Current and future ocean chemistry influences shell growth and gene expression in atlantid heteropods

Atlantid heteropods (sea elephants) are the only predatory, aragonite shelled, marine holozooplankton. Despite relying heavily on their shell as a swimming appendage and sharing the same qualities that make shelled pteropods susceptible to OA, atlantids have not been considered in any ocean acidification (OA) research. Here we present the first study on the effects of OA on atlantids at morphological and molecular levels across three pH scenarios: mid-1970's, ambient and 2050. Morphological responses to decreasing pH were generally negative, but not uniform. Calcification was reduced from mid-1970s to ambient conditions, but longer, thinner shell was grown under 2050 conditions. At the molecular level, differential gene expression also revealed a complex response, with genes involved in growth and biomineralization being down-regulated with decreasing pH. Shell thickness was consistently higher across all treatments compared to shell grown in the natural environment. This may be due to replete food in the experiments and suggests that plentiful food could mitigate some negative effects of OA.