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

The impacts of climate change and ocean acidification on marine species will be complex and involve both direct and indirect effects (Kroeker et al. 2013). As with other environmental stressors, the consequences of ocean acidification (OA) are likely to be variable, not only among species but also within species, with some individuals showing greater tolerance to low pH conditions (Doney et al. 2009). Phenotypic plasticity may play an important role for species faced with environmental fluctuations (Hofmann et al. 2010). However, ultimately it is genetic diversity within a species that determines its potential for adaptive evolution when subjected to altered selective pressures (Merila and Hendry 2014). If the ability to cope with low pH conditions is heritable, selection for tolerant genotypes can allow species to adapt to their changing environment (Schlegel et al. 2012). Currently, the relative roles of genetic variation and phenotypic plasticity in response to climate change are not well understood (Merila and Hendry 2014). Recent studies have found intraspecific variation in response to OA (Gazeau et al. 2010; Caldwell et al. 2011; Schlegel et al. 2012; Reusch 2014); however, many seemingly adaptive responses to the environment are likely to be environmentally induced phenotypic plasticity (Dupont and Thorndyke 2009). Inherent adaptive flexibility may be critical in an organism’s ability to cope with environmental change (Hofmann et al. 2010), yet rapid change may test the limits of a species’ potential to persist (Donnelly et al. 2012). With a rapid change in oceanic pH, the pressure for adaptation will intensify (Hofmann et al. 2010; Schlegel et al. 2012; Doney et al. 2012; Hoegh-Guldberg and Bruno 2010). As a first step, quantifying intraspecific genetic variability is vital to Abstract 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.