Acestrocephalus pallidus, new species

Fig.14

Holotype. MZUSP 35624, immature female, 60.5 mm SL, Brazil, Amazonas: rio Madeira, ilha do Puruzinho, by Expedição Permanente da Amazônia, 15 December 1975.

Paratypes. MZUSP 89586, 3, 52-60 mm SL, taken with holotype. MZUSP 76623, 2, 50 and 54.5 mm SL, Rondônia: rio Machado, Cururu, by Michael Goulding, 2 September 1980 ; MZUSP 63311, 10, 48.4-62 mm SL, 73471, 30, 40-66 mm SL, 73487, l, 42.7 mm SL, 76627, 3, 41-46 mm SL, rio Machado, Paracaúba, by Michael Goulding, 4 September 1980 .

Diagnosis. Acestrocephalus pallidus is distinguished from A. boehlkei, A. maculosus and A. stigmatus by lacking the dark humeral blotch characteristic of these species. Acestrocephalus pallidus has the eye smaller than A. sardina and A. nigrifasciatus (Fig. 4) and fewer maxillary teeth (30-38 vs 40-46. See Fig. 6). It differs from A. acutus by having only scattered dark chromatophores instead of the typical oblong dark blotch on the caudal-fin base of that species and the first pleural rib, the cavity anterior to it and the obliquus inferioris muscle visible vs entirely covered by the fibers of the obliquus superioris muscle in A. acutus (compare Figs. 5f and 5g).

Description. Morphometrics of holotype and additional specimens presented in Table 14. Meristic and morphometric data based on all examined lots because no statistical differences found among population samples studied. Body relatively small (SL= 40-66 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=60, including holotype. Posterior most ray unbranched, n=60. Adipose fin present. Anal-fin rays iv,33 (iv or v, usually iv unbranched, branched rays mean=33.5, range 32-36, n=60, posterior ray split to its base and counted as 1).Moderately developed anterior anal-fin lobe including anterior unbranched rays and first 10-11branched rays. No hooks on anal-fin rays, but sexual mature males not represented in the samples. Pectoral-fin rays i,13 (anterior unbranched ray i, n=60 branched rays mean=13, range 11-15, n=60). Posterior tips of longest pectoral-fin rays reaching slightly beyond origin of pelvic fin. Pelvic fin i,7, n=60. No hooks on pelvic-fin rays. Distal tips of longest pelvic-fin rays reaching anal-fin origin. Principal caudal-fin ray count 10/9, n=60.

Lateral line complete, perforated scales 72 (mean=71.6, range 68-74, n=60). Scale rows above lateral line 13 (mean=12.2, range 12-13, n=60). Scale rows below lateral line 11 (mean=11, range 10-11, n=60). Scale rows around caudal peduncle 22 (mean=21.5, range 21-23, n=60).

Shape, size and arrangement of teeth on premaxilla, maxilla and dentary as in A. sardina . Outer row small conical teeth on premaxilla 9 (mean=7.8, range 7-10, n=60). Maxillary teeth 31 (mean=33.8, range 30-38, n=60), tending to increase in number with an increase in standard length (Fig.6). Posterior row dentary teeth 34 (mean=32.2, range 29-37, n=60), also tending to increase in number with an increase in standard length (Fig. 7). Inner row dentary teeth 9 (mean=10.8, range 9- 13, n=60).

Vertebrae 38 (mean=38.7, range 38-39, n= 47). Total number of gill-rakers on first gill-arch 7 (mean=6.4, range 6-7, n=60).

Shape of muscular hiatus of pseudotympanum, arrangement of associated muscles and remaining structures (Fig. 5g) as in A. sardina .

Color in alcohol. Body pale yellow with no distinct dark spots or blotches. Scatered dark chromatophores present on dorsal head and snout and along free edge of scales on dorsal trunk being slightly darker than remaining parts of body. Dark chromatophores on caudal-fin base scattered, visible only using stereomicroscope, not forming a blotch. Dark spot at dorsalfin origin weak and dark color on lower jaw mental area sometimes diffuse. Lateral body stripe largely consisting of guanine pigment extending from posterior section dorsal opercle to caudal-fin base. All fins pale.

Distribution. Known only from rio Madeira and rio Machado, a tributary of rio Madeira (Fig. 8).

Etymology. The species name pallidus, adjective, from the Latin for pale is in reference to the overall body color of this species.