Ocean Acidification Improves Marine Snail Prey Response to Predator but Not Shell Growth

by Jennifer Fields

Ocean acidification is known to have an impact on the calcified structure of many marine invertebrates. However, there is recent evidence to suggest that ocean acidification also impacts other key biological processes, such as survival, growth, and behavior. Manríquez et al. (2013) observed the impacts of ocean acidification on shell growth, survival, metabolism, and self-righting ability of a marine gastropod. The authors found there was no significant impact of ocean acidification on net shell growth, survival, or metabolism; however, increased CO2 resulted in faster self-righting times in the marine gastropod. Faster self-righting behavior can reduce the duration of vulnerability to predators and chance of being dislodged by waves of intertidal gastropods. The behavior could be a positive consequence of ocean acidification on marine invertebrates that use a turnover response as a common trait for avoid predation and wave removal. This adaptive trait could induce a co-evolution between predator and prey that would alter predator-prey dynamics within the whole intertidal ecosystem.

Ocean acidification, caused by the increase absorption of anthropogenic CO2 in the ocean surface, is usually described as a reduction in seawater pH and carbonate ion concentration. Recent research has suggested that ocean acidification not only affects calcification of calcifying organisms, but also the survival, growth, behavior, and metabolism of the organism. This study examined alterations in self-righting behavior, survival, growth, and metabolism of an intertidal gastropod snail under three different CO2 conditions. Self-righting, or the ability for an individual to return to its normal orientation after being disturbed, is an indication of whether the individual is exhibiting a stress response to chemical changes in its environment. The authors tested the gastropod’s self-righting under modern-day CO2 and two different elevated CO2 conditions with the presence and absence of a predator crab. To test the survival of the individuals, some snails were glued to the bottom of the container to prevent the self-righting behavior. Growth was measured on regular intervals over a period of 83 days of ocean acidification exposure. The overall shell size, buoyancy, and empty shell weight were monitored. After 100 days of exposure to ocean acidification conditions, the metabolism of control and both elevated CO2 groups was analyzed through measuring the oxygen consumption of the animals.

As predicted by the authors, there were significant impacts on the self-righting behavior of the gastropod; however, there was little alteration to the snail’s survival, growth, and metabolism. There was no difference in survival during the self-righting experiment between the groups. There was no significant difference in wet mass, size, buoyant weight, or empty shell weight suggesting that there was no effect of increased CO2 on the shell growth and calcification between the groups. The self-righting ability of the gastropod changed quite drastically with addition of more CO2. Self-righting was three times as fast in the elevated CO2 groups as it was in modern day CO2 conditions. In the presence of the crab predator, elevated CO2 snails tended to right themselves even faster than in the absence of the predator. This change in righting ability did not result in an increase in metabolism.

The results indicate that shorter self-righting times could be an adaptive trait that may potentially improve the chances of gastropod survival. The insignificant findings in size, wet weight, and buoyant weight changes in the gastropod suggest that there was no net effect of ocean acidification on shell calcification; thus, the effects of ocean acidification on calcification are more complex than expected. The study implies that species will vary in response to increased CO2 and decreased carbonate concentration through growth, reduction, or no net change in their shells. Faster righting responses may be caused by the stress from chemical changes of ocean acidification. This might mean that future ocean acidification could be more optimal for normal metabolic functions in this particular marine gastropod. But this faster self-righting under increased CO2 could also indicate increased predator wariness and vigilance due to the increased stress caused by ocean acidification. The improved self-righting behavior can decrease the duration of vulnerability to predators and cause faster contact with the substrate, reducing the chances of being dislodged by wave action in the intertidal zone. It is possible that faster self-righting can be a positive consequence of ocean acidification on this marine gastropod and other marine invertebrates that use this turnover response to avoid predation and wave dislodgement. The higher prey fitness caused by faster self-righting could induce selection pressure on the predator population to improve its predator fitness as well. This will surely have further implications on changes in predator-prey interactions within the intertidal ecosystem.

Manríquez, P.H., Jara, M.E., Mardone, M.L., Navarro, J.M., Torres, R., Lardies, M.A., Vargas, C.A., Duarte, C., Widdicombe, S., Salisbury, J., Lagos, N.A. 2013. Ocean Acidification Disrupts Prey Responses to Predator Cues but not Net Prey Shell Growth in Concholepas concholepas (loco). PLoS ONE published ahead of print July 3, 2013,doi:10.1371/journal.pone.0068643

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