Behavioral response of eastern oyster Crassostrea virginica larvae to a chemical settlement cue is not impaired by low pH

Ocean acidification poses a major threat to marine organisms, but little is known about how pH affects larval behavior. The transition from a pelagic larval stage to adult life in the benthos, known as settlement, is a critical period in invertebrate life-histories and is influenced by larval responses to multiple chemical and physical settlement cues. We investigated the effects of low pH on the swimming behavior during settlement of the important reef-forming eastern oyster Crassostrea virginica, by tracking larval behaviors in ambient (8.1) and low (7.4) pH conditions, in the presence and absence of a chemical settlement cue. We hypothesized that low pH would impede the ability of C. virginica larvae to interpret or respond to the cue. In a preliminary experiment, observations of swimming behavior in larvae of 3 different stages (pre-competent, competent-to-settle, and post-competent) in the presence of the settlement cue showed no significant effect of pH on downward velocity or proportion of larvae remaining on bottom. This result indicates the response to the cue was not impaired by low pH. There were, however, trends in the proportion of larvae remaining on the bottom or swimming in helices, both in the presence and absence of the cue, that suggested larvae in one or more stages may have become more active or exploratory in low pH. Results from a follow-up experiment confirmed the lack of a significant effect of pH on larval response to the cue, but did not demonstrate a consistent or significant effect of pH on larval activity or exploration.