Restructure of food chain through Euphausia pacifica

太平洋磷虾（Euphausiapacifica）是北太平洋北方海洋生态系统的重要组成部分，一直被认为是一种食草动物。然而，我们认为该物种在很大程度上依赖于浮游动物作为食物。本研究验证了这一假设，并通过E. pacifica重构食物链。该种对裸纤毛虫的摄食速率为1.01- 3.24mgC/h，最大值相当于太平洋真水蚤体碳的2.3%/d。这表明裸纤毛虫本身就可以满足该物种的食物需求。该物种不能摄取细菌，但可以摄取纤毛虫。通过对太平洋真核生物碳、氮稳定同位素比值的分析，了解其营养地位。颗粒有机质（POM）作为太平洋真水蚤的食物似乎比净浮游动物更重要，因为δ^<15>N，约。2‰，太平洋真海蜇与POM之间的遗传距离大于太平洋真海蜇与网栖动物之间的遗传距离。由于太平洋真水蚤的δ^ N值高于网捕浮游动物，其营养地位可能高于<15>网捕浮游动物。POM中可能含有裸纤毛虫和浮游植物，但其组成尚需进一步研究。磷虾一生都在蜕皮，太平洋磷虾每5天蜕皮一次。本文从昼夜垂直迁移、生殖周期和摄食活动等方面对该物种的蜕皮周期进行了研究。蜕皮的个体在夜间向上迁移时受到限制。成熟雌虫蜕皮个体数最少，说明成熟雌虫蜕皮间期延长。换羽个体的摄食活动全天都很低，而非换羽个体则表现出昼夜摄食节律，夜间高，白天低。

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.