Gnathophausia gracilis Willemoes-Suhm 1875

How to Distinguish from Similar Species: Gnathophausia gracilis can be most readily distinguished from other Gnathophausia species by the large spines projecting dorsally along the dorsal abdominal ridge, along with the sinuous dorsal margin of the carapace with an interrupted ridge. It also has a mid-dorsal spine projecting posterodorsally at an angle at the posterior end of the carapace.

Bathypelagic

Geographical Range: Worldwide in tropical and temperate seas. To approximately 40 degrees N in the eastern Pacific, 20 20 degrees S. Especially common in the eastern tropical Pacific.

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. Gnathophausia gracilis has a large antennal scale which is jointed near the antertior end. The dorsal margin of its carapace is sinuous, the dorsal ridge is interrupted, and it ends posteriorly with a posterodorsally-directed spine. The dorsal ridge on the abdominal segments has large dorsally-directed spines. Maximum length up to 14 mm, including rostrum. Females may attain sexual maturity by 5 cm in the Pacific, but many larger females than this are found both in the Pacific and the Atlantic which are not mature.

Biology/Natural History: This is the second most common Gnathophausia species at bathypelagic depths off southern California, after Neognathophausia ingens. The animal lives below the most severe part of the oxygen minimum layer off southern California, and probably experiences oxygen tensions down to about 10 mm Hg. It is about 80% water by weight. It is negatively buoyant so it must swim constantly throughout its life to maintain its depth in the water. This species seems to have the most primitive foregut of all the mysids examined. This species may attain sexual maturity at a smaller size in the Pacific than in the Atlantic. Gnathophausia gracilis 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 this parasite also retards sexual maturation such as retarded development of oostegites in females and feminizing changes in males, but these changes may not be as pronounced in G. gracilis.

Depth Range: 900-2500 m