Comparative morphology of the feeding basket of five species of Euphausia (Crustacea, Euphausiacea) in the western North Pacific, with some ecological considerations

The feeding basket morphology and stomach content analyses of five Euphausi a species (E. recurva, E. nana, E. pacifica, E. mutica and E. similis) were compared to elucidate their feeding ecology. The filter areas of feeding b asket of E, pacifica and E. nana were proportionally larger than those of o thers in the size classes 13-20 mm and <13 mm, respectively, suggesting a h igh filtering efficiency in these species at each size class. Based on the secondary setal distance, it is suggested that there are three types of fee ding basket in five Euphausia species. One is the fine mesh (<5 mu m) of E. pacifica and E. nana; a second is the medium (10-20 mu m) of E. recurva an d E. mutica; and the third is the coarse (20-30 mu m) of E. similis. The ab ility to feed on particles <5 mu m would give both species, E. pacifica and E. nana, a great advantage over other species when number of flagellates i s high. Stomach content analyses indicated a more omnivorous feeding mode i n E. mutica, E. recurva and E. similis and a more herbivorous in E. pacific a and E. nana. This is in agreement with morphological studies of feeding b askets. Increments in the primary and secondary setal distances with increa sing size of four Euphausia species, except E. pacifica, were evident, refl ecting interspecific differences in food particle sizes utilized. In E. pac ifica, however, the morphological similarity should produce intraspecific c ompetition in diet.