Epibiotic euglenoid flagellates increase the susceptibility of some zooplankton to fish predation

Abstract

The phototrophic flagellate, Colacium vesiculosum (Euglenophyceae), lives externally on freshwater zooplankton. In experimental tanks with supplemental nutrients, epibiotic prevalence (% zooplankters infested with epibionts) was not consistently different from control. Experimental reduction of the zooplankton population did not reduce epibiont frequencies; no substrate limitation is suggested. In tanks with planktivorous fish, Menidia beryllina, prevalence was significantly reduced on Daphnia, Ceriodaphnia, and cyclopoid copepods. The pump-filter-feeding fish, Tilapia aurea and Dorosoma cepedianum, reduced prevalence on Ceriodaphnia and copepod nauplii, but only Dorosoma reduced prevalence on cyclopoids. Impacts of fish predation appear to be related to the effects of Colacium on visibility and escape efficiency.