Effect of low pH on embryonic and larval traits in the estuarine semi-terrestrial crab Neohelice granulata

Abstract

The decrease in the pH of the oceans is one of the consequences of the release of CO2 (carbon dioxide) into the atmosphere by human activities. In estuaries, the magnitude to which pH will change and how the organisms that inhabit these environments would be affected is today relatively poorly known. We evaluated, by two laboratory experiments, whether the low pH affects early stages of the semi-terrestrial crab Neohelice granulata, an estuarine and ecosystem engineer species. Firstly, embryos and the spawning larvae were exposed to acidified seawater (ambient pH: 7.9 and 8, respectively; 7.5 and 6.9); secondly, ovigerous females were collected from the field and freshly hatched larvae were cultured under these pHs. We found a slight delay in the development time of embryos exposed to pH 6.9 but no difference in those reared under intermedial pH (7.5) with respect to the control and 6.9. We also found a reduction in egg volume and eye size exposed at pH 7.5 but with a recovery to normal size later in development. Heartbeats did not vary with pH. A delay in the duration of the intermolt period was observed for both larvae zoea I (ZI) and II (ZII) exposed to pH 6.9 in the first experiment, and only in ZI under this pH in the second one. Differences in body morphometry were found in the zoeas analyzed in the first experiment but not for the second. Measurements of changes in oxygen concentration conducted in the second experiment showed a reduction in the consumption of all zoeae stages when exposed to both low pHs. Mortality of larvae remained constant across treatments. Although it was found that the early stages of this species would be slightly/moderately affected by potential future scenarios of pH decrease, we cannot rule out major impacts at the population level, which requires further experiments to be carried out.