The gastric mill of euphausiid crustaceans: A comparison of eleven species

The morphology of the gastric mill of 11 species in the family Euphausiidae is described. Two distinct arrangements can be recognized with respect to presence or absence of lateral teeth. One is the well-developed gastric mill of Thysanoessa gregaria, T. inermis, T. macrura and Pseudeuphausia sinica; the other is the gastric mill without lateral teeth of Nematoscelis atlantica, N. difficilis, N. megalops, Nematobrachion flexipes, N. sexspinosum, Stylocheiron abbreviatum, and S. carinatum. This supports the close relationships of the four species of both genera, Thysanoessa and Pseudeuphausia on the one hand and seven species belonging to Nematobrachion, Nematoscelis, and Stylocheiron on the other hand. The hypothesis that species in the same genus have similar gastric mill morphology, which was once rejected in the genus Thysanopoda, is accepted in all five genera examined here. Functional morphology of euphausiid foreguts is discussed.