Neognathophausia ingens (Dohrn 1870)

How to Distinguish from Similar Species: The rostrum is shorter than that of Gnathophausia zoea and Neognathophausia gigas and is indistinctly denticulate. Has reduced or no supra-orbital spines. The spines at the posterolateral margin of the carapace are also shorter than those of Gnathophausia zoea. Unlike Gnathophausia gracilis, this species does not have prominent dorsal spines on the abdominal segments. Unlike Neognathophausia gigas, both the anterior and the posterior lobe of the pleural plates are spiniform.

Bathypelagic

Lophogastrids are bathypelagic, shrimplike crustaceans which differ from true shrimp in that their carapace overhangs but is not actually connected to the posterior thoracic segments. They are not decapods as shrimp are, and, for example, have only one set of maxillipeds instead of 3 and 7 pairs of pereopods instead of 5. The pleopods, with which they swim, are well developed. They have large thoracic gills but no statocysts. As a Peracaridan, female Lophogastrids have long thoracic endopods which are modified into a basket for carrying eggs and larvae. This species is the largest pelagic crustacean. Maximum length up to 35 cm. Most are less than 18 cm long.

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, 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. 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. 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. Both its metabolic rate and ammonia excretion decrease with starvation. Neognathophausia ingens swims primarily with the pleopods, with some participation by the thoracic exopods. Their activity levels are little affected by pressure. The species swims constantly and has a relatively high drag compared to fish, but swims at a speed which minimizes energy losses due to drag. Gnathophausia means "light-jaw". This species has a gland on its second maxillae 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. This species often lives in oxygen minimum layers, yet its metabolism is entirely aerobic. 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. Its low rate of aerobic metabolism help keep it from building up oxygen debt. It has greater gill surface area than do most crustaceans and fishes. The oxygen diffusion distance across the gills is 1.5 to 2.5 microns, comparable to that found in many fishes. 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. 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. 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. Predators include the Melanostominid fish Echiostoma barbatum, the Macrourid fish Macrouronus novaezelandiae, dwarf sperm whale, the Antillean beaked whale, in which it comprised 41% of the stomach contents of a beached individual, and Cuvier's beaked whale. 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.

Geographical Range: Worldwide in tropical and temperate seas, most common in tropical and subtropical zones. Common bathypelagically off California and West Africa. Less common in the eastern tropical Pacific and eastern tropical Atlantic than in the equatorial Indian Ocean, probably because of the extremely low oxygen levels in the eastern tropical Pacific at the depths G. ingens inhabits.

Depth Range: Usually around 500-900 m. Can be found down to 4000 m. Juveniles are usually in water from 5 to 8 degrees C.