The ecology of the Mediterranean stony coral Cladocora caespitosa (Linnaeus, 1767) in the Gulf of Trieste (northern Adriatic Sea): a 30-year long story

Cladocora caespitosa is an endemic coral of the Mediterranean Sea and an important carbonate bioconstructor that adds 3D complexity to the habitat, thus increasing marine biodiversity. Despite its important role in the ecosystem, the real status of the population along most of the Mediterranean coastline is still poorly investigated and very little is known about the resilience of the species. Using non-destructive visual surveys, colonies of C. caespitosa were investigated by SCUBA diving in 2013 and 2015 at seven sites of the northern Adriatic Sea (southern part of the Gulf of Trieste). Data about colony size, index of sphericity and corallite diameter were collected. Almost all biometrical parameters differed significantly among sampling sites, showing low occurrence of the larger size classes compared to the abundance of small-sized colonies. This pattern of distribution is typical of long-lived organisms. The positively skewed colony size distribution could be due to both a high mortality rate of small colonies unable to reach larger size classes, and to a high fragmentation rate of colonies, related to a strong hydrodynamic forces. The northern Adriatic population of C. caespitosa has previously been investigated by Schiller, who reported size and abundance data of colonies from one site, at a depth range of 2–5 m. We compared these data with our findings from the same sampling site, adding new information about the ecology of C. caespitosa. After a 30-year period, the comparison shows a change in the size distribution of colonies, with a decrease of the small class and an increase of the medium class of colonies. In view of these conclusions, further assessments are required in order to evaluate the trend of the northernmost C. caespitosa population in the Mediterranean Sea.