Biology/Natural History: Lophogastrids were formerly thought to be a type of mysid. In regions where this species is common, males do not reach the maximum size. After the instar at which the species reaches sexual maturity (at about 15 cm total length), the females undergo a growth spurt and the males seem to disappear from the population. This implies that the females may eat the males after copulation. Very large males seem to be found mainly in areas where the species is scarce and the male may not have encountered and bred with a female. While brooding eggs, the female sinks down to around 1000-1200 m depth. She carries the eggs and larvae for about 1.5 years, during which time she loses much organic body mass and is apparently not feeding (Childress and Price, 1983). The female's eggs account for 61% of the energy she has accumulated over her lifetime. Another 13% is used during brooding of her young, 6% in cast exoskeletons, and she only retains 20% of her original total energy content after brooding (but her water content is very high). The species reproduces only once, and the female dies shortly after release of the larvae. The species has 13 instars. Intermolt interval varies from 166 to 253 days, depending on the size (Childress and Price, 1978.

This species lives permanently below the euphotic zone. Although its water content increases in winter, suggesting fluctuations in food availability, its O:N ratio changes little indicating that its lipid levels remain high and it is not starving (Hiller-Adams and Childress, 1983). Both its metabolic rate and ammonia excretion decrease with starvation (Hiller-Adams and Childress 1983, Quetin et al., 1980).

Neognathophausia ingens swims primarily with the pleopods, with some participation by the thoracic exopods (Hessler, 1985). Their activity levels are little affected by pressure (Quetin and Childress, 1980). The species swims constantly and has a relatively high drag compared to fish (Cowles et al., 1985), but swims at a speed which minimizes energy losses due to drag (Cowles and Childress, 1988).

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 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).

This species often lives in oxygen minimum layers, yet its metabolism is entirely aerobic (Childress 1968, 1969, 1971, Cowles et al., 1991). To facilitate oxygen diffusion, it maintains a high rate of oxygen flow over its gills and extracts a very high percentage of the available oxygen (Childress, 1971). Its low rate of aerobic metabolism (Childress, 1971, Cowles, 1987, Cowles et al., 1991) help keep it from building up oxygen debt. It has greater gill surface area than do most crustaceans and fishes (Belman and Childress, 1976). The oxygen diffusion distance across the gills is 1.5 to 2.5 microns, comparable to that found in many fishes (Belman and Childress, 1976). It maintains relatively high rates of blood flow via large circulatory system components. Its heart rate is similar to that of other similarly-sized crustaceans, and the heart slows as oxygen limitation is reached (Belman and Childress, 1976). It appears that much of the oxygen in the blood is carried by hemocyanin, which has a high oxygen affinity and cooperativity and a large Bohr shift (Sanders and Childress, 1990). Species which live in areas with very low oxygen levels, such as off California, are able to live aerobically at lower oxygen levels than are those from higher oxygen levels such as Hawaii (Cowles et al., 1991).

Predators include the Melanostominid fish Echiostoma barbatum (Sutton and Hopkins, 1996), the Macrourid fish Macrouronus novaezelandiae (Clark, 1985), dwarf sperm whale (Cardona-Maldonado and Mignucci-Giannoni (1999), the Antillean beaked whale (Debrot, 1998), in which it comprised 41% of the stomach contents of a beached individual, and Cuvier's beaked whale (Palacios, 2003).

The rostrum and spines of small individuals are relatively longer than in large individuals. This led to small individuals originally having been named a separate species, Gnathophausia calcarata.

Gnathophausia ingens 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. It also retards sexual maturation such as retarded development of oostegites in females and feminizing changes in males.