Rhinotridens chromocaudatus, Datovo & Ochoa & Vita & Presti & Ohara & Pinna, 2023

Datovo, Alessio, Ochoa, Luz, Vita, George, Presti, Paulo, Ohara, William M. & Pinna, Mario C. C. de, 2023, A new genus and species of miniature tridentine catfish from the Amazon basin (Siluriformes: Trichomycteridae), Neotropical Ichthyology (e 230076) 21 (3), pp. 1-22 : 6-15

Rhinotridens chromocaudatus , new species urn:lsid:zoobank.org:act:4BD03912-3F75-4F97-AABE-09AE2D07E0D2

( Figs. 1–9 ; Tab. 1)

Holotype. MZUSP 128216 , 17.57 mm SL, Brazil, Amazonas State , Humaitá Municipality , rio Amazonas basin, rio Purus drainage, rio Ipixuna, mouth of Lago Comprido, 07°30’37”S 63°20’23”W, 21 Jul 2012, W. M. Ohara.

Paratypes. INPA 28592 , 25 eth (14.65–16.87 mm SL), Brazil, Amazonas State , Beruri Municipality , rio Amazonas basin, rio Purus drainage, igarapé Itabocão, lago Aiapuá, Reserva de Desenvolvimento Sustentável Piagaçu Purus, approx. 04°25’41”S 62°08’05”W, 14 Nov 2007, L . R. Py-Daniel, E. Ferreira, F. Rossoni & A. Galluch ; MCP 55013 , 6 eth (15.39–16.48 mm SL); MNRJ 54171, 6 eth (15.99–16.33 mm SL); MPEG 39635, 6 eth (15.59–17.16 mm SL); MZUSP 128217, 19 eth (16.10–17.48 mm SL), 1 µct (16.37 mm SL), 1 c&s (16.72 mm SL); UFRO-ICT 15431, 20 eth (15.31–16.40 mm SL), same data as holotype .

Non-types. INPA 33819 , 24 eth (14.74–16.95 mm SL), same data as INPA 28592 . INPA 42139 , 19 eth (15.28–17.81 mm SL), Brazil, Amazonas State , Tapauá Municipality , rio Amazonas basin, rio Purus drainage, rio Ipixuna drainage, igarapés dos Caetanos, Floresta Estadual Tapauá, 06°19’57” S 63°12’46”W, 17 Aug 2012 , T. Couto & M. Carvalho . INPA 42148 , 1 eth (16.57 mm SL), Brazil, Amazonas State , Tapauá Municipality , rio Amazonas basin, rio Purus drainage, rio Ipixuna drainage, igarapé dos Mutuns, Floresta Estadual Tapauá, 06°22’29”S 63°16’27”W, 18 Aug 2012 , T. Couto & M. Carvalho . INPA 56815 , 1 eth (16.21 mm SL), Brazil, Amazonas State , Beruri Municipality , rio Amazonas basin, rio Purus drainage, stream in lago Aiapuá, 04°26’16.84”S 62°07’24”W, 28 Sep 2008, E. Ferreira . INPA 56885 , 3 eth (15.38–15.56 mm SL) , INPA 56920 , 76 eth (14.12–16.85 mm SL), same data as INPA 56815 . UFRO-ICT 15405 , 4 eth (15.0– 16.3 mm SL), Brazil, Amazonas State , Humaitá Municipality , rio Amazonas basin, rio Purus drainage, unnamed tributary of rio Ipixuna, 07°32’3”S 63°21’8”W, 22 Jul 2012, W.M. Ohara . UFRO-ICT 15520 , 2 eth (16.0–16.0 mm SL), Brazil, Amazonas State , Humaitá Municipality , rio Amazonas basin, rio Purus drainage, rio Ipixuna, 07°31’19”S 63°21’00” W, 21 Jul 2012, W.M. Ohara . UFRO-ICT 15538 , 1 eth (15.5 mm SL), Brazil, Amazonas State , Humaitá Municipality , rio Amazonas basin, rio Purus drainage, rio Açuã, 08°11’47”S 63°51’45”W, 9 Aug 2012, W.M. Ohara .

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 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.

Pectoral-fin rays similar in length, resulting in slightly convex distal margin. Pectoral-fin rays i,4 (both sides of 6 spec, including holotype) or i,3,i (4 spec). Pelvic-fin rays i,4 (5 spec; holotype) or i,3,i (5 spec). Dorsal fin with distal margin gently convex, its origin slightly posterior to that of anal fin. Dorsal-fin rays i,7 (6 spec; holotype), i,6,i (3 spec) or rarely i,5,i (1 spec). Anal fin elongate, with distal margin straight to slight concave. Anal-fin rays iP,i,17 (4 spec, holotype) or iP,i,16 (2 spec), rarely iP,i,18 (2 spec), iP,i,19 (1 spec) or iiP,i,18 (1 spec). Caudal fin ranging from truncate to emarginate. Caudal-fin rays ixP,i,5 on dorsal lobe and xP,i,6 on ventral lobe. First procurrent rays of both caudal-fin lobes rudimentary.

Neurocranium. Skull roof poorly ossified, forming a single, large fontanel bordered by mesethmoid, frontal, sphenotic-prootic-pterosphenoid, and parieto-supraoccipital ( Figs. 3 , 4 , 7 ). Mesethmoid T-shaped in dorsal view, with cornua strongly bent ventrolaterally. Mesethmoid axis extremely thin and covered posterodorsally by frontal. Frontal elongate and slender posteriorly, not contacting its antimere sagittally. Sphenotic, prootic, and pterosphenoid fused. Parieto-supraoccipital with curved anterolateral projections bordering posterolateral portion of cranial fontanel. Pterotic with small lateral process. Vomer constricted at middle portion and split posteriorly into three laminar processes. Middle posterior process longest, overlapping ventrally part of parasphenoid. Vomer with lateral tubercle for articulation with autopalatine. Lateral ethmoid tiny, laminar, restricted to neurocranial floor. Orbitosphenoid rod-like, gently curved dorsally, lacking any foramen, and restricted to neurocranial floor. Parasphenoid not overlapping basioccipital posteriorly. Basioccipital lacking anterior processes and not sutured to parasphenoid.

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.

Suspensorium and opercular series. Autopalatine with broad, undivided anterior cartilage and two elongate processes: lateral and posterior ( Figs. 3-5 ). Quadrate with anterior portion laminar and posterior portion elongate. Metapterygoid absent. Hyomandibula with broad distal process directed anterodorsally. Preopercle with lateral condyle for articulation with interopercle. Interopercle expanded posteriorly and with dorsal concavity for articulation with opercle. Opercle with conspicuous adductor crest and dilatator process. Anteroventral process of opercle short.

Hyoid bar. Parurohyal with thin, elongate lateral arms and short posterior process ( Fig. 8 ). Ventral hypohyal with ventral fovea for articulation with parurohyal condyles. Dorsal hypohyal absent. Anterior ceratohyal constricted at midlength. Posterior ceratohyal short and compressed. Interhyal absent. Branchiostegal rays 6, approximately equal in length. Three branchiostegal rays articulating with anterior ceratohyal and three with posterior ceratohyal.

Gill arches. Basibranchials and hypobranchials completely cartilaginous ( Fig. 9 ). Basibranchial 1 globose and autogenous; basibranchials 2 and 3 conjoined in elongate anterior copula; basibranchial 4 nearly hexagonal. Hypobranchials 1 and 2 thinner than hypobranchial 3. Ceratobranchials ossified at middle portion, cartilaginous distally to tips. Ceratobranchials 1–3 with posterior laminar projections; ceratobranchial 5 more densely ossified than others in series and bearing two short strong teeth. Ceratobranchialhypobranchial articulations forming acute angle. Epibranchials 1–3 poorly ossified at middle portion, cartilaginous distally to tips. Epibranchial 1 with protruding uncinate process; epibranchials 2 and 3 slender, rod-like; epibranchial 4 more robust and densely ossified than others. Only cartilaginous pharyngobranchial 4 present. Large arched upper pharyngeal tooth plate associated with pharyngobranchial 4 and bearing 8–9 elongate, conical teeth.

Weberian apparatus and axial skeleton. Weberian apparatus mostly encapsulated with narrow lateral opening ( Figs. 3 , 7 ). Neck-like lateral constriction of the Weberian capsule absent. Post-Weberian vertebrae 38. First post-Weberian vertebra nearly half length of subsequent one. First complete haemal arch and spine on third post-Weberian vertebrae. Pleural ribs two, second one not contacting parapophysis.

Paired girdles. Posttemporo-supracleithrum tightly articulated with neurocranium ( Fig. 3 ). Cleithrum ossified only at its margins. Scapulo-coracoid mostly cartilaginous. Two pectoral radials fully cartilaginous.Basipterygia fused sagittally, mostly cartilaginous, with tiny ossifications restricted to distal tips of anterolateral and anteromedial spines. Pelvic splint absent ( Fig. 6 ).

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.

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) , 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).

Figures

FIGURE 1. Rhinotridens chromocaudatus, new genus and species, holotype, MZUSP 128216, 17.57 mm SL. Brazil, Amazonas, rio Ipixuna, tributary of the rio Purus, rio Solimões basin. Scale bar = 2.0 mm.

FIGURE 2 | Premaxilla and lower jaw of Rhinotridens chromocaudatus, µCT-scan images, paratype, MZUSP 128217, 16.37 mm SL; A. Dorsal view; B. Ventral view; C. medial view, left side. Symphyseal teeth highlighted in yellow. Ac, angular complex; Dt, dentary; Dth, dentary teeth; Lt, labial teeth; Pmt, premaxillary teeth; Pmx, premaxilla. Scale bar = 0.3 mm.

FIGURE 3 | Cranium, pectoral skeleton, and anterior portion of axial skeleton of Rhinotridens chromocaudatus, µCT-scan images, paratype, MZUSP 128217, 16.37 mm SL; A. Left lateral view; B. Dorsal view. Ac, angular complex; AC, anterior ceratohyal; Ap, autopalatine; Boc, basioccipital; Br, branchiostegal rays; Cl, cleithrum; Dt, dentary; Ep, epioccipital; Fr, frontal; Hy, hyomandibula; Iop, interopercle; Le, lateral ethmoid; Me, mesethmoid; Mx, maxilla; Op, opercle; Osph, orbitosphenoid; Pop, preopercle; Psph, parasphenoid; Psoc, parieto-supraoccipital; Ptr, pectoral-fin rays; Ptscl, postemporosupracleithrum; PC, posterior ceratohyal; Pmx, premaxilla; Pop, preopercle; Pt, pterotic; Pu, parurohyal; Qd, quadrate; Sph, sphenotic-prootic-pterosphenoid; Vh, ventral hypohyal; Vo, vomer; WeC, Weberian capsule. Scale bar = 0.3 mm.

FIGURE 4 | Cranium of Rhinotridens chromocaudatus, µCT-scan images, paratype, MZUSP 128217, 16.37 mm SL; anterior view; arrow indicates right mesethmoid cornu. AC, anterior ceratohyal; Ap, autopalatine; Boc, basioccipital; Br, branchiostegal rays; Dth, dentary teeth; FPV, first post-Weberian vertebrae; Fr, frontal; Hy, hyomandibula; Iop, interopercle; Lt, labial teeth; Me, mesethmoid; Mx, maxilla; Op, opercle; Pop, preopercle; Psoc, parieto-supraoccipital; PC, posterior ceratohyal; Pmx, premaxilla; Pop, preopercle; Pu, parurohyal; Sph, sphenotic-prootic-pterosphenoid; SPV, second post-Weberian vertebrae; Vh, ventral hypohyal. Scale bar = 0.25 mm.

FIGURE 5 | Suspensorium and opercular series of Rhinotridens chromocaudatus, left side, µCT-scan images, paratype, MZUSP 128217, 16.37 mm SL; A. Lateral view; B. Medial view; arrow indicates distal process of hyomandibula. Hy, hyomandibular; Iop, interopercle; Op, opercle; Pop, preopercle; Qd, quadrate. Scale bar = 0.2 mm.

FIGURE 6 | Pelvic girdle of Rhinotridens chromocaudatus, digital illustration of c&s specimen, ventral view, paratype, MZUSP 128217, 16.72 mm SL. Bpt, basipterygium. Scale bar = 0.15 mm.

FIGURE 7 | Neurocranium and associated structures of Rhinotridens chromocaudatus, µCT-scan images, paratype, MZUSP 128217, 16.37 mm SL; A. Left lateral view; B. Dorsal view; C. Ventral view. Boc, basioccipital; Ep, epioccipital; Fr, frontal; Le, lateral ethmoid; Me, mesethmoid; Osph, orbitosphenoid; Psph, parasphenoid; Psoc, parieto-supraoccipital; Ptscl, postemporo-supracleithrum; Pro, prootic; Pt, pterotic; Sph, sphenoticprootic-pterosphenoid; Vo, vomer; WeC, Weberian capsule. Scale bar = 0.3 mm.

FIGURE 8 | Hyoid arch of Rhinotridens chromocaudatus, µCT-scan images, paratype, MZUSP 128217, 16.37 mm SL;A. Ventral view; B. Dorsal view; anterior to left. AC, anterior ceratohyal; Br, branchiostegal rays; PC, posterior ceratohyal; Pu, parurohyal; Vh, ventral hypohyal. Scale bar = 0.3 mm.

FIGURE 9 | Gill arches of Rhinotridens chromocaudatus, digital illustration of c&s specimen, dorsal view, paratype, MZUSP 128217,16.72 mm SL; right dorsal elements not shown. Bb1-4, basibranchials 1 to 4; Cb1-5, ceratobranchials 1 to 5; Eb1-4, epibranchials 1 to 4; Hb1-3, hypobranchials 1 to 3; Pb4, pharyngobranchial 4; Utp, upper pharyngeal tooth plate. Scale bar = 0.1 mm.

FIGURE 10 | Geographic distribution of Rhinotridens chromocaudatus. White dot indicates the typelocality, each symbol may represent than one lot or locality.

FIGURE 11 | Type-locality of Rhinotridens chromocaudatus; 07°30’37”S 63°20’23”W, rio Ipixuna, rio Purus drainage, rio Amazonas basin, Humaitá Municipality, Amazonas State, Brazil.