Impact of microphallid trematodes on the survivorship, growth, and reproduction of an isopod (Cyathura carinata)

Crustaceans are second intermediate hosts to several microphallid species (Trematoda). Some of these parasites are potentially pathogenic or manipulative. A laboratory experiment was performed to assess the impact of microphallids on the survival, growth and fecundity of Cyathura carinata, a protogynous hermaphroditic isopod, widespread within European estuaries. For nearly 12 weeks, experimental populations of infected and non-infected isopods were kept at 25 °C. C. carinata carrying microphallid cysts showed higher mortality rates than non-infected specimens and were not able to produce embryos. The reduced fecundity of infected isopods could be caused by parasite-induced castration and/or by mating failure due to behavioural modifications in one of the sexes. It might also be associated with lower growth rates and lower moulting frequencies, since infected C. carinata were significantly smaller than the non-infected after 9 weeks. This may imply a setback for the isopods to achieve sexual maturity (which may also affect the population sex ratio) and for females to lay their eggs in the marsupia. Regardless of the mechanisms involved, microphallids may have severe consequences for their host populations, through negative effects on survival, growth and fecundity. For species with direct development, such as C. carinata, parasite-induced reproduction failure may contribute to temporal fluctuations of abundance. Based on the present results, it is recommended to include parasites as an important factor influencing host populations from shallow-water ecosystems.