Biology/Natural History: Reach sexual maturity at about 12 cm total length (including rostrum). Spination and relative length of rostrum is greater in smaller individuals, which originally led to small individuals being called the separate species G. drepanephora. This species is less common than is N. ingens off our coast (at least off California)

Lophogastrids were formerly thought to be a type of mysid. This species lives permanently below the euphotic zone. Neognathophausia gigas swims constantly, primarily with the pleopods, with some participation by the thoracic exopods (Cowles, personal observations).

Gnathophausia means "light-jaw". This species has a gland on its second maxillae (mouthparts) from which it spews a brilliantly luminescent cloud into the water when disturbed. Luminescence seems to be a function of diet, since animals of a related species, Neognathophausia ingens, maintained on non-luminescent food in the laboratory gradually lose their ability to luminesce, while if luminescent food is restored they can regain their luminescence (Frank et al., 1984).

Neognathophausia gigas is sometimes parasitized by an ellobiopsid flagellate protozoan, Amallocystis fascitus, which forms a cluster of white filaments on the ventral side of the anterior abdominal segment. The parasite seems to be associated with the main nerve ganglion in this segment, and is associated with hypertrophy of the ganglion. In N. ingens it also retards sexual maturation such as retarded development of oostegites in females and feminizing changes in males.