This is the second most common Gnathophausia
species at bathypelagic depths off southern California, after Neognathophausia ingens
(Cowles personal observation). 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 (6.5% of saturation) (Childress, 1975). It is about 80% water by weight. It is negatively buoyant (Childress and Nygaard, 1974) 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 (De Jong-Moreau and Casanova, 2001). This species may attain sexual maturity at a smaller size in the Pacific than in the Atlantic (Fage, 1941).
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.