Elevated temperature, but not acidification, reduces fertilization success in the small giant clam, Tridacna maxima

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Elevated temperature and decreased ocean pH (ocean acidification) are associated with anthropogenic climate change and can adversely affect fertilization and development in marine invertebrates. However, the potential synergistic impact of these stressors on fertilization success remains unresolved for many ecologically and economically important species including giant clams of the genus Tridacna. Individual and interactive effects of warming and acidification on fertilization (successful first cleavage) were investigated in the small giant clam, Tridacna maxima. Experiments were performed on gametes of T. maxima (collected in October 2015 from the island of Moorea, French Polynesia; 17.54° S, 149.83° W) fertilized under ambient conditions (27 °C, pH 8.1) and under conditions congruent with temperature and pH projections for the coming century (31 °C, pH 7.6). Fertilization success was low, but within previously reported levels, under ambient conditions (47.7 ± 3.4%) and was significantly reduced at elevated temperature per se and in combination with lowered pH (18.5 ± 4.4% and 21.2 ± 4.6%, respectively). However, acidification alone had no effect on fertilization success in T. maxima (48.2 ± 3.1%). These results indicate that although fertilization in T. maxima is resilient to lowered pH, it is strongly inhibited by elevated temperature. Populations of T. maxima may, therefore, be at risk of low reproductive success over the coming century as a result of rising ocean temperature.