A new genus and species of miniature tridentine catfish from the Amazon basin (Siluriformes: Trichomycteridae)

Abstract A new miniature tridentine catfish is described from the rio Purus drainage, Amazon basin, Brazil. It differs from all other tridentines in having several unique autapomorphies: conspicuous anteromedial protuberance in the snout; set of symphyseal premaxillary and dentary teeth inclined posteromedially; distal process of the hyomandibula directed anteriorly; rod-like orbitosphenoid ossified only ventral to the optic nerve; mesethmoid cornua inclined ventrolaterally; opercular and interopercular odontodophores separated by a large interspace; basipterygia fused sagittally; and conspicuous dark brown horizontal stripe in the middle of the caudal fin. The new taxon is hypothesized to be sister to the clade formed by Tridensimilis and Tridens. A detailed osteological description of the new taxon is provided based on X-ray microcomputed tomography (µCT-scans) data and on cleared and stained specimens. Our analysis also reveals that “Tridens” brevis, an enigmatic species that has been indecisively assigned to three different tridentine genera over the past 134 years, belongs to Tridentopsis.Consequently, Tridensimilis is a monotypic genus that currently includes only T. venezuelae.

So far, Tridentinae includes only miniaturized species (sensu Weitzman, Vari, 1988) distributed in the Amazon, Orinoco, and Paraguay river basins.Members of the subfamily exhibit several typical paedomorphic features, such as a poorly ossified cranial roof, poorly developed lateral-line canals, and the presence in adults of fully cartilaginous skeletal elements that are typically found only in larval or juvenile stages.
Despite the increasing number of descriptions of new freshwater fishes in the Neotropical region, the diversity of miniature fishes is still poorly known.This is due to several factors, including the difficulty of sampling this type of fauna using standard collection methods and, sometimes, the misidentification of specimens as juveniles of other taxa.Tridentine taxonomy has been characterized by long hiatuses of decades without the addition of new taxa (apart from the controversial allocation of Potamoglanis, see Discussion).A few years ago, de Pinna (2016) stated that "[w]ithout a doubt, the trichomycterid subfamily where the least progress was made [in terms of new taxa descriptions] in the last 40 years is the Tridentinae".We present here the first description in almost 80 years of an undoubtedly new tridentine genus from a tributary of the rio Purus, Amazon drainage.Our study includes detailed osteological analyses and remarks on important aspects of the taxonomy and systematics of the subfamily.

MATERIAL AND METHODS
Morphometric data were taken point-to-point with digital calipers on the left side of specimens to the nearest 0.1 mm under stereomicroscope.Measurements followed Tchernavin (1944), except for the following ones that were taken according to Dutra et al. (2012): caudal peduncle length (from the base of the last anal ray to the base of the median caudal rays); caudal peduncle depth (vertically through the middle of caudal peduncle length); body depth (vertically through the pectoral insertion); and eye diameter (horizontally from the anterior margin to the posterior margin of orbit).In the fin counts, soft (i.e., non-spinous) unsegmented or procurrent (sensu Arratia, 2008) rays of median fins are represented by lowercase Roman numerals followed by a superscript 'P' (e.g., iii P ), unbranched segmented soft rays by simple lower case Roman numerals (e.g., iii), and branched segmented rays by Arabic numerals (e.g., 3).The two posteriormost closely-set rays in dorsal and anal fins were counted as separate elements.Counts were recorded for the holotype and paratypes, with absolute frequencies for each value given in square brackets throughout the description, and holotype counts indicated with an asterisk (*).Numbers of branchiostegal rays, vertebrae, ribs, fin rays, number and position of dorsal-and anal-fin support elements, and other osteological features were obtained from paratypes CT-scanned and cleared and stained according to Taylor, Van Dyke (1985).Vertebral counts include only the post-Weberian vertebrae, with the urostyle (compound caudal centrum) counted as a single element.Anatomical terminology follows Datovo, Bockmann (2010) with the following modifications: angular complex (= angulo-articulo-retroarticular) instead of "angulo-articular" (Adriaens et al., 2010) and odontodophores instead of "patch of odontodes" (de Pinna, Dagosta, 2022).Only external morphology could be examined on specimens from some lots of the new taxon.These lots are therefore not designated as types, although their external characters allow their unequivocal taxonomic identification.
A paratype was scanned on Phoenix v|tome|x m microfocus microcomputed tomography system (General Electric Company) with a 300 kV µ-focus X-ray source.To improve image resolution a multiscan montage of the whole specimen was generated from three individual scans, totaling 1,440 images.X-ray projection images were recorded at 1,000 ms of time exposure per image, with 70 kV, 200 mA, and voxel resolution of 27 µm.Three-dimensional visualization as well as the analysis of the reconstructed data was performed using VGStudio MAX2.2.3 64 bit (Volume Graphics GmbH, Heidelberg, Germany).New genus and species of tridentine from Amazon Institutional acronyms follow Fricke, Eschmeyer (2023). Abbreviations: eth, ethanol preserved specimens;c&s, cleared and stained specimens;HL, head length;SL, standard length;spec, specimen;µct, µCT-scanned specimen (preserved in ethanol).

Diagnosis.
The new species differs from all other tridentines by having a conspicuous dark brown horizontal stripe in the middle of the caudal fin (Fig. 1).Ongoing studies indicate the existence of additional undescribed species of Rhinotridens that lack this caudal stripe (see Discussion).As a result, this character is preemptively proposed as autapomorphic for R. chromocaudatus rather than as a synapomorphy for the genus as a whole.
Description.External morphology.Morphometric data for holotype and paratypes given in Tab. 1. Body elongate, roughly cylindrical at pectoral girdle level, progressively more compressed towards caudal peduncle (Fig. 1).Dorsal profile slightly convex from tip of snout to dorsal fin, gently concave from that point to caudal peduncle.Ventral profile straight to gently convex from tip of snout to pectoral-fin origin, then slightly convex to pelvic-fin origin, straight from that point to anal-fin origin, concave from anal-fin origin to end of caudal peduncle.Anal opening shortly anterior to anal-fin origin.Greatest body depth shortly anterior to vertical through pelvic-fin origin.
Head depressed, longer than wide.Snout with round anteromedial protrusion particularly evident in dorsal and ventral views.Anterior nostril small and round, surrounded by short tube of integument, positioned closer to upper lip than to anterior margin of eye.Nasal barbel absent.Posterior nostril smaller than anterior one and located at midline between anterior nostril and eye.Mouth ventral, crescent-shaped, its corners slightly posterolaterally-oriented in ventral view.Anterior margin of upper lip gently rounded and continuous laterally with maxillary-barbel base.Lower lip thicker than upper one, with gently convex anterior margin and continuous laterally with rictal-barbel base.Bases of maxillary and rictal barbels continuous with lower lip.Maxillary barbel surpassing middle of eyeball, but not reaching its posterior margin.Rictal barbel slightly shorter than maxillary one.Eyes large and circular, covered by thick translucent skin not adhered to eyeball's surface.Eyes located laterally on head, at middle of HL.Interorbital space slightly convex and about same length as eyeball diameter.Greatest head width at level of interopercular odontodophores.Interopercle New genus and species of tridentine from Amazon with 6(4*)-8(1) conical odontodes.Opercle with 4(1)-6(1) conical odontodes, their posterior margins reaching vertical through base of first pectoral-fin ray.Branchiostegal membrane forming free fold ventrally across isthmus.
Jaws.Premaxilla large, tapering distally, with conspicuous anteromedial ascending process articulating with mesethmoid cornu (Figs.2-4).Premaxillary teeth arranged in three arched rows.Three or four additional rows of labial teeth implanted in upperlip connective tissue just anterior to premaxilla.Posteromedial margin of premaxilla with 4-6 large, posteromedially-oriented symphyseal teeth.Tiny maxilla paralleling concave posterior profile of premaxilla and providing support to maxillary and rictal barbels.Lower jaw much wider than long.Coronoid process formed mostly by angular complex (= angulo-articulo-retroarticular).Dentary with numerous teeth arranged in five rows.Anteromedial margin of dentary with three offset posteromedially-oriented symphyseal teeth.Meckel's cartilage small and located just ventral to the last row of dentary teeth.Coronomeckelian absent.
Median-fins supports.Dorsal-fin basal radials 8, distributed between neural spines of 21 st and 26 th post-Weberian vertebrae.Anal-fin basal radials 19, distributed between haemal spines of 19 th and 30 th post-Weberian vertebrae.Uroneural continuous with compound caudal centrum (PU1+U1).Parhypural and hypurals 1-2 fused, forming lower hypural plate.Single upper hypural plate, presumably formed by fused hypurals 3-5.Uroneural anterodorsal to upper hypural plate.Epurals absent.New genus and species of tridentine from Amazon Coloration in alcohol.Unpigmented body background uniformly white to pale yellow (Fig. 1).Dark brown melanophores distributed in specific regions of head, trunk, and fins.Melanophores on skin of head clustered around cranial fontanel, along sagittal region and lateral borders of snout, and on opercular region.Pigments on connective membrane covering brain visible externally through cranial fontanel and translucent skin.Melanophores scattered along skin of dorsosagittal region of trunk, more densely between occiput and dorsal fin.Midlateral line of trunk with thin line of melanophores on skin, gradually expanding toward caudal-fin base.Caudal fin with broad midlateral horizontal dark brown stripe.Other fins with scattered melanophores concentrated at their bases and others irregularly scattered amid interradial membranes.Melanophores at dorsal region of peritoneum externally visible through thin abdominal wall, forming ventrolateral stripe between pectoral region and anus.

Coloration in life.
Body background mostly translucent with a faint superficial iridescent blue tint.Dark pigmentation as described in "Coloration in alcohol".

Geographical distribution.
Rhinotridens chromocaudatus is known from three tributaries (rio Ipixuna, rio Açuã, and lago Aiapuá) of the rio Purus, Amazon basin, Brazil (Fig. 10).Ecological notes.The rio Ipixuna at the type-locality is a medium sized blackwater river (6.5 m wide) with slow water flow (Fig. 11).Sampling took place during ebb season when some beaches were already appearing.Specimens of Rhinotridens chromocaudatus were collected during the evening (18:00-20:00) in the middle-upper water column in moderate abundance near the margin.The bottom was muddy with patches of leaf litter.Rhinotridens chromocaudatus was captured with the catfishes Bunocephalus coracoideus (Cope, 1874), Corydoras robustus Nijssen & Isbrücker, 1980, Farlowella amazona (Günther, 1864), Mastiglanis asopos Bockmann, 1994, Microglanis poecilus Eigenmann, 1912, Ochmacanthus reinhardtii (Steindachner, 1882) (Eigenmann, 1918;Baskin, 1973;de Pinna, 1998;Datovo, Bockmann, 2010;Ochoa et al., 2017Ochoa et al., , 2020)).The first phylogenetic definition of the subfamily was presented by Baskin (1973), who demonstrated that its four traditional genera form a monophyletic group: Miuroglanis, Tridentopsis, Tridensimilis, and Tridens. Later, Henschel et al. (2017) erected Potamoglanis (formerly "Trichomyterus" hasemani group) as a new tridentine genus.However, Ochoa et al. (2017Ochoa et al. ( , 2020) ) did not recovered Potamoglanis as a tridentine, a fact recently recognized by authors of the original description of the genus (Henschel et al., 2023;Fig. 12A).Therefore, the synapomorphies for Tridentinae are those proposed by Baskin (1973), which to date remains as the only phylogenetic study to include all tridentine genera.Our analysis confirms the material basis of all eight synapomorphies for the subfamily originally proposed by Baskin (1973), but the phylogenetic implications of some of them need qualifications because of discoveries in the intervening years (Fig. 12B): (1) cranial roof mostly unossified, forming a single greatly enlarged fontanel (Figs.3B, 7B; vs. ossified cranial roof with none, one, or two small fontanels) -this condition occurs also in Potamoglanis and in the vandelliines Paravandellia Miranda Ribeiro, 1912 and Paracanthopoma Giltay, 1935 and may not be decisively synapomorphic for Tridentinae (de Pinna, 1989;DoNascimiento, 2013;Henschel et al., 2023); (2) maxilla extremely small, proportionally the smallest in the family (Figs.3B, 4; vs. larger maxilla) -the large cartilaginous maxilla reported in Henschel et al. (2023) for Tridens vitreus Henschel, Ohara & Costa, 2023 appears to be mistaken for the lateral palatine cartilage, since the maxilla in bony fishes is dermal and never preformed in cartilage; (3) eyes exposed ventrally (Fig. 1; vs. not exposed ventrally) -paralleled to a lesser extent in the stegophiline Haemomaster Myers, 1927; (4) opercular and interopercular odontodophores juxtaposed, being separated by a distance less than the depth of the opercular patch (vs.patches separated by a distance greater than the depth of the opercular patch) -this condition is not present in Rhinotridens (see below); (5) opercle with an anteroventral process shorter than the depth of its articular condyle with the hyomandibula (Figs.3A, 5; vs. process longer than the articular condyle); (6) origin of dorsal fin at same level or posterior to that of anal-fin in external view (Fig. 1; vs. dorsal-fin origin anterior to anal-fin origin) -this condition is paralleled in all species of Trichogenes Britski & Ortega, 1983, several species of Paracanthopoma, and some species of Potamoglanis (Henschel et al., 2017;de Pinna et al., 2020;de Pinna, Dagosta, 2022); (7) anterior portion of hyomandibula with a dorsal distal process (Figs.3A, 5; vs. process absent); and (8) anal fin with 15 or more rays (vs.12 or less rays) -convergent in Trichogeninae (de Pinna et al., 2020).These conditions are found in all previously known tridentine genera.Rhinotridens chromocaudatus shares all but the fourth character.Given its inferred phylogenetic position, this absence is most parsimoniously interpreted as a reversal (Fig. 12B; see below).
Baskin (1973) also proposed a hypothesis about the interrelationships among tridentine genera, with Miuroglanis, Tridentopsis, Tridensimilis, and Tridens in that order as successive sister groups.The clade formed by the three latter genera was supported by the following characters (Fig. 12B): (9) anal fin with 17 or more rays (corrected from 18 by DoNascimiento, 2013:449; vs. 15 or less); (10) anteriormost anal-fin pterygiophore inserting two or more vertebrae anterior to the insertion of the anteriormost dorsalfin pterygiophore (vs.one or more vertebrae posterior); (11) ventral exposure of eye equal or greater than dorsal exposure (Fig. 1; vs. smaller); and (12) eye distinctly larger than in Miuroglanis.We confirm the presence of these character states in the examined specimens of Tridentopsis, Tridensimilis, Rhinotridens, and Tridens. Finally, Baskin (1973) proposed five synapomorphies supporting the sister-group relationship between Tridensimilis and Tridens: (13) three to six opercular odontodes (Fig. 5A; vs. 10-15); (15) eyes facing more ventrally than dorsally (vs.equal or more dorsally; Fig. 1); ( 16) Weberian capsule with elongate, neck-like lateral constriction (vs.elongate constriction absent; Figs.3B, 7B, C; paralleled in several stegophilines); (17) anal-fin origin three or more vertebrae anterior to dorsal-fin origin (vs.two or less); and rictal barbel not externally visible.The recent description of two species of Tridens with small but externally distinguishable rictal barbels refutes the validity of the last character (Henschel et al., 2023).We confirm the presence of characters 13, 15, 16, and 17 in the specimens of Tridensimilis and Tridens that we examined.Of these characters, only character 13 is present in Rhinotridens chromocaudatus.We additionally found that Rhinotridens, Tridensimilis, and Tridens share ( 14 New genus and species of tridentine from Amazon or completely cartilaginous in adults (Fig. 6; vs. mostly ossified in juveniles and adults).Therefore, character evidence supports the hypothesis that the new species is the sister group to Tridens plus Tridensimilis.This being so, the most economical nomenclatural solution is to recognize it as representative of a new genus, Rhinotridens (Fig. 12B).
Several morphological features of Rhinotridens chromocaudatus are unique among tridentines (Fig. 12B): (4) opercular and interopercular odontodophores separated by a large interspace (Fig. 5; reversal of the condition that evolved at the base of the Tridentinae; see above); (18) snout with a conspicuous anteromedial protuberance (Fig. 1; vs. protuberance absent or limited to a gentle convexity); ( 19) mesethmoid cornua directed ventrolaterally (Fig. 4; vs. cornua horizontally straight); (20) symphyseal series of premaxillary and dentary teeth inclined posteromedially (Fig. 2; vs. not inclined); (21) distal process of hyomandibula directed anteriorly (Figs.3A, 5; vs. posteriorly); ( 22) rod-like orbitosphenoid ossified only ventral to the optic nerve (Figs. 3,7; vs. laminar and ossified around the nerve, with a foramen for the nerve exit); ( 23) basipterygia fused sagittally (Fig. 6; vs. separated); and a (24) dark brown stripe in the middle of the caudal fin (Fig. 1; vs. stripe absent).The five former characters are also present in one, probably more, additional undescribed tridentine species from the Amazon basin.Consequently, we interpreted characters 4, 18-23 as synapomorphic for Rhinotridens (Fig. 12B), and character 24 as an autapomorphy for R. chromocaudatus.The description of the other purportedly new Rhinotridens species requires sampling of additional specimens and should therefore be presented in a future study.The caudal fin stripe of R. chromocaudatus (Fig. 1) is unique among tridentines and relatively uncommon in Trichomycteridae.A similar caudal stripe occurs in the trichomycterines Trichomycterus barbouri (Eigenmann, 1911) Remarks on tridentine taxonomy.Uncertainties in the assignment of species to genera have historically characterized the description of new species in Tridentinae.Eigenmann, Eigenmann (1889) described the genus Miuroglanis to allocate M. platycephalus and Tridens to allocate T. melanops and T. brevis.The authors highlighted that the two later species were morphologically so disparate that they could belong to distinct genera.Tridens brevis was never illustrated, and its single type was lost shortly after its original description (see Eigenmann, 1918:370).Subsequent studies then transferred T. brevis to two other genera based solely on the textual descriptions of its external morphology provided by Eigenmann, Eigenmann (1889) and Eigenmann (1918).Myers (1925)  first pectoral-fin ray (= pectoral filament), a greater number of opercular odontodes, and an anal-fin origin only slightly anterior to the dorsal-fin origin.We agree with Baskin's (1973) interpretation and found additional corroborative evidence: the elongate maxillary and rictal barbels of T. brevis.These barbels were originally reported to extend "to the base of the pectoral" fin and the "gill opening", respectively (Eigenmann, Eigenmann, 1889, andEigenmann, 1918).Among tridentines, such elongated maxillary and rictal barbels are found only in species of Tridentopsis (vs.barbels considerably shorter, with the maxillary barbel falling short of the branchiostegal opening and the rictal barbel not reaching the posterior margin of the eyeball).The elongate maxillary and rictal barbels are possibly reversals of reductions in these structures that could be optimized as having evolved at the base of the clade Tridentinae + Stegophilinae + Vandelliinae (Baskin, 1973).Moreover, we examined specimens collected near the type-locality of T. brevis (ZUEC 15118) that fit the diagnostic characters originally reported for the species.These specimens clearly belong to Tridentopsis.Therefore, all evidence indicate that T. brevis is a Tridentopsis and, consequently, Tridensimilis is a monotypic genus containing only T. venezuelae.
After the description of Tridensimilis, forty-six years passed before a new species of Tridentopsis, T. cahuali Azpelicueta, 1990, was described from the rio Paraguay drainage (Azpelicueta 1990).More recently, two new species of Tridens have been described from the Amazon basin, T. chicomendesi Henschel &Costa, 2023, andT. vitreus (Henschel et al., 2023).With the description of Rhinotridens chromocaudatus, Tridentinae now contains ten valid species, being the third least diverse subfamily of Trichomycteridae.

FIGURE 10 |
FIGURE 10 | Geographic distribution of Rhinotridens chromocaudatus.White dot indicates the typelocality, each symbol may represent than one lot or locality.
, Physopyxis ananas Sousa & Rapp Py-Daniel, 2005, P. lyra Cope, 1872, Rineloricaria lanceolata (Günther, 1868), and Scoloplax baskini Rocha, de Oliveira & Rapp Py-Daniel, 2008.Etymology.From chroma, latinized form of the Greek word khrôma (χρῶμα), meaning color, and cauda, a Latin word meaning tail.In reference to the presence of the dark brown pigmentation in the middle of the caudal fin.An adjective.Conservation status.Rhinotridens chromocaudatus was captured in localities of the lower rio Purus, including within the Floresta Estadual Tapauá and Reserva Sustentável Piagaçu-Purus.No significant threats to the species have been identified in the area of occurrence.Consequently, R. chromocaudatus can be provisionally classified as Least Concern (LC) according to the categories and criteria of the International Union for Conservation of Nature (IUCN Standards and Petitions Committee, 2022).
created the genus Tridentopsis to include Tridentopsis pearsoni Myers, 1925 and transferred T. brevis into that genus as well, thus restricting Tridens to T. melanops.Tridentopsis tocantinsi LaMonte, 1939 was described more than a decade later(LaMonte, 1939).Schultz (1944)then described the genus Tridensimilis to allocate a new species, T. venezuelae Schultz, 1944, and transferred T. brevis to that genus.Baskin (1973) disagreed with this transfer, arguing that T. brevis had morphological characters not found in Tridensimilis venezuelae but common to Tridentopsis species, such as the elongate 19/22 ni.bio.br| scielo.br/niAlessio Datovo, Luz Ochoa, George Vita, Paulo Presti, William M. Ohara and Mario C. C. de Pinna