of Gabès revisited (Neogastropoda, Nassariidae). Mar. Biol. Res. 2017;13(4): 370–389.
- 28. Aissaoui C, Puillandre N, Bouchet P. New insights in the taxonomy of Mediterranean Diodora (Mollusca, Gastropoda, Fissurellidae). J Mar Biol Assoc U.K. 2017;97(7): 1527–1536.
- 29. Barco A, Aissaoui C, Houart R, Bonomolo G, Crocetta F, Oliverio M. Revision of the Ocinebrina aciculata species complex (Mollusca: Gastropoda: Muricidae) in the northeastern Atlantic Ocean and Mediterranean Sea. J Molluscan Stud. 2018;84: 19–29.
- 30. Katsanevakis S, Deriu I, D’Amico F, Nunes AL, Pelaez Sanchez S, Crocetta F, et al. European Alien Species Information Network (EASIN): supporting European policies and scientific research. Manag Biol Invasions. 2015;6(2): 147–157.
- 31. Zenetos A, Çinar ME, Crocetta F, Golani D, Rosso A, Servello G, et al. Uncertainties and validation of alien species catalogues: the Mediterranean as an example. Estuar Coast Shelf Sci. 2017;191: 171–187.
- 32. WoRMS Editorial Board; 2018 [cited 2018 Dec 04]. http://www.marinespecies.org at VLIZ.
- 33. Sambrook J, Fritsch EF, Maniatis T. Molecular cloning: a laboratory manual. 2nd ed. New York: Cold Spring Harbor Laboratory Press; 1989.
- 34. Folmer O, Black M, Hoeng W, Lutz R, Vrijenhoek R. DNA primers for amplification of mitochondrial cytochrome c oxidase subunit I from diverse metazoan invertebrates. J Mol Microbiol Biotechnol. 1994;3: 294–299.
- 35. Palumbi SR, Martin A, Romano S, Owen Macmillan W, Stice L, Grabowski G. The simple fool’s guide to PCR. Honolulu: Department of Zoology, University of Hawaii; 1991.
- 36. Colgan DJ, Mclauchlan A, Wilson GDF, Livingston S, Macaranas J, Edgecombe GD, et al. Molecular phylogenetics of the Arthropoda: relationships based on histone H3 and U2 snRNA DNA sequences. Aust J Zool. 1998;46: 419–437
- 37. Hall TA. BioEdit: a user-friendly biological sequence alignment editor and analysis program for Windows 95/98/NT. Nucleic Acids Symp Ser. 1999;41(41): 95–98.
- 38. Tamura K, Stecher G, Peterson D, Filipski A, Kumar S. MEGA6: Molecular Evolutionary Genetics Analysis version 6.0. Mol Biol Evol. 2013;30: 2725–2729. pmid:24132122
- 39. Talavera G, Castresana J. Improvement of phylogenies after removing divergent and ambiguously aligned blocks from protein sequence alignments. Syst Biol. 2007;56: 564–577. pmid:17654362
- 40. Darriba D, Taboada GL, Doallo R, Posada D. jModelTest 2: more models, new heuristics and parallel computing. Nature Methods. 2012;9(8):722.
- 41. Schwarz G. Estimating the dimension of a model. Ann Stat. 1978;6(2): 461–464.
- 42. Ronquist F, Huelsenbeck JP. MrBayes 3: Bayesian phylogenetic inference under mixed models. Bioinformatics. 2003;19: 1572–1574. pmid:12912839
- 43. Stamatakis A, Hoover P, Rougemont J. A rapid bootsrap algorithm for the RAxML web servers. Syst Biol. 2008;57: 758–771. pmid:18853362
- 44. Hillis DM, Bull JJ. An empirical test of bootstrapping as a method for assessing confidence in phylogenetic analysis. Syst Biol. 1993;42: 182–192.
- 45. Alfaro ME, Zoller S, Lutzoni F. Bayes or bootstraps? A simulation study comparing the performance of Bayesian Markov chain Monte Carlo sampling and bootstrapping in assessing phylogenetic confidence. Mol. Biol. Evol. 2003;20: 255–266. pmid:12598693
- 46. Morariu VI, Srinivasan BV, Raykar VC, Duraiswami R, Davis LS. Automatic online tuning for fast Gaussian summation. Adv Neural Inf Process Syst. 2008;21: 1113–1120.
- 47. Swofford DL. PAUP*. Phylogenetic Analysis Using Parsimony (*and other methods). Version 4. Sunderland: Sinauer Associates; 2002.
- 48. Zhang J, Kapli P, Pavlidis P, Stamatakis AA. General Species Delimitation Method with Applications to Phylogenetic Placements. Bioinformatics. 2013;29(22): 2869–2876. pmid:23990417
- 49. Puillandre N, Lambert A, Brouillet S, Achaz G. ABGD, Automatic Barcode Gap Discovery for primary species delimitation. Mol Ecol. 2011; 21(8): 1864–1877. pmid:21883587
- 50. Leuckart FS. Breves animalium quorundam maxima ex parte marinorum descriptiones. Heidelbergae: Typis A. Osswaldi; 1828.
- 51. Cervera JL, Templado J, Ballesteros M, Megina C. An annotated and updated checklist of the opistobranchs (Mollusca: Gastropoda) from Spain and Portugal (including islands and archipelagos). Bol Inst Esp Oceanogr. 2004;20(1–4): 5–111.
- 52. García-Gómez JC, Cervera JL, Díaz J. Familia Goniodorididae. In Gofas S, Moreno D, Salas C, editors. Moluscos Marinos de Andalucía—II. Málaga: Universidad de Málaga, Servicio de Publicaciones e Intercambio Científico;2011.
- 53. Ballesteros M. Lista actualizada de los opistobranquios (Mollusca: Gastropoda: Opisthobranchia) de las costas catalanas. Spira. 2007;2(3): 163–188.
- 54. Ballesteros M, Madrenas E, Pontes M. Actualización del catálogo de los moluscos opistobranquios (Gastropoda: Heterobanchia) de las costas catalanas. Spira. 2016; 6: 1–28.
- 55. Vayssière A. Recherches zoologiques et anatomiques sur les mollusques Opistobranches du Golfe de Marseille (suite et fin). Memoire No. 1. Marseille: Annales du Musee d'Histoire Naturelle de Marseille; 1901.
- 56. Pruvot-Fol A. Études des nudibranches de la Méditerranée (2e partie). Arch Zool Exp Gen. 1951;88(1): 1–80.
- 57. Schmekel RL. First Record of Okenia impexa Marcus, 1957 from the Western Atlantic in the Mediterranean. Veliger. 1979;21(3): 355–360.
- 58. Vayssière A. Mollusques de la France et des régions voisines, tome premier, amphineures, gasteropodes opisthobranches, heteropodes, marseniades et oncidiides. Paris: Encyclopedie Scientifique, publiee sous la direction du Dr. Toulouse, Bibliotheque de Zoologie Doin; 1913.
- 59. Vayssière A. Recherches zoologiques et anatomiques sur les Mollusques Opisthobranches du Golfe de Marseille, 2me Supplément. Ann Mus Hist Nat Marseille. 1919;17: 53–92.
- 60. Vicente N. Contribution a l'étude des gastéropodes opisthobranches du Golfe de Marseille. I. Systématique, ecologie, biologie. Recl trav Stn mar Endoume. 1967;42(58): 133–179.
- 61. Wirz-Mangold K, Wyss U. Faune Marine des Pyrénées-Orientales—Fascicule 3 —Opistobranches. Vie Milieu. 1958;suppl. 9(2): 1–71.
- 62. Haefelfinger HR. Catalogue des opisthobranches de la Rade de Villefranche-sur-Mer et ses environs (Alpes Maritimes). Rev Suisse Zool. 1960;67(3): 323–351.
- 63. Carus JV. Prodromus Faunae Mediterraneae sive descriptio animalium Maris Mediterranei incolarum quam comparata silva rerum quatenus innotuit adiectis locis et nominibus vulgaribus eorumque auctoribus in commondom Zoologorum. vol.II. Brachiostomata, Mollusca, Tunicata, Vertebrata. Stuttgart: E. Schweiizerbart’sche Verlagshandlung (E. Koch); 1893.
- 64. Graeffe E. Ubersicht der Fauna des Golfes von Triest nebst Notizen über Vorkommen, Lebensweise, Erscheinungs- und Laichzeit der einzelnen Arten. Volume VI. Mollusca. Arb Zool Inst Univ Wien. 1903;14: 89–136.
- 65. Ciriaco S, Poloniato D. Guida illustrata ai nudibranchi del Golfo di Trieste. Ministero dell’Ambiente e Della Tutela del Territorio e del Mare. Area Marina Protetta Miramare. Rome: Pandion edizioni; 2016.
- 66. Zenetos A, Mačić V, Jaklin A, Lipej L, Poursanidis D, Cattaneo‐Vietti R, et al. Adriatic ‘opisthobranchs’ (Gastropoda, Heterobranchia): shedding light on biodiversity issues. Mar Ecol. 2016;37(6): 1239–1255.
- 67. Mazzarelli GF. Note biologiche sugli opistobranchi del Golfo de Napoli. Partie seconda: nudibranchi. Atti Soc ital sci nat Mus civ stor nat Milano. 1903;42(3): 280–296.
- 68. Cattaneo-Vietti R, Chemello R. The opisthobranch fauna of a Mediterranean lagoon (Stagnone di Marsala, western Sicily). Malacologia. 1991;32(2): 291–299.
- 69. Prkić J, Petani A, Iglić D, Lanča L. Opisthobranchs of the Adriatic Sea. Photographic atlas and list of Croatian species. Zadar: Diving Club Saint Roko; 2018.
- 70. Koutsoubas D, Koukouras A, Thompson TE. Sacoglossa and Nudibranchia (Opisthobranchia) from the Aegean Sea: new records for the eastern Mediterranean fauna. Zoosystematics Evol. 1993;69(1): 155–163.
- 71. Cattaneo-Vietti R, Chemello R, Giannuzzi-Savelli R. Atlante dei nudibranchi del Mediterraneo. Rome:La Conchiglia; 1990.
- 72. Sabelli B, Giannuzzi-Savelli R, Bedulli D. Catalogo Annotato dei Molluschi Marini del Mediterraneo. Bologna: Libreria Naturalistica Bolognese; 1990–1992.
- 73. Cossignani T, Ardovini R. Malacologia mediterranea: atlante delle conchiglie del Mediterraneo: 7500 foto a colori. Ancona: L'informatore Piceno; 2011.
- 74. Trainito E, Doneddu M. Nudibranchi del Mediterraneo. Milano: Il Castello; 2014.
- 75. Philippi RA. Einige zoologische Notizen. 1. Zwei neue Arten von Euplocamus. Arch Naturgesch. 1839;5(1): 113–115.
- 76. Philippi RA. Zoologische Bemerkungen. Arch Naturgesch. 1841;7(1): 42–59.
- 77. Grube AE. Die Insel Lussin und ihre Meeresfauna, Nach einem sechswöchentlichen Aufenthalte. Breslau: Verlag von Ferdinand Hirt, Konigl, Universitats-Buchandler; 1864.
- 78. Templado J, Guerra A, Bedoya J, Moreno D, Remón JM, Maldonado M, Ramos MA. Fauna marina circalitoral del sur de la península Ibérica. Resultados de la Campaña Oceanográfica “Fauna I”. Madrid: Museo Nacional de Ciencias Naturales (CSIC); 1993.
- 79. Alder J. Catalogue of the Mollusca of Northumberland and Durham. Newcastle: Robert Currie & Co., Newcastle Upon Tyne; 1848.
- 80. Alder J, Hancock A. Notice of some new species of British Nudibranchiata. Ann Mag Nat Hist. 1854;14(80): 102–105.
- 81. Cattaneo-Vietti R. Bathymetric distribution of soft-bottom opisthobranchs along the Ligurian and Tuscany continental slope (western Mediterranean). In Proceedings of the Tenth International Malacological Congress, Tubingen. Vol. 1989; 1991. pp. 327–334.
- 82. Ihering H. Beiträge zur kenntnis der Nudibranchien des Mittelmeeres. Part 2. Malakozoologische Blätter N.F, 1886;8: 12–48.
- 83. Moreno D, Templado J. Nuevas aportaciones al conocimiento de los opistobranquios del sureste español. II. Iberus. 1998;16(2): 39–58.
- 84. Pruvot-Fol A. Mollusques opisthobranches. Fauna de France. 58. Paris: Paul Lechevalier; 1954.
- 85. Schmekel RL, Portmann A. Opisthobranchia des Mittelmeeres. Munich: Springer-Verlag; 1982.
- 86. Barletta G, Melone G. Nudibranchi del Promontorio di Portofino (Genova) (Gastropoda Nudibranchia). Natura. 1976;67(3–4): 203–236.
- 87. Betti F. Il regno dei nudibranchi: guida ai molluschi opistobranchi della riviera del Conero. Imola: La Mandragora; 2011.
- 88. Mifsud C, Cachia C. More additions and corrections to the check-list of the marine Mollusca of the Maltese Islands Triton. 2013;28: 9–12.
- 89. Schmekel RL. Ascoglossa, Notaspidea und Nudibranchia im Litoral des Golfes von Neapel. Rev Suisse Zool. 1968;75(6): 103–155.
- 90. Bergh LSR. The Opisthobranchia of South Africa. Trans R Soc S Afr. 1907;27(1): 1–144.
- 91. Betti F, Bava S, Cattaneo-Vietti R. Composition and seasonality of a heterobranch assemblage in a sublittoral, unconsolidated, wave-disturbed community in the Mediterranean Sea. J Molluscan Stud. 2017;83(3): 325–332.
- 92. Montagu G. Descriptions of several new or rare animals found on the south coast of Devonshire. Trans Linn Soc Lond. 1815;11: 1–26.
- 93. Mifsud C. The Molluscan species living in Maltese Posidonia meadows. La Conchiglia. 1998;30(287): 37–48.
- 94. Vogel R, Schultz L. Cargoa-Cupella New Genus and New Species of Nudibranch from Chesapeake Bay and the generic status of Okenia, Idalia and Idalla. Veliger. 1970;12: 388–393.
- 95. Templado J, Calvo M, García-Carrascosa A, Boisset F, Jiménez J. Flora y fauna de la Reserva Marina de las islas Columbretes. Madrid: Secretaría General de Pesca Marítima; Ministerio de Agricultura, Pesca y Alimentación; 2002.
- 96. Sordi M. Nuova specie di mollusco nudibranchio del Mediterraneo. Doriana. 1974;5(216): 1–5.
- 97. Marcus E. On opisthobranchia from Brazil (2). Zool J Linn Soc. 1957;43: 390–486.
- 98. Templado J. Datos sobre los opistobranquios del Cabo de Palos (Murcia). Boll Malacol. 1982;18(9–12): 247–254.
- 99. Smallwood WM. Notes on the Hydroids and Nudibranchs of Bermuda. Proc Zool Soc Lond. 1910;80: 137–147.
- 100. Pola M. The Identity of Okenia zoobotryon (Smallwood, 1910) (Nudibranchia: Goniodorididae): Redescription and proposed designation of a neotype. Am Malacol Bull. 2015;33(1): 72–77.
- 101. Lipej L, Trkov D, Mavrič B. Polži zaškrgarji slovenskega morja. Piran: Nacionalni inštitut za biologijo, Morska biološka postaja. 2018.
- 102. ICZN (International Commission on Zoological Nomenclature). Opinion 1014. Okenia Menke, 1830 (Mollusca, Opisthobranchia): placed on the official list of generic names. Bull Zool Nomencl. 1974;31: 13–15.
- 103. MolluscaBase. Okenia cupella (Vogel & Schultz, 1970). 2018. Accessed through: World Register of Marine Species at http://www.marinespecies.org/aphia.php?p=taxdetails&id=140036 on 2018-11-20.
- 104. MolluscaBase. Okenia impexa Er. Marcus, 1957. 2018. Accessed through: World Register of Marine Species at: http://www.marinespecies.org/aphia.php?p=taxdetails&id=224283 on 2018-11-20.
- 105. Rios EC. Brazilian marine mollusks iconography. Rio Grande: Fundacao 1068 Universidade do Rio Grande; 1975.
- 106. ICZN [International Commission on Zoological Nomenclature]. International code of zoological nomenclature. 4th Edition. International Trust for Zoological Nomenclature; 2012 [cited 2018 Dec 4]. http://www.iczn.org/ (accessed 1 November 2018).
- 107. Thompson TE. Molluscs: benthic opisthobranchs (Mollusca: Gastropoda) Key and notes for the identification of the species. Leiden, New York, København, Köln: The Linnean Society of London and The Estuarie & Brackish-water Sciences Association;1988.
- 108. Lance J. New Distributional Records of some Northeastern Pacific Opisthobranchia (Mollusca: Gastropoda) with Descriptions of Two New Species. Veliger. 1966;9(1): 69–81.
- 109. Behrens DW. Pacific Coast nudibranchs: supplement II. New species to the Pacific Coast and new information on the oldies. Proc. Calif. Acad. Sci. 2004;55(2): 11–54.
- 110. Goddard JH, Treneman N, Pence WE, Mason DE, Dobry PM, Green B, Hoover C. Nudibranch range shifts associated with the 2014 warm anomaly in the northeast pacific. Bull South Calif Acad Sci. 2016;115(1): 15–40.
- 111. Ev Marcus, Er Marcus. Dorid nudibranch from Curaçao and faunistically related regions. Stud Faun Cur Caribb Isl. 1970;33(1): 1–129.
- 112. Ortea J, Leopoldo M, Espinosa J. El género Okenia Menke, 1830 (Mollusca: Nudibranchia) en las islas Canarias con notas sobre Okenia zoobotryon (Smallwood, 1910) una especie en controversia permanente. Vieraea. 2009;37: 75–83.
- 113. Sales L, Queiroz V, Migotto AE. Rediscovery of Okenia impexa Marcus 1957 on the Brazilian coast: the first image of a living specimen from Brazil. Spixiana. 2016;39(2): 203–204.
- 114. Oztürk B, Doğan A, Bitlis-Bakir B, Salman A. Marine molluscs of the Turkish coasts: an updated checklist. Turk J Zool. 2014;38: 832–879.
- 115. Crocetta F, Mariottini P, Salvi D, Oliverio M. Does GenBank provide a reliable DNA barcode reference to identify small alien oysters invading the Mediterranean Sea? J Mar Biol Assoc U.K. 2015;95(1): 111–122.
- 116. Crocetta F, Tringali LP. Mapping alien Mollusca distribution in the Mediterranean Sea: the Lessepsian immigrant Retusa desgenettii (Audouin, 1826) reaches Turkey. Quat Int. 2015;390: 15–20.
- 117. Crocetta F, Poursanidis D, Tringali LP. Biodiversity of sea slugs and shelled relatives (Mollusca: Gastropoda) of the Cretan Archipelago (Greece), with taxonomic remarks on selected species. Quat Int. 2015;390: 56–68.
- 118. Gofas S, Luque AA, Templado J, Salas C. A national checklist of marine Mollusca in Spanish waters. Sci Mar 2017;81(2): 241–254.
- 119. Crocetta F, Tringali LP. Remarks on Philine striatula Monterosato, 1874 ex Jeffreys ms., with a survey on Philinidae J.E. Gray, 1850 (1815) sensu lato (Gastropoda: Cephalaspidea) recently ascribed to the Mediterranean fauna. Mar Biodivers. 2018;48(3): 1499–1510.
- 120. Zenetos A, Corsini-Foka M, Crocetta F, Gerovasileiou V, Karachle PK, Simboura N, et al. Deep cleaning of alien species records in the Greek Seas (2018 update). Manag Biol Invasion. 2018;9(3): 209–226.
- 121. Domínguez M, Pola M, Ramón M. A new species of Tambja (Mollusca, Gastropoda, Nudibranchia) from the Mediterranean Sea: description of the first species of the genus from the Balearic Islands and Malta. Helgol Mar Res. 2015;69(2): 205.
- 122. Furfaro G, Salvi D, Mancini E, Mariottini P. A multilocus view on Mediterranean aeolid nudibranchs (Mollusca): Systematics and cryptic diversity of Flabellinidae and Piseinotecidae. Mol Phylogenet Evol. 2018;118: 13–22. pmid:28888791
- 123. Peñas A, Rolán E, Luque AA, Templado J, Moreno D, Rubio F, Salas C, Sierra A, Gofas G. Moluscos marinos de la isla de Alborán. Iberus. 2006;24(1): 23–151.
- 124. Gofas S. Marine molluscs with a very restricted range in the Strait of Gibraltar. Diversity and Distributions; 1998;4: 255–266.
- 125. Gallardo C, Sánchez C, Gofas S, Salas C. Conservation issues for the endemic molluscs of the Strait of Gibraltar. Frontiers in Marine Science Conference. XIX° Iberian Symposium of Marine Biology Studies [Communication]; 2016.
- 126. Boyer F, Renda W. About the alleged endemism of marine gastropods in the Strait of Gibraltar. Xenophora Taxonomy. 2018;18: 24–36