Cetopsidium minutum (Eigenmann, 1912)

Vari, Richard P., Ferraris Jr, Carl J. & de Pinna, Mário C. C., 2005, The Neotropical whale catfishes (Siluriformes: Cetopsidae: Cetopsinae), a revisionary study, Neotropical Ichthyology 3 (2), pp. 127-238 : 136-138

Cetopsidium minutum (Eigenmann, 1912) Figs. 2 , 3 , Tables 2-8

Hemicetopsis minutus Eigenmann, 1912a: 211 , pl. 23, fig. 2 [type locality: British Guiana (= Guyana), Amatuk Cataract].– Henn, 1928: 81 [holotype depository].–Ibarra & Stewart, 1997: 44 [holotype depository].– Burgess, 1989: 292 [ Guyana].–[Not Mago-Leccia, 1970: 82; Fernández- Yépez, 1972: 19, fig. on page 20; Lasso et al., 1990: 117; Taphorn et al., 1997: 85]

Pseudocetopsis minutus .–Evers & Seidel, 2002: 741 [listing].

Pseudocetopsis minuta . –Vari & Ferraris, 2003: 250 [in check list; distribution, in part, only Guyana].

Diagnosis. Cetopsidium minutum differs from all other species of Cetopsidium in the length of the pelvic fin (tip of fin reaching posteriorly to the anal-fin origin versus only reaching or falling short of the vent, respectively). Cetopsidium minutum differs from all other species of Cetopsidium with the exception of C. pemon in the length of the pectoral fin (tip of fin reaching to the vertical through the pelvic-fin insertion versus falling distinctly short of that line, respectively). Cetopsidium minutum further differs from C. orientale and C. roae in the relative alignment of the dorsal and ventral profiles of the portion of the body posterior of the terminus of the base of the dorsal fin (converging posteriorly versus running in parallel, respectively). Cetopsidium minutum also differs from C. roae in the relative position of the vent (located proximate to the base of the anterior most anal-fin ray versus located distinctly anterior to the base of the anterior most anal-fin ray, respectively).

Description. Body elongate, barely compressed laterally anteriorly and becoming progressively distinctly-compressed posteriorly. Body depth at dorsal-fin origin approximately 0.17- 0.18 of SL, and decreasing slightly from dorsal-fin origin to caudal peduncle and approximately equal to, or slightly greater than, distance from anterior margin of orbit to posterior margin of opercle. Lateral line on body incomplete, unbranched, and midlateral; extending from vertical through pectoral-fin base posteriorly to region dorsal of base of anal fin but falling short of caudal peduncle. Dorsal profile of body straight to slightly convex from nape to dorsal-fin origin and straight from dorsal-fin origin to caudal-fin base. Ventral profile of body slightly convex along abdomen, approximately straight and slightly converging with dorsal profile of body posteriorly. Caudal-peduncle depth approximately equal to caudalpeduncle length.

Head in profile acutely triangular overall with bluntlypointed snout. Dorsal profile of head slightly convex from tip of snout to nape. Ventral profile of head slightly convex, more so anteriorly. Margin of snout in dorsal view rounded. Postorbital margins of head slightly convex on each side from dorsal view. Enlarged jaw musculature very evident externally on dorsal and lateral surfaces of postorbital portion of head.

Opercular membrane attaching to isthmus only anterior of vertical through pectoral-fin insertion. Opercular opening moderately-elongate; extending anteroventral of pectoral-fin insertion by distance approximately equal to one-half of HL and extending dorsal of pectoral-fin insertion by distance equal to diameter of eye.

Eye situated on lateral surface of head; located entirely dorsal to horizontal extending through pectoral-fin insertion; eye visible in dorsal view, but not in ventral view, of head. Middle of orbit at approximately anterior 0.29-0.30 of HL. Eye diameter approximately one-third of snout length. Interorbital width approximately equal to distance from tip of snout to slightly beyond posterior margin of orbit.Anterior narial opening circular, surrounded by short, anteriorly-directed, tubular rim of skin. Opening of anterior nares located slightly dorsal of horizontal extending through maxillary-barbel origin and at, or slightly ventral of, horizontal running through tip of snout. Distance between anterior nares approximately equal to distance from tip of snout to posterior margin of orbit. Posterior narial opening located on dorsal surface of head, situated dorsal to anterior one-third of orbit. Narial opening nearly round and with flap of skin extending along anterior one-half of rim of aperture, with flap highest anteriorly.

Mouth subterminal, its width approximately two-thirds of HL. Margin of lower jaw gently rounded, its posterior limit reaching to vertical through middle of orbit. Premaxillary tooth patch in form of gently-arched band, continuous across midline and with anterior margin convex and posterior margin concave and running in parallel to anterior margin. Teeth on premaxilla small, conical, and sharply-pointed. Premaxillary teeth arranged in two irregular rows of uniform sized teeth medially that taper to one row laterally. Vomerine teeth arranged in single, arched row with distinct gap in tooth series along midline. Vomerine teeth stout and conical, and either of uniform size or becoming progressively slightly larger laterally, but with even smallest teeth in series larger than teeth on premaxilla. Dentary teeth comparable in shape to, but larger in size than, premaxillary teeth and arranged in two or three, irregular rows medially that taper to one row laterally.

Maxillary barbel slender, its length slightly greater than distance from anterior margin of orbit to posterior margin of opercle, and slightly greater than two-thirds of HL; barbel origin located ventral to anterior margin of orbit. Medial mental barbel slightly shorter than lateral mental barbel, with latter shorter than maxillary barbel. Medial mental-barbel origin located along vertical through rictus. Lateral mental-barbel origin situated slightly posterior of vertical through medial mental-barbel origin. Tips of adpressed mental barbels extending to, or slightly past, posterior margin of opercle.

Dorsal fin moderately large overall with length of dorsalfin base approximately 0.39-0.41 of HL. Length of longest branched dorsal-fin ray equal to approximately two-thirds of HL. Dorsal-fin spinelet present. First dorsal-fin ray spinous for basal one-half of length and flexible more distally, and with distal filament present in at least some specimens (see “Remarks”). Distal margin of dorsal fin convex, with second branched ray longest. Dorsal-fin origin located at approximately anterior 0.30-0.32 of SL and along vertical extending through middle of adpressed pectoral fin. Tip of adpressed dorsal fin falling short of vertical through vent. Posterior most dorsal-fin ray with slight, basal, posterior, membranous attachment to body.

Caudal fin deeply-forked, symmetrical; tips of lobes bluntly pointed. Length of longest caudal-fin ray approximately two times length of middle fin rays.

Base of anal fin moderately long. Anal-fin origin located somewhat posterior of middle of SL but falling short of middle of TL. Posterior most anal-fin ray with slight, membranous attachment to body.

Pelvic fin moderate sized; distal margin slightly convex with middle fin rays longest. Pelvic-fin insertion located anterior to middle of SL and along vertical passing through posterior terminus of base of dorsal fin. Tip of adpressed pelvic fin extending past middle of SL but falling short of posterior margin of vent. Medial most pelvic-fin ray with membranous attachment to body along basal one-half of its length.

Pectoral-fin length approximately two-thirds of HL. Pectoral-fin margin distinctly convex with middle ray longest. First pectoral-fin ray spinous with smooth margins; spinous portion of ray short, with length slightly more than one-half that of first branched ray.

Coloration in alcohol. Overall coloration of head and body pale to dusky, slightly darker dorsally. Abdomen pale. Body with small, scattered, rounded, dark spots on dorsal, dorsolateral, and lateral surfaces; spots extending further ventrally on posterior portion of body and somewhat more concentrated on anterolateral surface of body. Ventral surface of body pale. Head with scattered, dark spots on snout, upper lip, and dorsal surface. Spots more concentrated in region posterodorsal to orbit and with few, scattered, dark spots laterally. Ventral portions of head pale other than for few, scattered, small, dark chromatophores on ventral surface of lower jaw.

Dorsal fin pale except for scattered, dark pigmentation along base of fin. Anal fin pale. Caudal fin with few scattered, dark spots basally and remainder of fin pale. Pelvic and pectoral fins pale.

Maxillary barbel with few spots of dark, basal, pigmentation and otherwise pale. Mental barbels pale.

Sexual dimorphism. No sexual dimorphism was observed in the two examined specimens of Cetopsidium minutum .

Distribution. Cetopsidium minutum occurs in the Potaro River and middle portion of the Essequibo River of Guyana ( Fig. 2 ).

Remarks. Despite the repeated citations of Cetopsidium minutum in various genera from numerous localities over a broad expanse ranging from the Guianas to adjoining regions, our analysis indicates that only two of the specimens examined in this study are actually that species, both of which originated in the Essequibo River system of Guiana.

The report of Hemicetopsis minutus (= Cetopsidium minutum herein) from the central portions of the río Orinoco basin by Fernández-Yépez (1972: 19, fig. on page 20) is apparently based on specimens of Cetopsidium morenoi of smaller body sizes. Pseudocetopsis minuta was reported by Ferreira (1995: 52, table 3) from the rio Trombetas in the rio Amazonas basin, Pará, Brazil. The material that served as the basis of that record has proved to be an undescribed species, Cetopsidium ferreirai , that is described in this paper. Records of Pseudocetopsis cf. minutus by LeBail et al. (2000: 146) from localities in the Fleuve Maroni, Fleuve Iracoubo, Fleuve Comté, Fleuve Oyapock of French Guiana were based on material that is rather Cetopsidium orientale (Vari et al., 2003: 693) .

Mago-Leccia (1970: 82) and Taphorn et al. (1997: 85) reported Hemicetopsis minutus (= Cetopsidium minutum herein) for Venezuela, but our studies failed to demonstrate the occurrence of that species within that country. Although the upper portions of the río Cuyuni that drains eastern Venezuela is a part of the broader Essequibo River basin in which Cetopsidium minutum occurs, that species has not been reported from the Venezuelan portion of the río Cuyuni ( Machado-Allison et al., 2000, table 1).

Material examined. 2 specimens (18-21 mm SL). Guyana. Amatuk (5°18’N, 59°18’W), FMNH 53262 , 1 (18, holotype of Hemicetopsis minutus ) . Siparuni VIII-2: Essequibo River at Essequibo campsite (04°45’41"N, 58°45’53"W), ANSP 175839 , 1 (21).

Figures

Fig. 2. Map of central and northern South America showing geographic distribution of Cetopsidium ferreirai (1 = holotype locality), Cetopsidium minutum (2 = holotype locality of Hemicetopsis minutus), Cetopsidium morenoi, Cetopsidium orientale (3 = holotype locality of Pseudocetopsis orientale), Cetopsidium pemon (4 = holotype locality), and Cetopsidium roae (5 = holotype locality) (some symbols represent more that one locality and/or lot of specimens).

Fig. 3. Cetopsidium minutum, ANSP 175839, 21 mm SL; Guyana, Siparuni VIII-2, Essequibo River at Essequibo campsite (4°45’41"N, 58°45’53"W).