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

Abstract

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 freeliving 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.

First study on communities of parasites in Triportheus rotundatus, a Characidae fish from the Amazon River system (Brazil)

“…The abundance and diversity of the invertebrate fauna are also key components in the formation of a parasite community in fish populations (MOREIRA et al, 2009;MORLEY, 2012;TAVARES-DIAS et al, 2015). Moreover, environments with abundance of aquatic vegetation (e.g.…”

“…Moreover, environments with abundance of aquatic vegetation (e.g. macrophytes), as the ecosystem of this study (THOMAZ et al, 2004), may influence the abundance of parasites (MOREIRA et al, 2009;MORLEY, 2012), because its floodplains areas are widely used for shelter and feeding by many fish species. Consequently, this increases the possibility of the fish finding different infective stages of the parasite species in the environment.…”

“…Moreover, the knowledge about parasites community infracommunities and their relationship with the host fish is of great importance, because the parasites also play a key role in ecosystems by regulating the abundance or density of natural fish populations, thus stabilizing food web and host community structures (MOREIRA et al, 2009;MORLEY, 2012;TAVARES-DIAS et al, 2013;OLIVEIRA et al, 2016 (MACHADO, 1959;KRITSKY et al, 1992;COHEN et al, 2013;COSTA-PEREIRA et al, 2014;OLIVEIRA et al, 2016). However, there are no studies on the parasites of T. rotundatus.…”

Avian Schistosomes and Outbreaks of Cercarial Dermatitis

“…At least in temperate climates, schistosome cercariae occur seasonally once the ambient temperature is high enough for snail activity and thus cercarial emergence (271). Also, snails infected during late summer can survive through the winter season and serve as a source of cercariae in spring (13,272,273).…”

“…As far as trophic interactions of organisms are concerned, predation by small aquatic animals may represent a promising means of biological control. Schistosome miracidia and cercariae may serve as prey for larval aquatic insects, crustaceans, oligochaetes, shrimps, and fish (271,(333)(334)(335)(336)(337)(338). In particular, the annelid Chaetogaster limnaei sensu lato, living commensally or parasitically on the shell surface or in the mantle and pulmonary cavities of freshwater snails, may act as an efficient predator and prevent penetration of miracidia (339,340) or feed on cercariae as they are released from the snail (341,342).…”