Estimating intraspecific diversity at different geographic scales in the Mediterranean Sea: the case of Spongia officinalis (Porifera: Demospongiae)

Identifying spatial patterns of intraspecific variation of marine sessile invertebrates is crucial for the establishment of comprehensive conservation strategies, particularly given the threat of recent climate change effects on their coastal Mediterranean habitats. The harvested bath sponge Spongia officinalis, having recently undergone several mass mortality incidents, is a model species in this context since it is distributed throughout the Mediterranean. A sample set from three distinct regions of the Mediterranean was analyzed, corresponding to its main biogeographic zones (eastern - western Mediterranean and the Alboran Sea), thus allowing for the identification of barriers to gene flow occurring along the basin. A supplementary network of sampling locations inside each region, separated by a distance range of 10 to 500 kilometers was additionally surveyed for fine-scale estimation of diversity patterns. Variation was evaluated with partial mitochondrial cytochrome oxidase subunit I sequences, along with a set of high-resolution polymorphic microsatellite loci we recently developed for the species. While mitochondrial markers distinguished individuals only between the main Mediterranean basin and the Alboran Sea, microsatellites showed a more complex pattern, providing evidence for additional separation across the biogeographic zones, and revealed considerable structuring at the level of different geographic locations.