Early life stages of Mediterranean corals are more sensitive to ocean acidification than adults

Description

Volcanic CO2 vents are ideal sites to study the long-term effects of ocean acidification on long-lived calcified species, such as corals. At the Island of Ischia (Italy), we investigated, both in situand ex situ, whether past exposure to low pH confers physiological tolerance to ocean acidificationin different life stages and affects sexual reproduction. Adult colonies of Astroides calycularis(azooxanthellate) and Cladocora caespitosa (zooxanthellate), and larvae of A. calycularis weresampled at CO2 vent and ambient pH sites. They were then maintained under controlled conditionsin the laboratory, during two distinct 6-month experiments, under ambient (pHT 8.0), low (pHT7.7) and extreme low pH (pHT 7.5, only for larvae from CO2 vents). Adult net calcification andrespiration of both species, and net and gross photosynthesis of C. caespitosa were unaffected bythe pH treatments regardless of their environmental history. Larvae of A. calycularis from the CO2vents were smaller and had a lower survivorship and settlement success than larvae from theambient pH site regardless of experimental pH treatment. Colonies of C. caespitosa were smallerand less abundant near CO2 vents. Histological sections of C. caespitosa from the ventsdemonstrated an asynchrony of spawning between male and female, which could have majoreffects on fertilization. Our results suggest that adult colonies of A. calycularis and C. caespitosaexhibit tolerance to acidification. However, long exposition to low pH appears to decrease sexualreproduction and early life stages development, which are both of particular relevance for thepersistence of healthy populations.

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