Population Genetic Structure of Corallium rubrum in the Mediterranean Sea: Diversity, Phylogeography, and Bathymetric Patterns

Abstract

Among Mediterranean cnidarians, the octocoral Corallium rubrum is the most harvested species, mostly owing to the exploitation of its red skeleton for jewellery purposes. Red coral colonies can be found on vertical cliffs, in caves and in crevices from 20 to 100 m depths. Red coral also occurs in deeper water where it dwells on scattered boulders, rocky outcrops and seamounts down to about 1,000 m depth. Most red coral banks above 80 m depth have been overharvested all over the Mediterranean Sea. Nowadays commercial harvesting occurs mainly between 80 and 130 m, where older, large-sized and highly valued colonies still occur. Habitat features affect larval dispersal and genetic structuring of populations across spatial scales and depth gradients. In this chapter, knowledge on population genetic structure and diversity in C. rubrum is summarized. In Octocorallia, mitochondrial markers have extremely low polymorphism levels and do not reveal genetic patterns in shallow water populations. Conversely, they reveal weak phylogeographic patterns of structure in deep water populations. Microsatellite loci showed strong genetic structure in shallow water populations from Mediterranean basin scale down to populations separated by 10 m. In the Western Mediterranean the pattern of genetic structure reflects a combination of regional clustering and isolation by distance. Marked chaotic structuring was detected when downscaling the studies to very small spatial distances (1 m). Between colonies within a 0.5 m2 square, significant spatial genetic structure was found. Based on this result, the estimated effective larval dispersal ranges between 20 and 30 cm. This also suggests that breeding units are restricted in space and are composed of related individuals. A similar small-scale heterogeneity pattern of genetic structure was also found among populations inside submarine caves. Depth appeared to be one of the factors affecting genetic diversity, with lower diversity observed in Tyrrhenian and Ligurian deep populations. A threshold in genetic diversity occurs across 40–50 m depth. Moreover, patterns of structure differ between shallow and deep-water populations. Genetic data clearly show that connectivity in C. rubrum is limited, and suggest that deep red coral banks cannot act as refugia for shallow water populations as hypothesized for some tropical species. Studies on the response to thermal stress suggest that genetic structure could be related to differences in adaptive abilities, with major implications concerning the ability of the species to respond to future climate change. Conservation of red coral in the Mediterranean Sea must take into account that the species consists of an array of metapopulations, structured both geographically and by depth, which have to be considered as discrete management units