Early life stages of a Mediterranean coral are vulnerable to ocean warming and acidification

The ability of coral populations to recover from disturbance depends on larval dispersion and recruitment. While ocean warming and acidification effects on adult corals are well documented, information on early life stages is comparatively scarce. Here, we investigate whether ocean warming and acidification can affect the larval and recruit development of the Mediterranean azooxanthellate coral Astroides calycularis. Larvae and recruits were raised for 9 months at ambient (23 ∘C) and warm (26 ∘C) temperatures and ambient (8.0) and low pH (7.7, on the total scale). The timing of the larval metamorphosis, growth of the recruit polyp by linear extension and budding, and skeletal characteristics of the 9-month-old polyps were monitored. Settlement and metamorphosis were more successful and hastened under a warm temperature. In contrast, low pH delayed the metamorphosis and affected the growth of the recruits by reducing the calcified area of attachment to the substrate as well as by diminishing the skeleton volume and the number of septa. However, skeleton density was higher under low pH and ambient temperature. The warm temperature and low-pH treatment had a negative impact on the survival, settlement, and growth of recruits. This study provides evidence of the threat represented by ocean warming and acidification for the larval recruitment and the growth of recruits of A. calycularis.