ABSTRACT
As currently understood, the sub-order Argentinoidei generally includes three families, the Argentinidae, Microstomatidae, and Opistho-proctidae with the Bathylagidae as a subfamily of the Microstomatidae following Johnson & Patterson (1997). The two microstomatid sub-families (Microstomatinae and Bathylaginae) are treated as separate chapters here because of different authorship. Most argentinoids are meso-to bathypelagic in oceanic waters over the outer continental shelf and slope. In members of the Argentinoidei, the dorsal and anal fins develop in the median finfold (rather than along the body wall as is typical of most teleosts) and are connected to the trunk by a series of hyaline strands (Ahlstrom et al. 1984). The pectoral fins develop first, followed by the caudal, dorsal and anal, and pelvic fins. All argentinoids have a deflected terminal gut that hangs free from the body. All regional members of the Argentinoidei have 15 or fewer dorsal rays, and most have fewer than 15 anal rays and an adipose fin.
