Living in the waterfalls: A new species of Trichomycterus (Siluriformes: Trichomycteridae) from Tabay stream, Misiones, Argentina

A new species assigned to the genus Trichomycterus from the area of the waterfalls of Tabay stream, Paraná River basin, Misiones, Argentina, is described. Trichomycterus ytororo sp. nov. is distinguished from all other species in the genus by the presence of 31–35 dorsal procurrent caudal-fin rays and the combination of some external characters such as: coloration, number of pectoral–fin rays and pores of the laterosensory canals. The new taxon belongs to a presumably monophyletic group of species composed of T. crassicaudatus, T. igobi, and T. stawiarski based on the presence of 24 or more thickly ossified and rigid procurrent caudal-fin rays with a slender distal tip extending along the tips of at least ten neural spines.

Trichomycterus is currently the largest genus within Trichomycteridae, containing over 170 valid species [6], which are distributed in practically all South America, Costa Rica  Panamá in Central America [7], and even found in an offshore island in the Pacific Ocean [8].
Its geographical distribution ranges from lowlands of the Atlantic forest in southern Brazil, to mid-elevation streams on both sides of the Andean cordilleras, as well as the Patagonian region in Argentina [9]. Most species inhabit fast-flowing streams [10]. Their range of habitats is also remarkably diverse, including unusual freshwater habitats, such as sand-bottom streams, semipermanent pools, high-altitude Andean streams, and desolate creeks in Tierra del Fuego [11]. During recent samplings in northeastern Argentina we collected specimens of a remarkable new species of Trichomycterus from waterfalls in the Paraná River basin, which are described herein as a new species. The new taxon exhibits conspicuous characters that indicate a close relationship with T. crassicaudatus Wosiacki

Material and methods
Specimens were collected with hand nets at Tabay stream, Jardín América, Misiones, Argentina. Appropriate actions were taken to minimize pain or discomfort of fish. Specimens were euthanized by immersion in an anesthetic solution (0.1% 2-phenoxyethanol), and then fixed in a 4% formaldehyde for one week, washed in water for one day and transferred to a 70% ethanol solution for preservation. These steps are approved by the ethical use animals of Fundación Miguel Lillo (FML) that consider animal welfare laws. Field studies did not involve endangered species. Collection permit was granted by Ministerio de Ecología of Misiones (Disp 074/15). Measurements were taken point-to-point as linear distances with digital calipers to the nearest 0.1 mm. Number of vertebrae, ribs, odontodes, unsegmented dorsal-and anal-fin rays, and caudal-fin procurrent rays were taken from 11 radiographed specimens (rd) and two cleared and stained [12] specimens (c&s) whenever possible. Branchiostegal rays were counted in c&s specimens. The number of post-Weberian vertebrae includes the compound caudal centrum counted as a single element. Anteriormost unbranched fin rays are represented by lower case Roman numerals and branched fin rays by Arabic numerals. The musculature of the dorsolateral head was observed in one paratype (ms). Counst of the holotype are given in parentheses. Meristic and morphometric data, nomenclature in the osteology, myology and cephalic laterosensory system follows the most recent contributions in that sense [13,14,15,16]. The new taxon plus the putative close species T. crassicaudatus, T. igobi, and T. stawiarski are treated as Trichomycterus stawiarski group in the text. Species from the Trichomycterus hasemani group were not included in the comparisons considering their wellsupported relationship with non-trichomycterinae taxa [2,17]. Institutional abbreviations: CI-FML (Colección ictiológica Fundación Miguel Lillo, San Miguel de Tucumán). LGEP (Laboratorio de Genética Evolutiva-Peces, Posadas). IBIGEO-I (Instituto de Bio y Geociencias del NOA-Ictiología, Salta, Argentina)

Nomenclatural acts
The electronic edition of this work follows with the requirements of the 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:act:D4B81E7D-0D04-4907-852B-A36931736CBE Diagnosis. Trichomycterus ytororo is distinguishable from all congeners by the presence of 31-35 dorsal caudal procurrent rays (Fig 3; vs. 29 or less). In addition, Trichomycterus ytororo exhibits two characters shared only by three congeners (Table 1; T. crassicaudatus, T. igobi, and T. stawiarski): the procurrent caudal-fin rays thickly ossified and rigid with a slender distal   [35][36], lower number of ventral procurrent caudal-fin rays (13 vs. 17-18), and the dorsal procurrent caudal-fin rays extending over the tips of 13 neural spines (vs. dorsal procurrent caudal-fin rays extending over the tips of 12 neural spines); and from T. stawiarski by the lower number of ventral procurrent caudal-fin rays (13 vs. 17), the higher number of pectoral-fin rays (i,7 vs. i, 6), and lower number of vertebrae (37-38 vs. 39).

Results
In addition to the above mentioned characters, Trichomycterus ytororo is distinguishable from congeners inhabiting the Paraná-Paraguay system by: 1) the number of pectoral-fin rays 2) First pectoral-fin ray not prolonged as filament (vs. first pectoral-fin ray prolonged as a long filament beyond the distal margin of the fin in T. aguarague Fernández & Osinaga and T. taroba. 3) The color pattern of lateral surface of body composed of black blotches over a pale yellow background extending to the mid-length of caudal fin (vs. lateral surface of body gray in T. plumbeus Wosiacki & Garavello and T. papilliferus; with well-defined spots in T. iheringi (Eigenmann); and proximal region of the caudal fin with a vertical unpigmented band in T. diabolus Bockmann, Casatti & de Pinna).
Description. Morphometric data of holotype and paratypes in Table 2. Holotype and paratypes illustrated in Figs 1-3.
Body relative short, approximately rounded in transverse section at pectoral-fin level and laterally compressed towards tail. Dorsal profile from anterior margin of snout to dorsal-fin origin slightly convex; approximately straight from caudal peduncle to last caudal-fin rays. Ventral profile from tip of snout to pelvic fin slightly convex; straight from this point to last caudal-fin rays. Head relatively small, almost as long as wide, depressed from lateral view and trapezoidal from dorsal view. Snout broad, distal margin slightly convex from dorsal view. Dorsal and ventral profile of head slightly convex.
Eyes dorsally located on head but also visible from lateral view, anteroposteriorly elliptical, covered by thin and transparent skin. Anterior nostril smaller than eye, surrounded by fleshy flap of integument posterolaterally continuous with nasal barbel; distance between anterior nostrils slightly shorter than interorbital distance. Posterior nostril smaller than anterior one, partially surrounded anteriorly, laterally, and medially by thin flap of skin. Posterior nostrils more closely positioned to each other than anterior ones.
All barbels wider at bases and gradually narrowing towards tips. Nasal barbel extending to eye, not surpassing its posterior margin. Laterosensory canals with simple (non-dendritic) tubes ending in single pores. Nasal and frontal canals of supraorbital line fused to each other, with three pores (s1, s3, and s6); one paratype with extra pores between pores s3 and s6 on both sides and one paratype with four pores between pores s3 and s6 on left side. Posterior segment of frontal, sphenotic, and otic canals fused to each other. Antorbital segment of infraorbital line absent. Sphenotic canal with two pores (i10 and i11). Otic, postotic and scapular canals with preoperculo-mandibular and pterotic branches short, each one with one associated pore. Trunk canal with 4-7 pores; fourth pore located at vertical through mid-length of pectoral fin.
Pectoral fin with i,7 rays (i,7) and rounded distal margin; third branched ray longest; first one not prolonged as filament. Dorsal fin located posteriorly to mid-length of body and inserted in a conspicuous concavity of dorsal surface of body. Dorsal fin with two or three unsegmented rays; ii,7 (ii,7) with distal margin rounded; first branched ray longest. Dorsal-fin pterygiophores eight, first one inserting anterior to neural spine of 16 th or 17 th vertebra (16 th ). Anal fin smaller in size than dorsal fin; anterior insertion located slightly posterior to dorsalfin base. Anal fin with two unsegmented rays; i,5 rays (i,5) with distal margin rounded; first  (21 st ). Pelvic fin inserted anterior to vertical through dorsal-fin origin, not covering urogenital opening. Pelvic-fin rays i,4 (i,4); third and fourth rays longest; contralateral medial rays not overlapping in relaxed fins. Caudal fin with i,5 rays on upper hypural plate and i,6-7 rays on lower hypural plate (Fig 3) (i,6) with distal margin straight to slightly rounded. Smaller specimens (less than 67.3 mm SL) with anteriormost three caudal-fin rays of upper hypural plate slightly longer. Upper caudal plate with hypurals 4 and 5 fused together and hypural 3 autogenous; lower caudal plate with parahypural, hypurals 1 and 2 fused (Fig 3). Procurrent caudal-fin rays thickly ossified and rigid; 31-35 dorsally (holotype 34) and 13 ventrally (13) (Fig 3). Dorsal procurrent caudal-fin rays extending over tips of thirteen neural spines (13) and ventral procurrent caudal-fin rays extending dorsal to tips of five to six hemal spines (Fig 3) (holotype six). Vertebrae 37-38 (38). Ribs 11-12 pairs (11); first to sixth rib thicker. Color in alcohol. Sides and dorsal surface of body and head with marbeled color pattern composed of black blotches irregular in shape and size, overlapped by superficial layer of small and less pigmented spots over pale yellow background, forming marbled color pattern (Fig 1). Blotches of dorsal surface of body and head larger, becoming smaller towards to ventrolateral region. Ventral surface of head and anteriormost portion of trunk pale yellow. Ventral surface of caudal peduncle region with inconspicuous and faint black blotches. Barbels with scattered black blotches dorsally and pale yellow ventrally. Opercular and interopercular patches of odontodes pale yellow. Pectoral, dorsal and anal fins with black blotches in proximal portion and whitish band distally. Pelvic fin hyaline with few inconspicuous black blotches. Caudal fin with black blotches in proximal portion becoming inconspicuous towards distal margin.
Color in life. Overall coloration of specimens in life similar to specimens after fixation with exception of brighter yellow background and distal portions of fins hyaline (Fig 2).
Ecological notes. The Tabay stream basin through 192 km from its headwaters at Campo Viera to its mouth on the Paraná River, at Jardín América (Fig 4). The stream bed is mainly composed of basaltic bedrock, in which sections with waterfalls and pools alternates all along its run. At the type locality (Tabay waterfall; Fig 5), the stream is surrounded by remnants of the Paranaense riparian forest, with its left margin degraded due to a camping site. This waterfall consists of three consecutive falls, the main one is 10m high and 20-50m wide, which drains into a narrow gorge. All specimens of Trichomycterus ytororo were captured above the waterfalls at shallow areas (about 1 meter depth or less) or in rapids witha predominantly rocky bottom and strong current. Trichomycterus davisi was the single congener recorded at the type locality, which was not collected syntopically with T. ytororo.
Distribution. Trichomycterus ytororo is so far known only from its type locality, the Tabay waterfalls in the Tabay stream (Fig 5), a tributary of the left bank of the Paraná River, province of Misiones, northeast of Argentina (Fig 4).
Etymology. The specific epithet ytororo derived from the indigenous language Guaraní ("ytororõ") which means "waterfall" in reference to the habitat occupied by the new species. A noun in apposition.

Discussion
The new species proposed herein is allocated in the genus Trichomycterus (subfamily Trichomycterinae) due to the presence of all the synapomorphies for the clade that includes all the trichomycterids excepting the Copionodontinae and Trichogeninae and lacks those derived characters that support the clade comprising the subfamilies Glanapteryginae, Sarcoglanidinae, Stegophilinae, Tridentinae, and Vandelliinae [3]. The presence of the levator internus 4 muscle originating from the neurocranial floor and the dorsal face of the posttemporosupracleithrum, a synapomorphy for Trichomycterinae latu sensu [13], in the new taxon also supports its inclusion in the subfamily. Among the Trichomycterinae, the new taxon also lacks the diagnostic characters of Bullockia, Eremophilus, Hatcheria, Ituglanis, Rhizosomichthys, Scleronema, and Silvinichthys [3,4,18,19,20,21,22,23]. In view of these facts, the inclusion of the new species in the genus Trichomycterus is the most plausible action for the moment.
The non-monophyletic nature of the group and the long and complicated taxonomic history has placed Trichomycterus the most formidable problem in the systematics of the Tricomycteridae [3]. Many species of Trichomycterus have uninformative descriptions and are associated to old type material, limiting the applicability of the name only to the types [24]. Currently, the genus contains over 170 species [6], however, its known diversity remains increasing along the recent years. Even with these obstacles, species groups within Trichomycterus were defined based on morphological characteristics [11,25,26,27,28,29,30,31], some of them constantly redefined and re-diagnosed, such as the Trichomycterus brasiliensis species complex [14,32,33], which, to date, its monophyly could not be consistently assessed [34]. Besides, these characters and all species included on these groups were not placed into a comprehensive phylogenetic analysis, therefore with the possibility of representing artificial groups due to possible parallelisms.
The external and internal characters present in T. ytororo do not fit with any of the previous suggested species complexes with the exception of the group comprising T. crassicaudatus, T. igobi, and T. stawiarski. This group was proposed based on the presence of the procurrent caudal-fin rays thickly ossified and rigid with a slender distal tip; the dorsal procurrent caudalfin rays extending along the tips of at least 10 neural spines; and 10-11 branchiostegal rays [11] The two first conditions are shared with in T. ytororo (Table 1, Fig 3) and contrast with the procurrent caudal-fin rays thin and flexible [11,35] extending usually for seven or eight neural spines tips, as observed in other species within the genus [11]. In addition, the species of the Trichomycterus stawiarski group have numerous procurrent dorsal caudal-fin rays (24 or more), in comparison with other congeners (Table 1, Fig 3). Although the number of the caudal-fin procurrent rays are not available for all species, the closest value was observed in T. steindachneri DoNascimiento, Prada-Pedreros & Guerrero-Kommritz [24] Besides these osteological traits, the four species show a similar color pattern consisting of two distinct layers of pigmentation: deep layer with large black blotches irregular in shape and size, overlapped by a superficial layer of small and less pigmented spots [11,35] (Figs 1 and 2). In addition, the blotches are more concentrated on the dorsum of the trunk, becoming more scattered towards ventral region. However, considering the high variation of the color pattern within the genus, only an analysis including an extensive comparative material of Trichomcyterus and other Trichomycterinae in a phylogenetic context could provide an adequate framework for interpreting these characteristics of coloration. Trichomycterus ytororo has the extensor tentaculi muscle originating on both suspensorium and neurocranium (Table 1), trait only observed in T. stawiarski and T. davisi within the genus [13,36]. At the same time, myological data for T. crassicaudatus and T. igobi and the most species on genus are not available. As consequence, the origin of this character remains uncertain considering the well-supported relationship among T. crassicaudatus, T. igobi and T. stawiarski [36].
The putative monophyly of the clade formed by T. crassicaudatus, + T. igobi, + T. stawiarski [11] would be also supported by the presence of 10-11 branchiostegal rays, contrasting with other trichomycterids which have usually 5-8 branchiostegal rays, a plesiomorphic condition [11]. Based on this continuous character (10-11 branchiostegal rays), the recently described species, T. perkos, was proposed as possible sister taxon of this clade [36]. Trichomycterus ytororo has nine branchiostegal rays, a condition not present T. igobi, T. crassicaudatus and T. stawiarski also unusual among trichomycterids [35] Even so, the aforementioned presumably derived character states shared by the species herein proposed as belonging to the Trichomycterus stawiarski group (Table 1) suggest their close relationship and the reduced number of branchiostegal rays in T. ytororo could be interpreted as a homoplastic feature. However, a comprehensive phylogenetic analysis of the genus is still needed to assess if those character states are more parsimoniously attributed to common ancestry or parallelisms and to test the monophyly of this putative clade.
Trichomycterus crassicaudatus, T. igobi, and T stawiarski are endemic to the Iguazú (or Iguaçu, in Brazil) River basin, a left bank tributary of the Paraná River, while T. ytororo inhabits a downstream section of the same basin (Fig 5). Curiously, all these species are known from few collecting sites or only from their type localities (as T. ytororo), possibly indicating some ecological constraints in their distributions.