ABSTRACT
Opisthoproctids are highly specialized, relatively rare fishes from oceanic mid-waters that have an elongate or tubular snout, small terminal mouth, and enlarged pectoral or pelvic fins. The pelvic fins are placed near mid-body and most taxa have tubular, dorsally-directed eyes. Opisthoproctids have a general tendency for delayed metamorphosis and paedomorphosis (Moser 1996d). Larvae of slender-bodied Opisthoproctids have an elongate gut while deeper-bodied taxa have a relatively shorter gut. All Opisthoproctids have a sac-like stomach that exits through a constricted pylorus to the intestine (Ahlstrom et al. 1984). Deeper-bodied Opisthoproctids have a relatively larger head with a pronounced hump or bend at the nape (Ahlstrom et al. 1984). Opisthoproctids are easily recognized by their well-developed tubular eyes and precocious pelvic fins (Moser 1996d), and distinctive pigmentation patterns unique to each genus (Ahlstrom et al. 1984). Seven species are found in our area (Carter & Hartel 2003d) and their meristic characters are given in Table Argentinoidei 2. Illustrations of early stages are shown in Figure Opisthoproctidae 1.
