Restructure of food chain through Euphausia pacifica

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Euphausia pacifica plays an important role in marine ecosystems of northern North Pacific Ocean and has been thought as a herbivore. However, we suggested that this species relies on zooplankton as food to a great extent. The present study tested the hypothesis and restructured the food chain through E.pacifica. The feeding rate of the species on cultured naked ciliates was 1.01-3.24mg C/h, the maximum value of which corresponds to 2.3% of body carbon of E.pacifica per day. It suggests that naked ciliates alone can fulfill food requirement of the species. The species cannot ingest bacteria but can ingest ciliates. Therefore, it plays a role in linking microbial loops to grazing food chains.We analyzed C and N stable isotope ratios of E.pacifica in order to know its trophic position. Particulate organic matter (POM) seems more important as food for E.pacifica than net zooplankton since the difference in δ^<15>N, ca. 2‰, between E.pacifica and POM was larger than that between E.pacifica and net zoolankton. The trophic position of the species may be higher than net zooplankton because E.pacifica showed higher δ^<15>N than net zooplankton. POM may contain naked ciliates as well as phytoplankton, but further study is needed to clarify its constituent.Euphausiids molt throughout their life span, and so does E.pacifica every 5 days. We investigated molt cycle of the species in relation to diel vertical migration, reproductive cycle and feeding activity. Molting individuals were restricted in their upward migration at night. The percentage of molting individuals was least in mature females, which suggests that intermolt period is prolonged in these females. Feeding activity of molting individuals was very low throughout the day, while non-molting individuals showed diel feeding rhythm, being high at night and low in the daytime.