Lack of genetic variation in the response of a trematode parasite to ocean acidification

Ocean acidification is already having measurable impacts on marine ecosystems. Intraspecific variation in the responses of marine organisms to ocean acidification can reveal genetic differences in tolerance to low pH conditions and determine the potential for a species to adapt to a changing environment. This study tests for the existence of genetic variation in both the transmission success of the trematode Maritrema novaezealandense to its second intermediate amphipod host, Paracalliope novizealandiae, and the extent of parasite-induced mortality in that host, in response to decreasing pH. Eight parasite genotypes were tested in a custom-built ocean acidification simulation system, at 8.1 pH (current ocean conditions) and under conditions of 7.4 pH (worst-case scenario future prediction). The parasites had significantly higher infection success in the more acidic treatment, but there was no significant difference among genotypes in how infection success was affected by pH. In contrast, some parasite genotypes induced higher mortality in amphipods than other genotypes, but this genetic effect was also independent of pH. Overall, our results reveal no significant intergenotype variation in how the parasite responds to ocean acidification with respect to two key traits, infection success and parasite-induced host mortality, suggesting limited potential for adaptation in the face of acidifying conditions.