Deep Connections: Eastern Mediterranean Sponge Grounds as Refugia

Mesophotic sponge grounds (MSGs) are recognized as local biodiversity hotspots in the Eastern Mediterranean and have been suggested to serve as refugia for threatened species from shallower habitats. This study focused on the broadcasting sponge Axinella polypoides, a species that inhabits both shallow and mesophotic environments. The primary goal was to evaluate its genetic connectivity across two distinct scales: geographically, by comparing populations between the Eastern and Western Mediterranean, and bathymetrically, by examining shallow versus MSG populations along the Israeli coast. Understanding these connections was assessed to understand their role as a refuge.

Utilizing ddRADseq technology, an initial analysis of 106 samples yielded 2,295 SNPs, revealing significant genetic differentiation between Eastern and Western Mediterranean populations. Interestingly, within the Eastern populations, a state of panmixia was observed. A reduced dataset, comprising 95 samples and 5,400 SNPs, subsequently revealed subtle genetic substructure differences between shallow and mesophotic populations, particularly at Achziv. Moreover, shallow populations were generally less genetically divergent than mesophotic ones, except for the Rosh-Carmel mesophotic population, which showed higher gene flow with shallow populations. The Herzliya shallow population was identified as a key gene source.

These compelling results underscore the Eastern Mediterranean MSG's role as refugia for A. polypoides, and possibly for species with similar life-history traits. The intricate genetic links between shallow and deep habitats demonstrate a dynamic ecosystem crucial for maintaining genetic diversity. As lead author Morav notes in the paper, "Our findings emphasize the importance of protecting these habitats, especially given the increased anthropogenic and climate-driven pressures."