The effect of in-situ ocean acidification on the community structure of marine invertebrates with a focus on the gastropod Columbella rustica

Laboratory experiments show that ocean acidification can be detrimental to the growth and calcification of many organisms at the species level. However, the effects of ocean acidification at the ecosystem-scale are less studied and harder to predict due to the diversity of responses of taxa. Areas with naturally high CO2 levels are becoming increasingly important in determining community level responses of ecosystems to ocean acidification. This study adds to previous work conducted along a natural acidification gradient caused by cool volcanic vents in Ischia, Italy. Researchers found an increase in tolerant crustacean taxa but were unable to separate this increase from habitat-dependent causes. To eliminate this confounding variable, benthic invertebrate communities were characterized using artificial substrate to standardize the habitat. The primary objective, to examine the processes underlying community reorganization of mobile invertebrates caused by ocean acidification, was achieved by sorting 18 samples collected along the gradient to the lowest taxonomic level possible. To assess the physiological consequences of acidification for the dominant gastropod species, the characteristics of the dominant gastropod in the system, Columbella rustica, were surveyed for differences in size, shell condition and shell and body weight between the ambient and low pH zones.
Our results highlight the differing responses of invertebrate taxa to ocean acidification, showing a general decrease in calcifying organisms at lower pH and an increase in those taxa that exhibit little to no calcification. When the habitat was held constant, the increasing abundance of crustacean taxa seen in previous studies at these vents (when the habitat varied with the pH) was not observed. These results suggest that the increased abundance of small crustacean grazers at extreme low pH reported in a previous study is likely associated with changes in the structural characteristics of the habitat and cannot be attributed to a direct benefit of low pH to these taxa. Preliminary results also indicate that calcifying gastropods can cope with increased acidity, but at a pH threshold, the snails lose their ability to cope with increasing acidity through adaptive mechanisms.