Will shellfish be unable to grow in oceans of the future?
—Quantifying the threat of ocean acidification to predict its impact on shellfish larvae

1. Key Points

• As ocean acidification worsens, concerns over its effects on calcifying organisms^※1, including shellfish and coral, are increasing. To date, research has been unable to quantify the impact of ocean acidification on calcifying organisms, largely because of their incredibly small shells.
• In the present study, mollusk larvae with tiny shells measuring approximately 0.1 mm were raised in an environment designed to simulate severe ocean acidification. Subsequently, a high-resolution microfocus X-ray computed tomography (MXCT)^※2 scanner was used to obtain three-dimensional measurements of the shells. For the first time, globally, changes in shell morphology, in terms of reduced shell thickness, size, and density, were quantified precisely.
• In addition, gene expression in genomic domains involved in shell formation was reduced significantly, which marks a significant step toward comprehensive understanding of the effects of ocean acidification on organisms, with respect to biological responses and the impact on shells.
• The methodology employed in the present study could also be applied to other organisms with calcified shells or skeletons, such as bivalves and corals. Such studies could facilitate environmental impact assessment, marine conservation, and fisheries resource management in future.

Fig. 1. Predicting the future using a technique to measure the shell density of shellfish larvae.

2. Overview

As global warming intensifies, so does ocean acidification. Such ocean acidification poses a serious threat to marine ecosystems. It not only lowers the pH of seawater but also reduces the “aragonite saturation state (Ω_aragonaite)^{※3, ※4} ”. Aragonite is a crystalline form of calcium carbonate. When the “aragonite saturation state” is 1 or higher, it indicates a supersaturated state; conversely, values below 1 indicate an unsaturated state. The value is as an indicator of how easily organisms can form aragonite-based shells or skeletons. When the aragonite saturation state decreases, it is more difficult for organisms to form aragonite shells or skeletons. However, evaluating the effects of ocean acidification on early developmental stages of organisms such as plankton and mollusk larvae with aragonite shells, has proven challenging. Their shells are exceptionally small (approximately 0.1 mm in diameter and only a few micrometers^※5 thick), which makes precise quantitative assessment more challenging than with mature specimens.

To address the challenges above, Keisuke Shimizu (Associate Researcher) and Katsunori Kimoto (Acting Group Leader) from the Japan Agency for Marine-Earth Science and Technology Research Institute for Global Change Earth Surface System Research Center, alongside Masahide Wakita (Associate Researcher, Mutsu Research Institute), carried out joint research with Takenori Sasaki (Associate Professor, University Museum at the University of Tokyo). The team analyzed the morphology of the shells of shellfish larvae, using high-resolution MXCT and scanning electron microscopy (SEM)^※6, as well as gene expression. Globally, the research team is the first to successfully visualize and quantify the effects of decreased aragonite saturation on the growth and structure of extremely small shells (approximately 0.1 mm) composed of aragonite crystals, using shell density (which is analogous to human bone density) as a novel growth marker. In addition, the findings suggest that a decrease in aragonite saturation may both directly impact shells and influence gene expression domains involved in shell formation in shellfish larvae. The findings could facilitate prediction of the effects of environmental change (such as global warming and acidification) on calcifying organisms such as shellfish and corals.

These results have been published in the Journal of Molluscan Studies on September 18 (Japan time). The research was conducted with the support of a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (KAKENHI; JP23H02299).