Population genomic structure of the sea urchin Diadema africanum, a relevant species in the rocky reef systems across the Macaronesian archipelagos

The sea urchin Diadema africanum is a macro-herbivore found in the rocky reef systems of the West African region and Macaronesian archipelagos. Over several decades, high densities of this species have generated marine barrens in certain areas at the Canary Islands. In contrast, more recently, during the last few years, the species has suffered mass mortality events that continue to the present day. In this study, we used 9,109 Single Nucleotide Polymorphisms (SNPs) and a fragment of a mitochondrial gene to evaluate the species' population structure, effects of mass mortalities on its diversity, and potential local adaptation across the Canary Islands and Cabo Verde. Our research provides compelling evidence of low genomic diversity and homogeneity across the studied area for neutral markers, along with recent demographic fluctuations. The high connectivity among distant areas likely allows a rapid recovering of the populations from local mortality events. Interestingly, we also observed genomic sub-structure from 405 SNPs identified as candidate loci under selection for seawater temperature. The lack of divergence among distant sites and the low diversity found can be attributed to the species' divergence from a small ancestral genomic pool, followed by a contemporary demographic expansion, and ongoinggene flow.