Ocean acidification alters the dissolved inorganic carbon chemistry of seawater and can reduce the calcification rates of tropical corals. Here we explore the effect of altering seawater pCO2 on the skeletal morphology of 4 genotypes of massive Porites spp. which display widely different calcification rates. Increasing seawater pCO2 causes significant changes in in the skeletal morphology of all Porites spp. studied regardless of whether or not calcification was significantly affected by seawater pCO2. Both the median calyx size and the proportion of skeletal surface occupied by the calices decreased significantly at 750 µatm compared to 400 µatm indicating that polyp size shrinks in this genus in response to ocean acidification. The coenosteum, connecting calices, expands to occupy a larger proportion of the coral surface to compensate for this decrease in calyx area. At high seawater pCO2 the spines deposited at the skeletal surface became more numerous and the trabeculae (vertical skeletal pillars) became significantly thinner in 2 of the 4 genotypes. The effect of high seawater pCO2 is most pronounced in the fastest growing coral and the regular placement of trabeculae and synapticulae is disturbed in this genotype resulting in a skeleton that is more randomly organised. The study demonstrates that ocean acidification decreases the polyp size and fundamentally alters the architecture of the skeleton in this major reef building species from the Indo-Pacific Ocean.

In order to allow full comparability with other ocean acidification data sets, the R package seacarb (Gattuso et al, 2021) was used to compute a complete and consistent set of carbonate system variables, as described by Nisumaa et al. (2010). In this dataset the original values were archived in addition with the recalculated parameters (see related PI). The date of carbonate chemistry calculation by seacarb is 2022-07-07.