Acestrocephalus stigmatus, new species

Fig. 11

Acestrocephalus sardina; Menezes, 1976: 39 (in part, only specimen from rio das Mortes, MZUSP 10422).

Holotype. MNRJ 28718 (80 mm SL), mature female, Brazil, Mato Grosso: Nova Xavantina, rio das Mortes, rio Araguaia basin, 14º40’9"S, 52º21’12"W, by P.A. Buckup, A.Aranda, F. Silva and C. Figueiredo, 29 January 2002.

Paratypes. MNRJ 24997, 1, 82 mm SL, taken with holotype. MNRJ 17610, 2, 78.5 and 93 mm SL, Goiás: Colinas do Sul, pools in rio Tocantins below reservoir of Serra da Mesa, 13º49’S, 48º11’W, by D.F. Moraes, D.A. Halboth, O.T. Oyakawa et al.,1996. MZUSP 10422, 98 mm SL, Mato Grosso: rio das Mortes, Xavantina, 14º40’S, 52º21’W, by RS/RGS Xavantina Expedition, March 1974; MZUSP 62846, 1, 92.5 mm SL, Nova Xavantina: rio das Mortes in Capitariguara, south of Nova Xavantina, 14º30’22"S, 51º51’89", by C.H. Melo, A.M. Batistella and H.A. Silva, October, 1997.

Non-type material. MZUSP 89811, 1, 73.4 mm SL, Brazil, Mato Grosso: Paranatinga, rio Culuene, rio Xingu drainage, 13º49’00"S, 53º15’00"W; MZUSP 88699, 1, 59 mm SL, Gaúcha do Norte: rio Curisevo, rio Xingu drainage, 13º12’58"S, 53º29’53"W.

Diagnosis. Acestrocephalus stigmatus can be distinguished from all congeners except A. boehlkei and A. maculosus by the presence of a dark humeral blotch. From A. boehlkei it differs in having fewer anal-fin rays (29-31 vs 34-36) and fewer scale rows around caudal peduncle (22-23 vs 25). It has more anal-fin rays (29-31) than A. maculosus (25-27). The muscular hiatus of the pseudotympanum in A. stigmatus (Fig. 5 d) is narrower than that of A. boehlkei, but much more developed than that of A. maculosus (Fig. 5c).

Description. Morphometrics of the holotype and additional specimens presented in Table 11. Meristic and morphometric data based on two lots from two widely separate localities, however no statistical differences were found between samples. Body moderately small (SL= 78.5-98 mm). Body form, dorsal and ventral body profiles, shape of snout and mouth and extension of maxilla as in A. sardina.

Dorsal-fin rays ii, 9 in all specimens, n=6, including holotype. Posterior most ray unbranched, n=6. Adipose fin present.Anal-fin rays iv,30 (unbranched rays iv, n=6, branched rays mean=30, range 29-31, n=6, posterior ray split to its base and counted as 1). Moderately developed anterior anal-fin lobe including anterior unbranched rays and first 7-8 branched rays. Two sexually mature males (MNRJ 17610) with bilateral hooks on posterior anterior branched rays. One specimen (93 mm SL) has 10 hooks on second branched ray, 9 on third, 7 on fourth and 2 on fifth. Pectoral-fin rays i,13 (anterior unbranched ray i, n=6, branched rays mean=13.3, range 13-14, n=6). Posterior tips of longest pectoral-fin rays reaching to about one third of pelvic fin length. Pelvic-fin rays i,7, n=6. No hooks on pelvic-fin rays of sexually mature males. Distal tips of longest pelvic-fin rays reaching posterior border of anus. Principal caudal-fin ray count 10/9, n=6.

Lateral line complete, perforated scales 70 (mean=72.1, range 70-74, n=6). Scale rows above lateral line 13 (mean=13.3, range 13-14, n=6). Scale rows below lateral line 11 (mean=11.5, range 11-12, n=6). Scale rows around caudal peduncle 22 (mean=22.5, range 22-23, n=6).

Shape size and arrangement of teeth on premaxilla, maxilla and dentary as in A. sardina. Outer row small conical teeth on premaxilla 8 (mean=7.6, range 7-9, n=6). Maxillary teeth 39 (mean=39.6, range 39-40, n=6). Posterior row dentary teeth 35 (mean=35.6, range 33-40, n=6). Inner row dentary teeth 10 (mean=10.5, range 9-12, n=6.

Vertebrae 38 (mean=38.4, range 38-39, n=6). Total number of developed gill-rakers on first gill-arch 6 (mean= 5.6, range 5-6, n=6).

Muscular hiatus of pseudotympanum longer than deep, but arrangement of associated muscles without major modifications (Fig. 5d). First and second pleural ribs slightly less exposed than in A. sardina (Compare figs. 5d and 5a) and obliquus inferioris muscle visible.

Color in alcohol. Body pale yellow darker dorsally especially on head and snout. Premaxilla and anterior maxilla densely pigmented with dark chromatophores. Dark chromatophores also concentrated along lower jaw upper edge extending onto bases of anterior canines. Free edges of scales on dorsolateral trunk bordered with row of dark chromatophores. Dark blotch at humeral region irregularly shaped, deeper than long. Lateral body stripe represented by thin layer of dark chromatophores, unconspicuous in some specimens sometimes obscured by guanine pigment. Lateral body stripe extending from behind dorsal opercle across humeral dark blotch to caudal peduncle. Oblong dark blotch encompassing posterior caudal peduncle and caudal-fin base, extending to bases of middle caudal-fin rays. Mental area of lower jaw with diffuse dark patch of dark chromatophores. Dorsal-fin origin with black spot extending onto base of first unbranched ray. All fins pale with very few chromatophores except dorsal and pectoral fins where dark chromatophores are mostly concentrated along proximal half of first unbranched ray (pectoral) or along entire length of first two unbranched rays (dorsal).

Distribution. Specimens of this species are known from the rio Tocantins, rio das Mortes, a tributary of rio Araguaia (Fig. 8) and from the rio Xingu. It is sympatric with A. maculosus and A. acutus in the upper Tocantins basin.

Remarks. The two specimens from the rio Xingu drainage, not included in the type series did not show significant differences with respect to the type series, but additional specimens are needed for a more thorough evaluation.

Etymology. The species name stigmatus, adjective, is derived from the Greek word “stigma” meaning mark, spot, in reference to the dark spot found at the humeral region of this species.