Seawater carbonate chemistry and shell morphometrics during CO2 acidified seawater laboratory experiment and field experiment of flat tree oysters, Isognomon alatus (Gmelin, 1791)

Seawater changing chemistry has consequences on coastal ecosystems and their living resources. Future projections suggest the pH could drop 0.2-0.3 pH units by the year 2100 under a business-as-usual (BAU) CO2 emission scenario. Marine calcifying organisms such as corals, calcifying algae, crustaceans, mussels, oysters and clams are most likely to be impacted by ocean acidification. The Isognomon alatus (flat tree oyster) is an important species that can be negatively affected by the lowering of seawater pH. Isognomon alatus is an important food source, a substrate for other benthic organisms (e.g., stone crab, Menippe mercenaria) and contribute to nutrients recycling in coastal ecosystems. The study was conducted to test the impacts acidified seawater CO2 on the growth of I. alatus under controlled laboratory conditions as well as field experiment. The Isognomon alatus lost weight and experienced negative growth rates of –0.56 +- 0.36 mg /g/day under average pH values of 7.8 expected by the end of this century compared to a loss of –0.26 +- 0.23 mg/g/day under ambient pH (value 8.1) conditions. In contrast, I. alatus incubated in a field experiment showed a gain in weight and positive growth of 3.30 +- 0.23 mg/g/day despite exposure to pH levels (7.4) during low tide significantly lower than those experienced in the laboratory. Overall, the results showed concern on the impacts of acidification flat tree oyster (Bivalvia:Isognomonidae). A decline of calcifying bivalves populations can impact coastal ecosystems function and indirectly affect the human beings that depend on them as a food source.