Seasonal occurrence and role of protistan parasites in coastal marine zooplankton

The prevalence of parasites and epibionts associated with zooplankton in coastal waters of the NW Mediterranean Sea was studied through fortnightly sampling from July 2003 to January 2005. The most commonly found parasites were dinoflagellates of the generaSyndiniumandBlastodiniuminfecting a range of planktonic copepod species, for the most part females. The lethal parasiteS. turbowas found in the calanoid copepodParacalanus parvusall year round, but primarily during summer and autumn. Up to 7% of femaleP. parvuswere visibly infected withS. turbo, corresponding to an estimated parasite-induced mortality rate of up to 0.15 d–1. The parasitic castratorsBlastodiniumspp. were generally found only from summer to late autumn. The most commonBlastodiniumspecies wasB. manginiparasitizing the cyclopoid copepodOncaeacf.scottodicarloiwith up to 17% of females visibly infected. This corresponded to an estimated impairment of the reproductive rate of 0.16 d–1. The epibiotic diatomSceptronema orientaleinfested on average 34% of adults of the harpacticoid copepodEuterpina acutifronsfrom late summer until early winter, but was absent during spring and early summer. Occurrence of the dinoflagellateOodiniumsp., a parasite of appendicularians, was seasonally very restricted with a peak of 14% ofOikopleurasp. infected in late summer. Other parasites such asParadiniumspp.,Ellobiopsisspp. and the epibiotic diatomPseudohimantidium pacificumwere found more sporadically on various copepod species. The results indicate that parasitism in marine zooplankton is a common phenomenon that has the potential to play an important role in the population dynamics of marine zooplankton.