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The colony formation in Mediterranean coral Corallium rubrum is initiated by a larva that metamorphoses into the first polyp of the emerging colony approximately two weeks after settlement. The primary polyp then sets up a slow process that eventually, at least after a few years, gives rise to a tree-like rigid colony structure on which other polyps flourish. For a mature colony, this axial skeleton provides support for new polyps. However, the first emergence of the characteristic axial skeleton can take two years or more from the larva stage. The early colony morphology, instead, is shaped exclusively by the polyps' abundant deposition of sclerites, a magnesian calcite biomineral that has a different granularity from the distinctive red-coloured skeleton. With the appearance of the first polyp, a growing sclerite heap in a mesoglea layer provides a base for the emerging colony. In this paper, to elucidate the mechanical processes of early skeleton development in C. rubrum colonies, we present a computational model whereby the mesoglea layer provides a diffusion medium for the sclerites that the polyps deposit. We show that our stochastic model with three parameters captures the dynamic variability observed in measurements on living colonies. Our simulation results provide evidence for a diffusion process whereby the interplay between polyp budding and sclerite deposition are the main determinants of structure in early colony formation. Our model demonstrates that the frequency of budding events in an early colony can be described as a function of the available mesoglea surface whereas the number of polyps on the colony plays a secondary role in determining this frequency. We show that these model predictions are confirmed by direct observations on the colonies in our sample. Moreover, our results indicate that diffusion is a prevalent mechanism of colony development also at later stages of a colony's life span. • A stochastic model of early colony development in Corallium rubrum is proposed. • Samples of 1- and 2-year-old colonies are used for model fitting and verification. • Simulations based on diffusion quantitatively capture early colony development. • The frequency of polyp budding is only indirectly affected by the number of polyps. • The budding frequency remains unaffected with the availability of sufficient surface. [ABSTRACT FROM AUTHOR]

Abstract: In recent years, some of the ramifications of the ocean acidification problematic derived from the anthropogenic rising of atmospheric CO2 have been widely studied. In particular, the potential effects of a lowering pH on tropical coral reefs have received special attention. However, only a few studies have focused on testing the effects of ocean acidification in corals from the Mediterranean Sea, despite the fact that this basin is especially sensitive to increasing atmospheric CO2. In this context, we investigated the response to ocean acidification of the two zooxanthellate coral species capable of constituting the main framework of the community, the endemic Cladocora caespitosa and the non-native Oculina patagonica. To this end, we examined the response of both species to pCO2 concentrations expected by the end of the century, 800ppm, vs the present levels. Calcification rate measurements after 92days of exposure to low pH conditions showed the same negative response in both species, a decrease of 32–35% compared to corals reared under control conditions. In addition, we detected in both species a correlation between the calcification rate of colonies in control conditions and the degree of impairment of the same colonies at low pH. Independent of species, faster growing colonies were more affected by decreased pH. After this period of decreased pH, we conducted a recovery experiment, in which corals reared in the acidic treatment were brought back to control conditions. In this case, normal calcification rates were reached in both species. Overall, our results suggest that O. patagonica and C. caespitosa will both be affected detrimentally by progressive ocean acidification in the near future. They do not display differences in response between native and non-native species but do manifest differential responses depending on calcification rate, pointing to a role of the coral genetics in determining the response of corals to ocean acidification. [Copyright &y& Elsevier]