Pachyurus stewarti, Lilian Casatti & Ning Labbish Chao, 2002

Lilian Casatti & Ning Labbish Chao, 2002, A new species of Pachyurus Agassiz 1831 (Teleostei: Perciformes: Sciaenidae) from the Río Napo basin, Eastern Ecuador., Zootaxa 38, pp. 1-7 : 2-6

Pachyurus stewarti n. sp.

(Fig. 1, Table 1)

Holotype. - FMNH 101957 (191.7 mm SL), Ecuador, Sucumbios, Rio Aguarico, in the confluence with Rio Pushino , 0º2.6’N 76º54.4’W, collected by D. Stewart, 18 September 1983 .

Paratypes. All localities in Ecuador, collected by D. Stewart. FMNH 94410 (2, 100.0- 115.0 mm SL), Napo, Rio Tiputini at mouth in Rio Napo , 48º9’S 32.5’W, 21 June 1982; FMNH 94416 (149.1 mm SL), Napo, Rio Napo , 0º32.2’S 77º3.5’W, 25 September 1981; FMNH 101956 (116.2 mm SL), Sucumbios, Rio Aguarico, above the Rio Shushufindi , 0º17.0’S 76º25.5’W, 24 November 1983 .

Diagnosis. A species of Pachyurus with a unique coloration pattern of about twelve brown vertical bars and ellipsoid blotches on the side of trunk and a thick horizontal stripe from the middle of the caudal peduncle to the front of the caudal-fin base. Additional comparisons among Pachyurus species found in the Rio Amazonas basin are given in Table 2.

Description. Morphometric and meristic data presented in Table 1. Body elongate, dorsal profile moderately arched, ventral profile nearly straight from prepelvic region to anal-fin origin. Maximum body depth at dorsal-fin origin. Head low, pointed; mouth inferior and horizontal (gape width 5.2-5.6 in head length). Maxilla extending beyond vertical through anterior edge of eye. Underside of lower jaw with 5 mental pores, 1 median and 2 posterior pairs. Teeth uniformly small, villiform, set in band on jaws.

Snout pointed, longer than diameter of eye; its lower edge with three small rostral pores and five marginal pores; nostrils closely set, anterior circular and posterior crescentshaped. Eye elliptical, horizontal diameter longer than vertical height. Laterosensory canal segments in infraorbitals, preopercle, and lower jaw visible externally. Preopercle margin slightly serrate with 7-9 short spines. Posterior margin of postemporal bone covered with small ctenoid scales, having form of bony flap above dorsal limit of gill slit. Gill rakers short and slender; 5-7+11-12=16-19.

Dorsal fins: IX-X + I-28-29 (usually X + I-29), first spine very small (less than 1/5 length of second spine); longest spine falling short of base of second dorsal fin spine when depressed; notch present between first and second dorsal fins. Pectoral fin: I + 18-19, falcate, its tip falling sligtly short of vertical through tip of pelvic fin. Pelvic fin: I + 5, first soft ray prolonged as filament, its tip close to anus in smaller specimens, more distant from that point in larger specimens. Anal fin: II + 7, truncate, first spine reduced, approximately equal in length to first dorsal fin spine; second spine rigid, long, and as long as first soft ray. Principal caudal-fin rays: i+9-8+i; caudal fin rhomboidal, with upper lobe slightly convex and lower lobe concave.

Scales usually ctenoid (cycloid on snout, lachrymal, second infraorbital, and preopercle). Anterior half of lateral line curved dorsally, posterior half running straight to posterior margin of caudal fin; 53-56 pored lateral-line scales from supracleithrum to hypural joint; 9-11 scales rows above lateral line; 9-10 (usually 10) scales rows below lateral line. Bases of second dorsal and anal fins covered by 1 or 2 rows of small ctenoid scales. Caudal fin almost completely covered by scales.

Gas bladder spindle-shaped, fusiform, without appendages; drumming muscles absent in juvenile ( FMNH 94410, 100 mm SL). Peritoneal membrane pale. Sagitta ovoid, moderately thick, with a tadpole shaped sulcus on inner surface with a pear-shaped head portion (ostium) and a long J-shaped tail (cauda) not deeply grooved. Vertebrae 10+15=25.

Color in alcohol. Head and dorsal portion of body tan, ventral region light tan (Fig. 1). Inside surface of opercle with brownish chomatophores, also visible externally as dark blotch. Trunk with about twelve brown, vertical bars and ellipsoid blotches in the region from the rear of opercle to caudal peduncle; posterior blotches often merged along lateral line. Longitudinal stripe sometimes present under soft dorsal-fin base and extending to dorsal edge of caudal peduncle. Tip of spinous dorsal fin dusky; soft dorsal fin with slightly darker margin, membranes between soft rays dotted with brown chromatophores and forming 2 or 3 longitudinal bands on fin. Pectoral, pelvic, anal, and caudal fins yellowish-pale.

Distribution. Only known from the Río Napo and Río Aguarico, eastern Ecuador, in the western portion of the Rio Amazonas basin.

Etymology. Named in honor of Dr. Donald Stewart (formerly at FMNH) for his contributions to the knowledge of Neotropical fishes.

Remarks. Our decision to assign this new species to the genus Pachyurus is due to its lack of the appendages on the gas bladder together with the absence of mental barbels. Pachyurus is also characterized by having a lachrymal with a thick dorsal margin (Casatti 2001), a feature not examined herein because of the few specimens available for dissection.

The specimens of Pachyurus stewarti were collected during expeditions to the Río Napo basin, Ecuador, in late 1981 and 1983 (Stewart et al. 1987) and are the Pachyurinae known from this drainage, despite the presence of five Pachyurus species elsewhere in the Rio Amazonas basin ( P. schomburgkii Guenther , P. junki Soares & Casatti , P. gabrielensis Casatti , and P. calhamazon Casatti , cf. Casatti 2001). Although some morphometric and meristic features of P. stewarti overlap those found in other Pachyurus species, the color pattern of the new species is a striking diagnostic character that unequivocally distinguishes it from its congeners (see Table 2).