Acestrocephalus acutus, Menezes, 2006

Menezes, Naércio A., 2006, Description of five new species of Acestrocephalus Eigenmann and redescription of A. sardina and A. boehlkei (Characiformes: Characidae), Neotropical Ichthyology 4 (4), pp. 385-400 : 397-398

Acestrocephalus acutus , new species

Fig.13

Holotype. MZUSP 31650 , mature female, 80 mm SL, Brazil, Pará: Caldeirão, igarapé do Cinzento, rio Itacaiúnas , by Michael Goulding , November , 1983.

Paratypes. MZUSP 40830 , 3 , 73.5 -101.0 mm SL, Brazil, Tocantins: Arraias, rio Bezerra near bridge on road GO-112, rio Tocantins drainage, by José C. Oliveira, 8 January 1989 ; MZUSP 82105 , 3 , 50-68 mm SL, Porto Alegre do Tocantins: rio Manuel Alves da Natividade , rio Tocantins drainage, by Cristiano R. Moreira, J.C. Nolasco , and M. Ávila, 20 July 2002 , MZUSP 40691 , 6 , 51.5-75 mm SL, Goiás: Monte Alegre de Goiás, rio Bezerra , right bank tributary of rio Paraná, rio Tocantins drainage, by José C. Oliveira and Wilson, J.M. Costa, 22 September 1989 ; MCP 15855, 1 , 85 mm SL, Niquelândia : ribeirão da Laje, rio Tocantins drainage, by R. E. Reis, J. Pezzi, E. Pereira and L.A. Porto, 17 July 1992 ; MNRJ 28716 , 1 , 83 mm SL, Niquelândia: rio do Peixe , right bank tributary of rio Maranhão, rio Tocantins drainage, 14º28’S, 48º45’W, by Gustavo Nunan and D.F. Moraes, 08 October 1985 ; MNRJ 17612 , 28 , 39.4-98 mm SL, 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 alii, 1996 ; MNRJ 28717 , 1, 103.5 mm SL, Minaçu, Cavalcante: rio Tocantins in Serra da Mesa , 14º40’9"S, 52º21’12"W, by P. Buckup, A. Aranda, F. Silva and C. Figueiredo, 29 January 2002 ; MNRJ 18315 , 2 , 28.5 and 72.0 mm SL, Colinas do Sul : pools in left bank of rio Tocantins above Ponte Rubião, by D.F. Moraes, D.A. Halboth, O . T. Oyakawa et al., 28 October 1996 ; MNRJ 21136 , 1 , 78 mm SL, Formosa: tributaries of rio Paraná , north of Formosa, 15º28’52"S, 47º20’33"W, by F.A.G. Melo, P.A. Buckup and M . R.S. Melo , 21 November 2000 .

Non type material. MZUSP 63009 , 4 , 87-96.5 mm SL, Brazil, Mato Grosso: Paranatinga, rio Teles Pires , fazenda do Antônio Bezerra, 14º26’S, 54º36’W, November 1997 ; MCP 30 485, 2, 76 and 82 mm SL, Nova Canaã do Norte: rio Tels Pires on road between Nova Canaã do Norte and Alta Floresta, 10º14’17"S, 55º48’28"W, 22 January 2002 ; MCP 30481, 1 , 39.3 mm SL, Tabaporã: rio Teles Pires on road MT-220 between Sinop and Porto dos Gaúchos, 11º39’15"S, 55º39’18"W, 20 January 2002 .

Diagnosis. Among the species of Acestrocephalus without dark humeral blotch, A. acutus can be distinguished from A. sardina and A. nigrifasciatus by having a smaller eye (28.0- 31.7 vs 32.6-36.5 in A. nigrifasciatus and 31.3-37.4 in A. sardina ; compare tables 3, 12 and 13) and an oblong dark blotch at caudal base instead of the triangular dark blotch present on the caudal base of those two species (compare Figs. 3 , 9 , and 12). It differs from A. pallidus by the presence of the oblong dark blotch vs absence of dark blotch at caudal base. The shape of the muscular hiatus of the pseudotympanum and the muscular arrangement around it in A. acutus and A. pallidus are also different (compare figs. 5f and 5g). Acestrocephalus acutus differs from A. anomalus by having more teeth in the posterior row on dentary (28-36 vs 23-28) and fewer teeth in the external tooth row on premaxilla (6-8 vs 8-11).

Description. Morphometrics of holotype and additional specimens presented in Table 13. Meristic and morphometric data based on all lots in type series. No statistical differences found among population samples studied. Body relatively large (SL= 50-103.5 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=47, including holotype. Posterior most ray unbranched, n=47. Adipose fin present. Anal-fin rays iv,31 (iv or v, usually iv unbranched, branched rays mean=32, range 29-35, n=47, posterior ray split to its base and counted as 1). Modestly developed anterior anal-fin lobe including anterior unbranched rays and first 8-9 branched rays. Three sexually mature males with bilateral hooks on posterior anterior branched rays. One sexually mature male (MCP 15855, 85 mm SL) has hooks on first 14 branched anal-fin rays, number of hooks per ray increasing from first (5) to sixth (14) then decreasing from seventh (12) through tenth (5), 13th (3) and 14th (1). Pectoral-fin rays i,12 (anterior unbranched ray i, n=47, branched rays mean 12.7, range 11-14, n=47). Posterior tips of longest pectoral-fin rays reaching to about one third of pelvic-fin length. Pelvic-fin rays i,7, n=47. No pelvic-fin ray hooks on sexually mature males. Distal tips of longest pelvic-fin rays reaching anterior border of anus. Principal caudal-fin ray count 10/9, n=47.

Lateral line complete, perforated scales 74 (mean=73.8, range 70-77, n=46). Scale rows above lateral line 13 (mean=12.9, range 12-14, n=46). Scale rows below lateral line 11 (mean=11.4, range 10-14, n = 46). Scale rows around caudal peduncle 23 (mean=22.8, range 22-24, n=46).

Shape, size and arrangement of teeth on premaxilla, maxilla and dentary as in A. sardina . Outer row small conical teeth on premaxilla 9 (mean=8.3, range 7-10, n=47). Maxillary teeth 36 (mean=33.6, range 28-38, n=47). Fig. 6 suggests a tendency for the number of maxillary teeth to increase in number according to an increase in standard length. Posterior row dentary teeth 33 (mean=30.5, range 28-36, n=47), tending to increase in number with an increase in standard length ( Fig.7). Inner row dentary teeth 11 (mean=11.2, range 10-13, n=47).

Vertebrae 38 (mean=38.7, range 37-39, n=28). Total number of gill-rakers on first gill-arch 7 (mean=6.6, range 6-8, n= 47).

Branchiostegal rays 4; 3 originating on anterior and one on posterior ceratohyal.

Muscular hiatus of pseudotympanum reduced, constricted by dorsal and ventral fibers of obliquus superioris muscle ( Fig. 5f ). Cavity anterior to first pleural rib, this as well as obliquus inferioris muscle not visible. Second pleural rib partially exposed.

Color in alcohol. Body pale yellow slightly darker dorsally due to presence of dark chromatophores along free edge of scales, lighter ventrally. Dorsal part of head and snout more densely pigmented with dark chromatophores. Lateral head and opercular bones with scattered dark chromatophores. Anterior upper edge of lower jaw also heavily pigmented with dark chromatophores. Dark lateral body stripe extending from

Characters Holotype N Range Mean SD Standard length 80.0 47 50.0-103.5

Body depth 26.6 41 23.5-28.6 25.3 1.1 Snout to dorsal-fin origin 48.7 47 48.7-54.2 51.4 1.0 Snout to pectoral-fin origin 30.7 47 27.6-31.5 29.7 0.9 Snout to pelvic-fin origin 43.2 47 41.0-45.4 42.5 1.1 Snout to anal-fin origin 59.0 47 56.1-61.4 58.0 1.3 Caudal peduncle depth 8.1 47 7.5-8.8 8.1 0.3 Caudal peduncle length 8.7 47 8.1-11.0 9.4 0.5 Pectoral-fin length 17.7 47 14.7-18.3 16.4 0.7 Pelvic-fin length 15.0 47 12.6-15.5 13.7 0.8 Dorsal-fin base length 11.2 47 11.0-12.8 11.7 0.4 Dorsal-fin height 25.3 47 21.2-26.6 24.2 1.1 Anal-fin base length 35.6 47 32.7-37.6 35.4 1.0 Anal-fin height 17.5 47 14.0-18.6 15.7 1.3 Eye to dorsal-fin origin 34.1 47 34.1-37.8 36.2 1.0 Dorsal-fin origin to caudal-fin base 51.5 47 48.6-53.2 51.0 1.0 Head length 29.7 47 29.0-33.3 30.4 0.8 Horizontal eye diameter 31.1 47 28.0-31.7 29.4 0.8 Snout length 30.2 47 27.1-30.4 28.6 0.7 Least interorbital distance 18.0 47 16.0-19.7 17.6 0.7

behind upper opercle to caudal peduncle, wider from below dorsal-fin origin to origin of adipose fin, narrower from below dorsal-fin origin anteriorly and on caudal peduncle. Oblong dark blotch on anterior caudal base joining dark lateral stripe, extending posteriorly to bases of middle caudal-fin rays. Mental area of lower jaw with conspicuous dark blotch in some specimens. Black spot at origin of dorsal fin very conspicuous, anteriorly extending over two or three predorsal scales, laterally over three or four scales of first horizontal scale row. All fins pale, first unbranched dorsal and pectoral fin rays with scattered dark chromatophores.

Distribution. Acestrocephalus acutus is known from the rio Tocantins and rio Tapajós drainages. It is sympatric with A. maculosus and A. stigmatus in the upper rio Tocantins ( Fig. 8 ).

Remarks. The specimens from rio Teles Pires in the rio Tapajós drainage, not included as type material, are morphologically undistiguinshable from the holotype and paratypes, but examination of more specimens is required for better comparison.

Etymology. The species name acutus , adjective, from Latin meaning sharp is in reference to the pointed nature of the snout of this species.

Figures

Fig. 13. Acestrocephalus acutus, MZUSP 31650, holotype, mature female, 80 mm SL, igarapé do Cinzento, rio Itacaiunas, rio Tocantins drainage.

Fig. 3. Acestrocephalus sardina, MZUSP 81209, 93 mm SL, immature female, rio Tiquié, rio Negro drainage.

Fig. 9. Acestrocephalus boehlkei, MZUSP 12993, paratype, 85 mm SL, mature male, Río Punino, Río Napo drainage.

Fig. 6. Scatter plot of number of maxillary teeth on standard length for specimens of Acestrocephalus sardina, A. boehlkei. Acestrocephalus nigrifasciatus, A. pallidus, and A. acutus. The figure is not intended to show species differences therefore regression lines were not drawn and regression analyses were not performed. The primary purpose is to express the tendencies of the number of teeth on the maxilla to increase according to an increase in standard length. For the species represented by large number of specimens the data points are widely scattered.

Fig. 5. Schematic drawings of external view of muscles covering the anterior portion of the swim bladder inA – Acestrocephalus sardina (MZUSP 29241); B – A. boehlkei (MZUSP 38699); C – A.maculosus (MNRJ 12657); D – A. stigmatus (MZUSP 10422); E – A. nigrifasciatus (MCP 30420); F – A. acutus (MNRJ 17612); G – A. pallidus (MZUSP 73471).Anterior towards left; 1 plr (first pleural rib); 2 plr (second pleural rib); os (obliquus superioris); ls (lateralis superficialis); oi (obliquus inferioris). Not drawn to scale.

Fig. 8. Map of northern and central South America showing the geographic distribution of the species of Acestrocephalus. Some symbols may represent more than one locality.