Planktonic ciliates: Diverse ecological function in seawater

Abstract

Planktonic ciliates are a representative group of microzooplankton, which play important roles in planktonic food webs. Based on results in previous various studies, quantitative information on roles as consumers of pico- and nanoplankton, as nutrient regenerators and as prey for various marine organisms were reviewed. The role as grazers of bacteria may be limited in small ciliates while autotrophic picoplankton is an essential food source for ciliates and can sustain populations of ciliates in oligotrophic waters. The grazing impact of ciliates is considered to greatly influence population dynamics of nanoplankton. Feeding responses of meso- and macrozooplankton suggested that ciliates become an essential food source for them. These findings show that ciliates play an important role in intermediation of energy flow from pico- and nanoplankton to larger plankton feeders. In addition, nutrient regeneration by ciliates is one of the important resources to support the production system not only in oligotrophic waters but also in other various waters. Furthermore, ciliates play roles as grazers for some harmful algal bloom species; the grazing impact may influence development and decay of the blooms. Since ciliates can also become food sources for bivalves and for polyp and ephyra stages of the moon jellyfish Aurelia aurita , they possibly contribute to production of bivalve aquaculture and occurrence of jellyfish blooms. These findings support that ciliates have diverse ecological functions in planktonic food webs.