Trichomycterus astromycterus, REIS ET AL., 2019

Reis, Vinícius & Pinna, Mário De, 2023, Diversity and systematics of Trichomycterus Valenciennes 1832 (Siluriformes: Trichomycteridae) in the Rio Doce Basin: iterating DNA, phylogeny and classical taxonomy, Zoological Journal of the Linnean Society 197, pp. 344-441 : 374-379

TRICHOMYCTERUS ASTROMYCTERUS REIS ET AL., 2019

( FIG. 14)

Trichomycterus astromycterus Reis, de Pinna & Pessali, 2019: 2 , figs 1–8 [type locality: Rio do Peixe, tributary of Rio Doce Basin , 20°11’40.32”S, 42°51’8.47”W, Rio Doce Municipality , Minas Gerais, Brazil; holotype: MZUSP 124559, paratypes: MZUSP 123341 (1), MZUSP 123361 (13, 1 c&s), MZUSP 124760 (37, 3 c&s)]; Reis, 2018: 14 (morphology, relationships); ETYFish Project, 2020 (etymology); Reis et al., 2020: 2 (diversity); Fernandez et al., 2021: 15 (phylogenetic relationships).

Diagnosis: Autapomorphically diagnosed by the following characters, unique in Trichomycteridae : (1) distally expanded maxilla (Reis et al., 2019: fig. 3) (vs. not expanded); (2) mesethmoid cornua thick and short, shorter than half width of premaxilla (vs. mesethmoid cornua slender and long, equal or longer than half width of premaxilla); and (3) long anterior process of the vomer (vs. short). Slightly asymmetrical caudal fin, with lower lobe longer than upper one further distinguishes the new species from all congeners (vs. with upper and lower lobes equal). In addition, it is distinguished from all congeners from south-eastern South America by the combination of the following traits: (1) eight or nine branched dorsal-fin rays (rarely six or seven) (vs. invariably seven); (2) seven to nine dorsal pterygiophores (vs. invariably eight); (3) narrow caudal peduncle (mean 9.3% vs. 10.1–15.1%); (4) short barbels, not surpassing anterior margin of eyes (vs. long barbels surpassing eyes); and (5) eyes dorsal and protruding (vs. more laterally located and not protruding). Further distinguished from all congeners from the Rio Doce Basin by highly discrepant numbers of dorsal (20–23) and ventral (9–13) proccurent caudal-fin rays (vs. less dissimilar numbers in the two series, 13–25 dorsally, 11–17 ventrally).

Description: Morphometric data for specimens examined is presented in Table 5. Body short and stout, trunk roughly round in cross-section near head, then slightly deeper than wide and abruptly compressed at caudal peduncle, tapering to caudal fin. Dorsal profile of body gently convex to dorsal-fin origin, then straight or slightly convex along caudal peduncle to caudal-fin origin. Ventral profile convex from gular region to vent, due partly to abdominal distension, then straight to anal-fin origin and straight or slightly concave along caudal peduncle to caudal-fin base. Caudal peduncle region markedly less deep than rest of body. Dorsal profile of caudal peduncle including vestigial fin fold or adipose fin, poorly defined and confluent with caudal fin.

Head longer than wide and depressed. Mouth inferior, located distant from anterior margin of snout. Upper jaw markedly longer than lower, with premaxillary dentition almost entirely exposed in ventral view. Upper lip broad, wide and fleshy in ventral view, its ventral surface covered with numerous large papillae. Each papilla formed by cluster of small villi. Lateral portion of upper lip continuous with base of rictal barbel. Lower lip small, approximately half as wide as upper one, median concavity slightly dividing it into right and left portions. Lower lip with uniform coveling of tiny villi, velvet-like in aspect and not clustered into large papillae. Region between upper and lower lips with well-differentiated fleshy lobe, adpressed posteriorly to cheek musculature.

Dentary and premaxillary teeth similar to each other in shape and size. Dentary teeth approximately 45–55 in number, arranged in three irregular rows extending to base of coronoid process, with size of individual teeth increasing markedly towards symphysis and from posterior to anterior rows, with medialmost anterior tooth approximately seven times the length of lateralmost posterior tooth. In cleared and stained and CT-scanned paratypes, dentary teeth of first row mostly narrowly incisiform near symphysis, with remaining teeth conical (Reis et al., 2019: fig. 7). Area of premaxillary teeth smaller than that of dentary, with 30–40 teeth arranged irregularly over entire ventral surface of premaxilla. In cleared and stained, and CT-scanned (in original description, Reis et al., 2019) paratypes, premaxillary teeth near symphysis mostly narrowly incisiform, especially anteriorly, with remaining teeth conical. In considerably larger holotype, most jaw teeth incisiform.

Eyes large, protruding, positioned dorsally on head, without free orbital rim and covered with transparent skin. Eyes located slightly posterior to half of HL, closer to midline than to lateral border of head in dorsal view. Anterior naris surrounded by tube of integument directed anterolaterally, continuous posterolaterally with nasal barbel. Part of integument rim extending along posterior margin of barbel forming conspicuous flap for proximal 25% of barbel. Posterior naris slightly closer together than anterior ones, surrounded by tube of integument incomplete posteriorly. Maxillary barbel extremely wide at base and narrowing markedly towards fine tip, reaching slightly beyond middle of eye when completely extended. Rictal barbel inserted immediately ventral to maxillary barbel, but with base 1/3 as wide, its tip not reaching base of interopercular patch of odontodes when completely extended. Nasal barbel keel-like, originating on posterolateral region of anterior naris, reaching about 60% of distance between its base and anterior margin of eye. Interopercular patch of odontodes small in overall area, slightly larger than eye in lateral aspect, oval in shape and with well-developed odontodes, visible in ventral aspect of head but not conspicuously so. Interopercular patch of odontodes extending from vertical through posterior border of eye anteriorly to slightly to anterior to vertical through pectoral-fin base posteriorly. Odontodes arranged in two irregular series, with those on mesial series much larger than those on lateral one; odontodes gradually larger posteriorly in both series, with those posteriorly on mesial row largest. Interopercular odontodes 23–24. Opercular patch of odontodes on dorsolateral surface of posterior part of head, anterodorsally to pectoral-fin base, roundish in shape and slightly smaller than eye in dorsal aspect of head. Opercular odontodes 11–16, each one sunk in individual slit of integument, progressively larger posteriorly, all with fine tips, with largest ones curved distally and claw-like. Entire patch surrounded by rim of integument.

Pectoral fin large (tip of first ray extending to middle of distance between occiput and pelvic-fin base), gently convex or sinusoidal in distal margin, its base immediately posterior and ventral to opercular patch of odontodes. Pectoral-fin rays I + 7 or I + 8 with counts asymmetrical in some specimens. First pectoral-fin ray (unbranched) longer than all others, prolonged as filament beyond fin margin. Other rays progressively shorter, their tips following continuous line along fin margin. Second ray sometimes longer than posterior ones. Pelvic fin with convex distal profile, its origin slightly posterior to middle of SL and anterior to vertical through dorsal-fin origin, surpassing anal and urogenital openings by approximately half-fin length, but falling short of anal-fin origin also by half-fin length. Pelvic-fin rays I + 4, first ray (unbranched) shorter than others. Long and slender pelvic splint extending for almost half-length of first pelvic-fin ray. Dorsal fin long, its distal margin sinusoidal. Dorsal-fin origin closer to base of caudal-fin than to tip of snout. Dorsal-fin rays ii + II + 6 (1), ii + II + 7 (2), ii + II + 8 (2) or ii + II + 9 (2).Anal fin smaller and shorter than dorsal fin, its distal margin gently convex. Anal-fin origin slightly anterior to vertical through end of dorsal-fin base. Anal-fin rays i + II + 5 (6) or ii + II + 5 (1). Caudal fin bilobed with round corners, with lower lobe longer than upper one and 6 + 7 principal rays. Caudal-fin procurrent rays 20–23 dorsally and 9–13 ventrally, plus one segmented non-principal ray ventrally in some specimens. Adipose fin absent or modified into low integument fold extending between end of dorsal fin and caudal-fin origin. Vertebrae 32(2), 33(3), 34(1) or 35(1). First dorsal-fin pterygiophore immediately anterior to neural spine of 12 th (3), 13 th (2) or 14 th (2) vertebra, first anal-fin pterygiophore immediately anterior to neural spine of 17 th (3), 18 th (2) or 19 th (2) vertebra. Ribs 8 (2), 9 (4) or 10 (1). Branchiostegal rays 7. Dorsal-fin pterygiophores 7 (1), 8 (2) or 9 (4). Anal-fin pterygiophores 6 (7).

Cephalic lateral line canals with simple, non-dendritic tubes ending in single pores. Supraorbital canal mostly in frontal bone. Supraorbital pores invariably present: s1 mesial to nasal-barbel base and autopalatine, s3 elongate, mesial to posterior nostril and anterior to frontal and s6 single or closely set, posteromedial to eye and at midlength of frontal. Infraorbital laterosensory canal incomplete, with four pores, i1 and i3 anteriorly and i10 and i11 posteriorly. Canal extending from sphenotic posteriorly to terminal pore located ventroposteriorly to eye. Infraorbital pores: i1 ventro-lateral to nasal-barbel base and autopalatine, i3 ventrolateral to posterior nostril and anterior to frontal, with its anterior border open, i10 and i11 posterior to eye, and anterolateral to anterior process of sphenotic. Otic canal without pores. Postotic pores po1 (anteromedial to opercular patch of odontodes and lateral to articulation between sphenotic-prootic-pterosphenoid and pterotic) and po2 (medial to opercular patch of odontodes and lateral to articulation between pterotic and posttemporo-supracleithrum). Lateral line of trunk anteriorly continuous with postotic canal and reduced to short, non-ossified tube in all four type specimens. Lateral line pores ll1, ll2 and ll3 dorsomedially to pectoral-fin base and posterior to posttemporo-supracleithrum.

Coloration in ethanol: Dorsum and sides of body with large roundish marks roughly arranged in rows, 12–15 laterally and 7–9 dorsally, progressively smaller and less aligned posteriorly. Narrow scattering of smaller spots ventral to large lateral ones, roughly following interface between myomeres and abdominal cavity and extending posteriorly onto ventral part of caudal peduncle. Conspicuous dark spot dorsal to base of pelvic fin. Abdominal region white.

Overall colour of head similar to, and slightly denser than, that of dorsum. Cheeks and snout with closely set, dark fields. Dorsal part of opercular patch of odontodes surrounded with dark field. Integument amidst opercular odontodes interspersed with dark streaks, contrasting with white odontode slits. Region of head corresponding to skull roof almost continuously dark. Sensory pores on head white. Bases of nostrils extremely dark. Ventral surface of head white, except for sparse irregular melanophores close to angle of mouth and mentum. Dorsal surface of upper lip less dark than rest of head. Ventral part of upper lip and lower lip white or with sparse dark fields. Base of maxillary barbel darky pigmented dorsally, especially posteriorly. Dark fields extending onto basal third of dorsal surface of barbel, with remainder of barbel white on both sides. Rictal barbel mostly white, with few small elongate dark fields near base. Nasal barbel dark along nearly its entire length, on both sides. Base of pectoral fin with dense dark spots on dorsal surface, extending to half of fin as elongate fields along fin rays. Ventral surface of pectoral fin lacking dark pigment. Dorsal and anal fins with heavy dark covering over basal third or half of individual rays, fading distally. Fin web lacking dark chromatophores. Pelvic fins white. Basal 2/3 of caudal-fin rays darkly dashed, with individual dashes vertically aligned and forming pattern of three or four vertical stripes on fin, progressively less pronounced distally. Margin of caudal fin white.

Colour pattern of smallest juvenile (smallest examined specimen 12 mm SL) differing from that of adults in having only six dark spots along midline and by lacking dark maculae on the dorsal part of body.

Remarks: Trichomycterus astromycterus is a remarkable species differing from all trichomycterines in a number of peculiar traits (cf. diagnosis above). Its external morphology superficially resembles species of Microcambeva (Microcambevinae, formerly Sarcoglanidinae), but investigation of its anatomy and COI leave no doubt that it is unrelated to those subgroups but to Trichomycterinae instead. Also, as pointed out in the original description (Reis et al., 2019), T. astromycterus shares some morphological convergences with Bullockia maldonadoi (Eigenmann, 1920) . The numerous and conspicuous autapomorphies of T. astromycterus distinguish it not just from all Trichomycterus in the Rio Doce Basin , but from all other congeners in eastern South America. Other peculiarities are not necessarily autapomorphic, but still stand out as highly unusual. For example, its variation in number of dorsal-fin pterygiophores (seven to nine, modally nine) is anomalous. Species of Trichomycterus otherwise have a constant number of eight such elements. We know of only one species having such numerous dorsal-fin pterygiophores, T. santaeritae , the holotype of which has 11 dorsal-fin pterygiophores (but more information is needed on that species for further comparisons; see remarks on T. alternatus above). Such a high degree of phenotypic divergence might suggest that T. astromycterus represents a separate genus under traditional generic concepts in Trichomycteridae . Nevertheless, no morphological evidence was found that might place it outside of the phylogenetic range currently circumscribed to species of Trichomycterus ( de Pinna, 1998; Wosiacki, 2002; Ochoa et al., 2017, 2020). This placement is here corroborated by COI data, which place the species nested in T. alternatus (see remarks on T. alternatus above, and discussion below). Trichomycterus astromycterus is definitely a taxon requiring more study, especially because of its combination of pronounced phenotypic differentiation (which extends to the morphology of juveniles) associated with no significant COI divergence (see Discussion).

Material of T. astromycterus comes from two different habitats. At the Rio do Peixe locality, adult specimens were found among rocks in an extremely fast-running water sector. At the Rio Santo Antônio locality, specimens were collected deeply buried in sand banks, in sectors with moderate current. Juvenile specimens were only found in the Rio do Peixe locality (16 specimens, 12.0– 20.9 mm SL). They were found buried in sand banks in the margin of the stream in quiet sectors. The smallest specimen examined of T. astromycterus differs from adults in coloration (see description above), longer pectoral fin, reaching origin of pelvic fin, presence of a prominent dorsal fin fold (which remains until 15 mm SL; Fig. 15) and cephalic laterosensory canals mostly open (except the symphysial canal of pore S6, which is already closed, ending in a single pore).

G e o g r a p h i c a l d i s t r i b u t i o n: Tr i c h o m y c t e r u s astromycterus was collected in tributaries of the Upper and Middle Rio Doce Basin. All localities are near the main channel of the Rio Doce ( Fig. 16).

Type material examined: Holotype: MZUSP 124559, 52 mm SL; Brazil, Minas Gerais, Rio Doce , Rio do Peixe River tributary of Piranga River (20°11’40.32”S 42°51’8.47”W); col. V.J.C. Reis, M. de Pinna, G. Ballen & G.F. de Pinna. Paratype: MZUSP 123341, 1, 46.1 mm SL; Brazil, Minas Gerais, Naque Municipality, Rio Santo Antônio River , tributary of Rio Doce Basin (19°14’0.76”S 42°19’26.52”W); col. T.C. Pessali & E.P. Estevam, 1 September 2017. MZUSP 123361, 13, 29.3–38.5 mm SL, 2 c&s, 34.3–35.7 mm SL; Brazil, Minas Gerais, Naque Municipality, Rio Santo Antônio , tributary of Rio Doce Basin (19°14’0.76”S 42°19’26.52”W); col. T.C. Pessali, E.P. Estevam & V.J.C. Reis, 25 October 2017. MZUSP 123760, 37, 11.1– 51.7 mm SL, 2 c&s, 44.1–45.8 mm SL; collected with holotype.