Gnathiids are quickly recognized by the presence of only six free pereonites and five pairs of pereopods, the first pereonite being fused to the cephalon (with its appendages functioning as a second pair of maxillipeds, or pylopods) and the seventh pereonite being greatly reduced and without legs. The pleon is abruptly narrower than the pereon, always with five free pleonites (plus the pleotelson). Adult males have broad flattened heads with grossly enlarged mandibles that project in the front. Females have small narrow heads and no mandibles at all. In both sexes the eyes are well developed and frequently on short processes (ocular lobes). The embryos are incubated internally, distending the entire body cavity and displacing the internal organs.
Gnathiids occur from the littoral zone to the deep sea, and they are often numerous in shallow soft-bottom benthic samples. Adults probably do not feed and are often found in association with sponges. Adults are benthic but the juvenile stage, called "praniza," is a temporary parasite on marine fishes. Praniza are good swimmers, whereas adults have only limited swimming capabilities. Females and juveniles cannot be identified, and the taxonomy of this suborder is based entirely on males. About ten genera and 125 species, in a single family (Gnathiidae), have been described worldwide. Eight species have been found in California waters, all but G. steveni (figure 71) being subtidal. Only two species have been reported from north of Point Conception, Gnathia tridens and Caecognathia crenulatifrons. For a key to all known California species see Wetzer and Brusca (1997).