Variations in growth and skeletal characteristics of a temperate non-zooxanthellate colonial coral naturally living at CO2 vents

Ocean acidification may impact ecosystems reliant on calcifying organisms, potentially reducing thesocioeconomic benefits these habitats provide. This study investigates the response of growth and skeletalproperties of the non-zooxanthellate stony coral Astroides calycularis living in naturally acidified seawater ata Mediterranean CO2 vent (Ischia, Italy). Unexpectedly, contrasting patterns of calcification were found atpolyp and colony levels. Polyp linear extension rate, bulk skeletal density and net calcification rate increasedwith acidification (i.e., skeletal porosity decreased). At the colony level, a decrease of net calcification ratewas observed in acidified conditions, with colonies that extended less, were smaller, and were composed byfewer polyps, thus partitioning the available energy among less polyps. As a result, the single polyps had ahigher amount of resources available for calcification than in control conditions, and all their growthparameters had higher values. This unforeseen pattern of response to acidification was observed in this studyfor the first time, and contrasts with what previously observed in the Mediterranean solitary coralBalanophyllia europaea,whose polyps invest the calcification resources in linear extension rate to reach thesize at sexual maturity at the expenses of skeletal resistance (i.e. bulk skeletal density). In the colonial A.calycularis asexual reproduction is possible and corals may invest more on skeletal resistance without theurge to reach sexual maturity.