Cercariae (Platyhelminthes: Trematoda) as neglected components of zooplankton communities in freshwater habitats

Zooplanktonic animals live in the open water of freshwater habitats, whilst trematodes are ubiquitous parasitic worms of aquatic vertebrates and invertebrates. A defining characteristic of trematode life cycles is the cercariae, a mainly planktonic free-living larval stage that emerges into the aquatic environment from infected molluscan hosts. Cercariae are lecithotrophic (non-feeding) larvae analogous of meroplankton. Although millions of planktonic cercariae enter freshwater habitats on a daily basis and are capable of forming into dense clouds, they are a largely neglected component of the zooplankton community, rarely mentioned in faunal studies. Nevertheless, there is increasing evidence to suggest that cercariae have important secondary roles in aquatic food webs and energy transfer. This article reacquaints freshwater biologists with cercariae, highlighting the key characteristics of their biology and population dynamics, their role in food webs, public health and veterinary importance, and the risk of increased population densities under the influence of climate change. The reasons for their neglect in freshwater biology is evaluated and considered to be associated with zooplankton sampling methodologies being unsuitable for collecting cercariae and the paucity of identification keys in the limnological literature.