Impact of ocean acidification on multiplication and caste organization of parasitic trematodes in their marine gastropod hosts
Supervisor: Robert Poulin (Department of Zoology, University of Otago, New Zealand)
Ocean acidification, a global phenomenon caused by an increase of atmospheric CO2, is predicted to impact the structure and functions of all marine ecosystems in this century. Now that we have achieved a better understanding of chemical seawater changes, studies focused on the impacts of ocean acidification on marine organisms have been increasing. However, these have to date ignored the presence of marine parasites, which are often neglected in marine ecological research although their diversity and influence within ecosystems are complex and important. For this reason, the impacts of ocean acidification on parasite-host interactions were investigated in the present study, testing the effects of low pH on different stages of two marine trematode species (Philophthalmus sp and Parorchis sp, both in the family Philophthalmidae) infecting two marine gastropods. The study used a potentiometric ocean acidification simulation system which automatically regulates pH through the injection of 100% CO2 gas into temperature-controlled seawater (12°C). The impacts of acidified seawater (pH 7.8 and 7.6) on three variables were examined: the production of free-living infective stages (cercariae), the survival of encysted resting stages (metacercariae), and the division of labour between two morphs (castes) of within-host stages. The results showed that acidified seawater generally caused an increase in cercarial production and a reduction in metacercarial survival. However, the ratio of the two castes within snail hosts seemed to be affected but further research will be needed to elucidate the precise effects of environmental changes on the division of labour for both trematode species. Moreover, the observed responses to ocean acidification differ between both species, indicating that the effects of reduced pH may be species-specific. These results suggest that ocean acidification may affect transmission success and parasite abundance of some trematode species, which should alter host-parasite interactions in marine ecosystems and consequently change population, community and ecosystem composition.

Keywords: Ocean acidification, host-parasite interaction, Trematoda, Philophthalmid species, cercariae, metacercariae, rediae, division of labour, gastropod