Tokyo University of Marine Science and Technology, Japan
Abstract: Ocean acidification (OA) refers to a reduction in the pH of seawater over an extended period. Ocean acidification decreases the growth and survival rates of calcifying organisms (e.g., mollusks, corals, and coralline algae) by disrupting calcification of the exoskeleton or shell. It also modifies animal behavior by impairing the nervous system. In addition to these direct effects, indirect effects could be considerable through changes in prey and/or predator abundance or composition. The Japanese common chiton Acanthopleura japonica is a mollusk species having eight shell plates, spicules, and lens that consist of aragonite (a calcium carbonate polymorph that is more soluble than calcite). Acanthopleura japonica plays the role of a keystone species in intertidal rocky shores; hence, the influence of OA on this species would be substantial for the communities. In this study, we evaluated the direct and indirect effects of OA on A. japonica.
First, we conducted field surveys in July, November 2022, February, and April 2023 using the CO2 seeps (areas simulating future acidification conditions) on Shikine Island, Tokyo, Japan. To evaluate OA effects on the population dynamics of A. japonica and surrounding environments, we compared A. japonica abundance, food resources (macro/micro-algae), and predator abundance (Thais clavigera, a common carnivorous whelk) between the CO2 seeps (acidified) and normal condition (reference) sites. We also measured survival rate, shell plate density, and predator defense responses after 28 days of rearing under the acidified condition in the laboratory.
The field survey revealed that A. japonica was almost eliminated from the acidified sites. In contrast, predator density was not significantly different between the acidified and reference sites, whereas food resources rather increased in the acidified sites. The field survey results suggested that A. japonica disappearance in the acidified sites was not because of food availability and predation pressure. The laboratory experiments showed that acidified seawater did not decrease survival rate and shell plate density, indicating the absence of these direct effects. However, the defensive response was significantly inhibited under the acidified condition, and this inhibition might increase susceptibility to predation by carnivores. Overall, our study demonstrated that OA could eliminate the A. japonica population from intertidal rocky shore habitats, possibly through increased predation by carnivores.
