Phylogeny and New Classification of Hydrothermal Vent and Seep Shrimps of the Family Alvinocarididae (Decapoda)

The paper addresses the phylogeny and classification of the hydrothermal vent shrimp family Alvinocarididae. Two morphological cladistic analyses were carried out, which use all 31 recognized species of Alvinocarididae as terminal taxa. As outgroups, two species were included, both representing major caridean clades: Acanthephyra purpurea (Acanthephyridae) and Alpheus echiurophilus (Alpheidae). For additional support of the clades we utilised available data on mitochondrial Cytochrome c Oxidase I gene (CO1) and 16S ribosomal markers. Both morphological and molecular methods resulted in similar tree topologies and nearly identical clades. We consider these clades as evolutionary units and thus erect two new subfamilies: Rimicaridinae (Alvinocaridinides, Manuscaris, Opaepele, Shinkaicaris, Rimicaris), Alvinocaridinae (Alvinocaris), whilst recognising Mirocaridinae (with genera Mirocaris and Nautilocaris) at subfamily level. One genus, Keldyshicaris could not be assigned to any subfamily and is thus left as incertae sedis. The monophyly of Alvinocardinae was supported by morphological data, but not supported by molecular data (two analyses); the monophyly of all subfamilies was supported both by morphological and molecular data. Chorocaris is herein synonymized with Rimicaris, whilst Opaepele vavilovi is herein transferred to a new genus Keldyshicaris. Morphological trends within Alvinocarididae are discussed and short biogeographical remarks are given. We provide emended diagnoses for all subfamilies and genera along with keys to all recognized species.


Introduction
Shrimps of the family Alvinocarididae inhabit deep-sea cold-seeps and hydrothermal vent areas around the world, and have been found in the Atlantic, Pacific, and Indian Oceans [1] within the depth range of 252 to 4960 m [2][3]. Most species of the family occur at hydrothermal vents, but a few are found in cold-seep areas [3]; one species, Alvinocaris longirostris, has been reported from both vents and seeps [4][5][6][7]. The first record of the family was based on a few specimens from the Galapagos Rift, which were described in 1982 as Alvinocaris lusca by Austin Williams and Fenner Chace [8]. Later the first author described a further two species of a new genus, Rimicaris from the hydrothermal vent field TAG [9]. One of these species was subsequently transferred to a new genus Chorocaris in 1990 [10]. In the middle of the 1990s, Russian and American scientists described two further genera Opaepele [11] and Mirocaris [12]; whilst more recently the genera Nautilocaris, Shinkaicaris, and Alvinocaridinides were described by Japanese and French researchers [2], [13], [14].
Due to drastic metamorphosis in ontogeny, the history of the family systematics has not been smooth and some taxa were later synonimised. For example, the genus Iorania [15] and the species Rimicaris aurantiaca [16] are now considered to be juveniles of Rimicaris exoculata. There was no consensus on the status of the new family Mirocarididae established for a single genus Mirocaris [12]; phylogenetics showed a significant distance between this group and the rest of Alvinocarididae [17][18], although taxonomists kept Mirocaris as a genus within Alvinocarididae [19]. Status for Opaepele vavilovi also remains unseratin [20].
Three new species and a new genus Manuscaris have recently been described from hydrothermal vents in the Pacific Ocean [21]. In this comprehensive study, partial sequences of mitochondrial COI were used, resulting in a minor change in the classification of Alvinocarididae, the transfer of Opaepele susannae into Chorocaris [21].
At present, 9 genera and 31 species are known within the family and a comprehensive phylogenetic analysis is needed to disentangle existing problems and to elucidate the status of all genera.
This task is eased by the presence of a significant amount of information on partial sequences of mitochondrial COI gene in GenBank, unusually rich for decapods. Much of this data was used in a previous comprehensive study [17], which confirmed three distinct clades consistent with morphology at that time: (1) Rimicaris/Chorocaris/Opaepele, (2) Alvinocaris, and (3) Mirocaris. Evolutionary relationships of vent-endemic shrimp species were shown to correlate neither with their current biogeographic distribution nor with the history of sea-floor spreading. Later studies have incorporated further molecular data for several recently described species and enhanced information for the species studied in [3], [17], [18], [21][22][23][24][25][26]. However, no attempt to carry out a complete phylogenetic analysis of the whole family Alvinocarididae has been carried out to date.
Combining both morphological and molecular evidence should shed light on the complex relationships in Alvinocarididae.
In this paper we summarize original and literature data about the composition, morphology, and genetic diversity of the family Alvinocarididae. Further, we (1) find and describe morphological characters, (2) perform cladistic morphological analyses, (3) analyze molecular data, (4) combine and compare morphological and molecular results, (5) discuss supported taxa, and (6) provide a new classification, emended diagnoses, and identification keys for all subfamilies, genera, and species.

Material for morphological analysis
Material was collected along the Mid-Atlantic Ridge during six cruises of R/V "Akademik Mstislav Keldysh" with the use of two deep-sea manned submersibles "Mir-1" and "Mir-2" (34th cruise, August-October 1994, 39th cruise, August-October 1996, 41st cruise, August-December 1998, 47th cruise, June-July 2002, 49th cruise August 2003, 50th cruise, August 2005). Seven vent fields were investigated during 1994-2005, including Menez Gwen (37. (14.752 N 44.9785 W). No specific permission was required for field studies in any of these locations. The field studies did not involve endangered or protected species.
Shrimps were collected using baited traps and suction samplers. Immediately after retrieval all specimens were sorted, measured, and preserved in 80% alcohol. Measurements follow established methods for shrimp morphological description [27]. Shrimp morphology and its temporal/spatial variations were thoroughly investigated for this material on the basis of 5861 individuals [28], [29]. A detailed description of this material and discussion of the various species may be found in [20], [29], [30].
Analysis of the morphology of all species within the family was made with the use the above original data and all other available literature data (Table 1).

Terminal taxa, outgroups, and characters used for morphological analysis
All thirty-one recognized species of Alvinocarididae were included as terminals. Outgroup selection was made on the basis of a comprehensive molecular study [31], which revealed two major clades of Caridea: (1) Alpheidae, Hippolytidae, Crangonidae, Glyphocrangonidae, Barbouriidae, Pandalidae, Hymenoceridae, Gnathophyllidae, and Palaemonidae and (2) Rhynchocinetidae, Oplophoridae, Nematocarcinidae, Alvinocarididae, Campylonotidae, Pasiphaeidae and Eugonatonotidae. The first outgroup species, Acanthephyra purpurea A. Milne-Edwards, 1881 [32], represents the first clade: Wong et al. [33] have shown that family Acanthephyridae is sister to Oplophoridae and advocate combining both families as Oplophoridae. We chose A. purpurea partly because this species is present in GenBank and could also be used as the outgroup in the molecular analysis. The second outgroup species, Alpheus echiurophilus Anker, Komai and Marin 2015 [34], belongs to Alpheidae and represents the second major clade of Caridea. Both species are ecologically very different (pelagic and burrowing) as well as morphologically and a comparison of cladograms is thus instructive.
Sixty-three morphological characters (ten multistate) were used in the analysis, and are listed in Table 2, along with character states, brief descriptions, and references to figures (see also . The data matrix is presented in Table 3.
All characters were unordered (non-additive) and equally weighted, missing data were scored as unknown. Characters were unordered, so the score given for each state (i.e., 0, l, 2) implies nothing about order in a transformation series [38]. Trees were generated in TNT under the implicit enumeration. Relative stability of clades was assessed by standard bootstrapping (sample with replacement) with 10000 pseudoreplicates and by Bremer support (algorithm TBR, saving up to 10000 trees up to 3 steps longer).

Molecular data
Both Mitochondrial Cytochrome c Oxidase I (CO1) and 16S ribosomal markers were selected for phylogenetic analyses, as only these markers have been sequenced for a representative number of alvinocaridid species, with CO1 sequences for 20 (out of 31) species available (Table 4). For the present phylogenetic analyses, we used all publicly available 271 CO1 sequences for individuals identified to species-level. Partial 16S sequences are only available for 10 alvinocaridid species (Table 4). For the present phylogenetic analyses, we used all 29 sequences available in the GeneBank.

Analysis of molecular data
Multiple alignments were made with the use of the Clustal W algorithm [39]. Six CO1 sequences were discarded after alignment, as they represented non-barcoding parts of the CO1 gene or were too short. The remaining 265 aligned sequences were trimmed according to the shortest sequences (Accession Numbers: KC840928-KC840940, HM125910-HM125956) with a total length of 471 bp. Amino acid sequences received from the nucleotide sequences had no stop codons within the open reading frame using the invertebrate mitochondrial code. All 16S sequences after alignment were trimmed according to the shortest sequences (Accession Numbers: AM087916-AM087925) with a total length of 286 bp. Phylogenetic analysis was performed using both Maximum Likelihood (ML) and Bayesian analyses. To root the resultant trees, Acanthephyra purpurea Awas used (also see section 2.2). The best-fit model selected using jModelTest 2.1.7 [40] was the Tamura-Nei model with a gamma distribution and invariable sites (TrN+G+I) for CO1 and the Hasegawa-Kishino-Yano model with a gamma distribution (HKY+G) for 16S data set. These models were used to generate ML gene trees in MEGA 5. Support for branches was assessed using bootstrap analyses with 1,000 replicates [41]. Bayesian phylogenetic analysis was made with the use of MrBayes v3.2.1 [42]. A general time-reversible model (GTR) of sequence evolution with a gamma distribution and invariable sites for CO1 data set was chosen as it represents the closest approximation of the Tamura-Nei model in MrBayes. HKY+G model was used for 16S data set. The Markov Chain Monte Carlo (MCMC) analysis was further used with the following settings: (1) for CO1-18 million generations, trees sampled every 5000 generation, and the first 900 trees discarded; (2) for 16S-1.5 million generations, trees sampled every 1000 generation, and the first 375 trees discarded. The average standard deviation of split frequencies between two runs of MCMC was less than 1% for each analysis, thus indicating convergence.

Phylogeny and New Classification of Alvinocarididae
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: XXXXXXX. 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, LOCKSS.

Morphological clades
Analysis 1 with Acanthephyra purpurea as the outgroup retrieved 36 minimal length trees of length 145 (Fig 4A). The basal clade Alvinocaris (pink in Fig 4A) forms a sister clade to the rest of the family and is followed by Keldyshicaris and two sister clades: Nautilocaris+ Mirocaris (blue) and Opaepele+Alvinocaridinides+Manuscaris+Shinkaicaris+Rimicaris (light green). Within the clade Alvinocaris, Alvinocaris niwa is the most basal, followed by the other species of Alvinocaris. Within the light green clade, there are three clades: Opaepele, Alvinocaridinides +Manuscaris+Shinkaicaris, and Rimicaris (green). After discard of all unsupported clades with Bremer support <3, all considered coloured clades persist (Fig 5A).
Analysis 2 with Alpheus echiurophilus as the outgroup retrieved 437 minimal length trees of length 146; the tree topology slightly differs from that in Analysis 1, but the principal clades are the same (Fig 4B, same colours). The clade Nautilocaris+ Mirocaris is basal and followed by three clades: (1) Opaepele+Alvinocaridinides+Manuscaris+Shinkaicaris+Rimicaris, (2) Keldyshicaris, and (3) Alvinocaris. After discard of all unsupported clades with Bremer support <3, all considered coloured clades persist (Fig 5B).  Table 4. GenBank accession numbers for COI and 16S sequences of species used for phylogenetic analyses.

Molecular clades
The molecular phylogenetic Analysis 1 with use of CO1 gene resulted in a tree (Fig 6A), which resembles the morphological tree except the branching of Alvinocaris (Fig 5B). In the morpho analysis Alvinocaris forms a single clade (not internally resolved), but in the genetic analysis three (COI) or two (16S) clades occur.
The clade Mirocaris+Nautilocaris (Fig 6A, coloured in blue) is supported both by ML bootstrap (98) and Bayesian posterior probability (100). This clade comprises two genera: Mirocaris (M. fortunata) and Nautilocaris (N. saintlaurentae). Genetic difference between the genera  The clade Alvinocaris komaii (Fig 6A, pink) comprises only one species of Alvinocaris. Position of this on the tree topology is unresolved and thus requires further research.
The clade Alvinocaris dissimilis +Alvinocaris stactophila +Alvinocaris chelis (Fig 6A,  pink) is supported by ML bootstrap (99) and Bayesian posterior probability (100). This clade comprises the rest of Alvinocaris used in Analysis 1 (Fig 6A, coloured in pink). The former two species are genetically identical, while the genetic distance (p-distance) between them and A. chelys reaches 0.5%.

The clades
The clade/subfamily Mirocaridinae (Fig 5A and 5B, blue) comprises three species and two genera: Nautilocaris and Mirocaris. This clade is supported by the following synapomorphies common for both morpho analyses (starting from the character number): (48)(49) presence of strap-like epipods terminating in a hook, on the fourth pereopod, (58-60) much reduced appendix interna in second to fourth pleopods The clade is well supported by molecular data (Fig 6A).
The clade/subfamily Rimicaridinae (Fig 5A and 5B, light green) comprises thirteen species and five genera: Opaepele, Alvinocaridinides, Manuscaris, Shinkaicaris, and Rimicaris. This clade is also supported by the following synapomorphies common for both morpho analyses: (28) entirely fused eyestalks without mould seam, (47,54,57) presence of two or more rows of accessory spinules on the dactyls of the third to fifth pereopods.
The clade is supported by molecular data (Fig 6A). The position of A. methanophila within Rimicaridinae is worthy of comment. This result is based on a single specimen of A. methanophila, which was collected at the Blake Ridge Diapir site, sequenced and deposited in GenBank (Accession No AY163260) under the name «Alvinocaris sp. TMS-2002». The shrimp was originally named «Blake Ridge shrimp» [43]. Later this specimen along with 33 other adult specimens was described as a new species A. methanophila [44]. Texeira et al [45] used the same sequence and included this in their clade "ESU 2" which comprised specimens from three genera. We feel that the resulting position of A. methanophila in a common clade with R. chacei and R. hybisae is a result of incorrect identification or processing the material.
Molecular analyses indicate close relation between Opaepele (O. loihi) and Rimicaris, however morphologically they are quite distinct and for now we keep both genera as valid. Both morphological analyses supported validity of remaining genera of the clade Rimicaridinae: Alvinocaridinides, Shinkaicaris, and Manuscaris.
The clade Rimicaris is nested within the clade Rimicaridinae and comprises all species of the currently recognised genera Chorocaris and Rimicaris (Fig 5A and 5B, green). This clade received very high support during morphological and molecular analyses and is also supported by the synapomorphies common for both morpho analyses: (19) dorsal organ under carapace extended beyond the postorbital region, (61-62) presence of two movable spines mesial to posterolateral tooth on uropodal exopod.
As this is a robust monophyletic clade (except terminal R. exoculata+R. kairei), we herein synonymize Chorocaris with Rimicaris, with Rimicaris being the older name. We note that the type species of Chorocaris, Chorocaris chacei, was initially described as Rimicaris [9]. Our analyses also show that the former Opaepele susannae was correctly removed from the genus Opaepele (and transferred to Chorocaris) [21].
Recent molecular studies by Texeira et al [45] based on studies from the Tropical Atlantic have previously showed a common clade for Rimicaris and Chorocaris. "These showed very low genetic divergence at levels similar to divergence between individuals of the same species. We posit that these taxa belong to the same genus, possibly even the same species" [45].
The clade is well supported by our molecular data (Fig 6A). A chequerwise arrangement of the species belonging to the former Rimicaris and Chorocaris within Rimicaridinae gives additional evidence for their genetic similarity and thus synonimising both genera.
One of the unexpected results of our molecular analyses is the position of specimens of R. chacei in two different and well supported clades (distance 7.9%). Those specimens which are closer to R. exoculata, were identified and sequenced by T.M. Shank (NN AF125395-397, AF125414-415 from Snake Pit and TAG [17]. The specimens closer to Opaepele loihi (NN KC840928-KC840940 from Logatchev and Lucky Strike) are from Teixeira et al. [45]. This division of R. chacei into two groups may be caused by various factors: • mitochondrial introgression in which mitochondrial DNA of R. exoculata may have been incorporated in the populations of C. chacei at Snake Pit and TAG. Similar effects were found in other marine Malacostraca [46], [47] and also in vent Bivalvia [48], [49], • existence of cryptic species of C. chacei recognized as a single morphological species.
The clade/subfamily Alvinocaridinae (Fig 5A and 5B, pink) comprises fourteen species of the genus Alvinocaris. This clade is also supported by the following synapomorphies common for both morpho analyses: (3,9) laterally compressed and ventrally carinate rostrum, (15) presence of postrostral dorsal carina extending beyond the midlength of carapace. Molecular analyses indicate presence of three supported clades of Alvinocaris (Fig 6A). Here we consider the genus and the subfamily as monophyletic on the basis of the two morphological analyses and leave the possibility of a polyphyletic origin of the clade to be resolved by more data in the future.
The clade Keldyshicaris comprises a single species, Keldyshicaris vavilovi. Both morphological analyses did not support monophyly of the former genus Opaepele and we suggest division of this genus into two monotypic genera, Opaepele with O. loihi (subfamily Rimicaridinae) and Keldyshicaris gen.n. with K. vavilovi n. comb. The status of this genus may be further clarified after receiving molecular data.

Morphological trends in Alvinocarididae
Mirocaridinae and Rimicaridinae share a degenerate rostrum, reduced external spines and the presence of the dorsal organ. This type of rostrum may be advantageous in the vicinity of shimmering waters and vent fluids where Mirocaridinae and Rimicaridinae occur. Predators are rare in these extreme biotopes where sulphides, heavy metals, and methane are actively leaching from surrounding rocks [50]. A shorter unarmed rostrum along with reduced spines reduces impact of strong turbulent water fluxes, which are common in the microbiotopes where the shrimps thrive [51].
Mirocaridinae further differ from other Alvinocarididae in having strap-like epipods terminating in a hook and in much reduced appendix interna in second to fourth pleopods. The strap-like pereopodal epipods are common for many caridean families and these characters may be retained from the ancestor. Modification in pleopodal characters may be related to specific traits in movement or copulation, which may be adaptive in the shimmering waters where Mirocaridinae thrive.
Rimicaridinae possess entirely fused eyestalks and presence of two or more rows of accessory spinules on the dactyls of the third to fifth pereopods-adaptations favouring anchoring close to the strong currents hydrothermal fluids. Within the clade Rimicaridinae, the genus Rimicaris shows further modification of external structures on the rostrum and frontal part of carapace, further development of the dorsal organ, and elaboration of uropodal exopod. The polarization of the characters 19 along with inflation of carapace and extension of scaphognathite shows deeper association of Rimicaris with vent fluids than any of other genera of the subfamily [51], [59], [60]. Presence of two strong movable spines instead of one (the polarization of characters 61-62) may indicate importance of the tail fan, which is used in the escape behaviour of shrimps ( [61]. The presence of additional spines may make this behaviour more efficient when high-temperature turbulent water fluxes can suddenly erupt from the rocks and damage shrimps [51]. Alvinocaridinae are characterized by a well-developed instead of reduced rostrum and postrostral carina, by a frontal ocular tubercle, and by the spination of the fourth-fifth pereopods. These traits are plausibly accounted for by their habitat at the periphery of hydrothermal vent fields [60]. In these habitats, predatory fish do occur and frontal armature may partly protect the shrimps from attacks. The frontal ocular tubercle indicates that the eyes may have additional chemo-or mechanosensory function facilitating orientation at the vent field by means of the frontal tubercle. If so, the tubercle may represent a sensory mechanism alternative to the photoreceptory dorsal organ of other Alvinocarididae. Distal movable spines on the merus of third and fourth pereopods (characters 45 and 51) are present in Alvinocaridinae, potentially enabling more efficient prey catching and sorting and processing the organic particles which the species live on.
Remarks. The type species of the genus Mirocaris is M. keldyshi, a junior subjective synonym of Chorocaris fortunata. Although the generic status of Mirocaris has largely been supported, the family Mirocarididae Vereshchaka, 1997 has not been recognized, but is herein resurrected at subfamily level [19]. Molecular data have been previously indicating high status of Mirocaris [17].
Genus Mirocaris Vereshchaka, 1997 [12] Diagnosis. Rostrum dorsoventrally compressed, not reaching end of first antennular segment, apically obtuse, dorsally and ventrally not carinate, smooth; carapace with antennal angle acute, pterygostomial tooth present; dorsal organ restricted to postorbital region; third abdominal segment not serrated; telson with sinuous row of movable dorsolateral spines. Anterior margin of cornea without tubercle; scaphognathite not much expanded, without heavily plumose bacteriophore setae; third maxilliped with distal movable spine on antepenultimate segment; first pereopod with grooming apparatus; second pereopod with a distal movable spine on ischium; ischia of third to fifth pereopods with or without movable spines; dactyli of third to fifth pereopods with a single row of accessory spinules; uropodal exopod with a single movable spine mesial to posterolateral tooth.
Remarks. The genus includes 2 species, one from the Atlantic, and the other from the Indian Ocean. Both species are so similar in morphology that supporting molecular data are necessary to prove the validity of M. indica. We keep here, however, a conservative approach and recognize both species.
Genus Nautilocaris Komai, Segonzac, 2004 [14] Diagnosis. Rostrum dorsoventrally compressed, overreaching end of first antennular segment, apically acute, dorsally not carinate, toothed, ventrally smooth; carapace with antennal angle acute, pterygostomial tooth present; dorsal organ restricted to postorbital region; third abdominal segment serrated; telson with sinuous row of movable dorsolateral spines. Anterior margin of cornea without tubercle; scaphognathite not much expanded, without heavily plumose bacteriophore setae; third maxilliped with a distal movable spine on antepenultimate segment; first pereopod with grooming apparatus; second pereopod with distal movable spine on ischium; ischia of third to fifth pereopods with or without movable spines; dactyli of third to fifth pereopods with a single row of accessory spinules; uropodal exopod with a single movable spine mesial to posterolateral tooth.
Remarks. Nautilocaris differs from the closely related genus Mirocaris in having a longer and denticulate rostrum and in the serrated pleura of the third abdominal somite.

Genus Alvinocaridinides Komai & Chan, 2010 [2]
Diagnosis. Rostrum dorsoventrally compressed, not reaching end of first antennular segment, apically blunt, dorsally toothed, ventrally smooth; carapace dorsally smooth, antennal angle acute, pterygostomial tooth present; dorsal organ restricted to postorbital region; third abdominal segment not serrated; telson with sinuous row of movable dorsolateral spines, posterior margin bearing 4 strong spines. Anterior margin of cornea without tubercle; scaphognathite not much expanded, without heavily plumose bacteriophore setae; third maxilliped with 1 distal movable spine on antepenultimate segment; epipods of first to fourth pereopods rudimentary; first pereopod with grooming apparatus; second pereopod without distal movable spine on ischium; ischia of third to fifth pereopods with or without movable spines; dactyli of third to fifth pereopods with two or more rows of accessory spinules; appendix interna in pleopods II-V developed; uropodal exopod with a single movable spine mesial to posterolateral tooth.
Type species.

Manuscaris Komai & Tsuchida, 2015 [21]
Diagnosis. Rostrum laterally compressed, reaching end of first antennular segment, apically acute, dorsally carinate, toothed, ventrally smooth; carapace dorsally toothed, pterygostomial tooth present; third abdominal segment serrated; telson with linear row of movable dorsolateral spines, posterior margin bearing 2-4 strong spines. Anterior margin of cornea without tubercle; scaphognathite not much expanded, without heavily plumose bacteriophore setae; third maxilliped with a distal movable spine on antepenultimate segment; epipods of first to fourth pereopods rudimentary; first pereopod with grooming apparatus; second pereopod with distal movable spine on ischium; ischia of third to fifth pereopods with or without movable spines; dactyli of third to fifth pereopods with two or more rows of accessory spinules; appendix interna in pleopods II-V developed; uropodal exopod with a single movable spine mesial to posterolateral tooth.
Remarks. This genus was recently erected [21] and our analyses support its validity.
Opaepele Williams and Dobbs, 1995 [11] Diagnosis. Rostrum dorsoventrally compressed, not reaching end of first antennular segment, apically blunt, dorsally and ventrally not carinate, notched or smooth; carapace dorsally smooth, antennal angle acute, pterygostomial tooth present; dorsal organ restricted to postorbital region; third abdominal segment serrated; telson with sinuous row of movable dorsolateral spines, posterior margin bearing 2-4 strong spines. Anterior margin of cornea without tubercle; scaphognathite not much expanded, without heavily plumose bacteriophore setae; third maxilliped with a distal movable spine on antepenultimate segment; epipods of first to fourth pereopods rudimentary; first pereopod with grooming apparatus; second pereopod without distal movable spine on ischium; ischia of third to fifth pereopods without movable spines; dactyli of third to fifth pereopods with two or more rows of accessory spinules; appendix interna in pleopods II-V developed; uropodal exopod with a single movable spine mesial to posterolateral tooth.
Genus Rimicaris Williams & Rona, 1986 [9] Diagnosis. Rostrum if present dorsoventrally compressed, not reaching end of first antennular segment, apically obtuse, dorsally and ventrally not carinate, smooth; carapace dorsally smooth, antennal angle blunt or acute, pterygostomial tooth present or absent; dorsal organ extending beyond the postorbital region; third abdominal segment smooth or serrated; telson with sinuous row of movable dorsolateral spines, posterior margin bearing 2-4 strong spines. Eyes fused entirely, anterior margin of cornea without developed tubercle; scaphognathite expanded, with or without heavily plumose bacteriophore setae; third maxilliped with 0-2 distal movable spines on antepenultimate segment; epipods of first to fourth pereopods rudimentary; first pereopod with or without grooming apparatus; second pereopod without distal movable spine on ischium; ischia of third to fifth pereopods with or without movable spines; dactyli of third to fifth pereopods with two or more rows of accessory spinules; appendix interna in pleopods II-V developed; uropodal exopod with two movable spines mesial to posterolateral tooth.
Remarks. The genus includes all species of the former genera Chorocaris sensu Komai and Tsuchida [21] and Rimicaris.
Rimicaris exoculata from the Atlantic and R. kairei from the Indian Ocean are very similar in morphology and are not statistically distinct on the molecular tree ( Fig 6A). We keep a conservative approach and recognize both species, while pointing out a need of additional research to confirm their validity.
Rimicaris vandoverae and R. paulexa are so similar morphologically that it is impossible to articulate sharp distinctions between them. The species are geographically isolated and the minor morphological distinctions may refer to an inter-population difference rather than to an inter-specific variability. Molecular data (now missing for R. paulexa) will help in understanding the status of these species.
As stated above, R. chacei may include at least two cryptic species. New sequences are desirable to clarify this situation.
Genus Shinkaicaris Komai & Segonzac, 2005 [13] Diagnosis. Rostrum laterally compressed, overreaching end of first antennular segment, apically acute, dorsally carinate, toothed, ventrally smooth; carapace dorsally toothed, antennal angle acute, pterygostomial tooth present; dorsal organ restricted to postorbital region; third abdominal segment not serrated; telson with sinuous row of movable dorsolateral spines, posterior margin bearing 2-4 strong spines. Anterior margin of cornea without tubercle; scaphognathite not much expanded, without heavily plumose bacteriophore setae; third maxilliped with a distal movable spine on antepenultimate segment; epipods of first to fourth pereopods rudimentary; first pereopod with grooming apparatus; second pereopod without distal movable spine on ischium; ischia of third to fifth pereopods without movable spines; dactyli of third to fifth pereopods with two or more rows of accessory spinules; appendix interna in pleopods II-V developed; uropodal exopod with a single movable spine mesial to posterolateral tooth.
Type Keldyshicaris gen.nov urn:lsid:zoobank.org:act:4D7D81BC-2C9A-45A0-93EC-D276E33FA174 Diagnosis. Rostrum dorsoventrally compressed, not reaching end of first antennular segment, apically blunt, dorsally not carinate, dorsally and ventrally notched; carapace dorsally smooth, antennal angle acute, pterygostomial tooth present; dorsal organ restricted to postorbital region; third abdominal segment serrated; telson with linear row of movable dorsolateral spines, posterior margin bearing 2-4 strong spines. Anterior margin of cornea with rudimentary tubercle; scaphognathite not much expanded, without heavily plumose bacteriophore setae; third maxilliped with a distal movable spine on antepenultimate segment; epipods of first to fourth pereopods rudimentary; first pereopod with grooming apparatus; second pereopod with distal movable spine on ischium; dactyli of third to fifth pereopods with two or more rows of accessory spinules; appendix interna in pleopods II-V developed; uropodal exopod with a single movable spine mesial to posterolateral tooth. Key to subfamilies, genera, and species of Alvinocarididae (Table 5)    19. Telson with sinuous row of dorsolateral spines and 2-4 spines on posterior margin; cornea without anterior tubercle; ischia of third and fifth pereopods without strong movable spines; dactyli of third to fifth pereopods with two or more rows of accessory spinules Opaepele, the only species Opaepele loihi.
-Telson with linear row of dorsolateral spines and >10 spines on posterior margin; cornea with anterior tubercle; ischia of third and fifth pereopods with strong movable spines; dactyli of third to fifth pereopods with a single row of accessory spinules