Spatial diversity patterns and environmental drivers of soft bottom annelids across marine caves (Alboran Sea, SW Mediterranean).

Marine caves are ecologically distinctive habitats affected by steep environmental gradients, yet the macroinfauna inhabiting their soft bottoms remains poorly understood. This study provides the first description of the diversity and composition of soft-bottom annelid assemblages in seven marine caves along Alboran Sea coast (SW Mediterranean Sea). We identified 76 different annelid morphospecies, most of them polychaetes belonging to 30 families. Community structure and species composition differed significantly among caves, supporting the idea of marked "individuality" of macrobenthic assemblages in cave sediments. Potential drivers of this community structure were evaluated (organic matter content, granulometry, trace, minor, and major elements and geographical distance). Most of the variation in annelid assemblages was not explained by the measured environmental variables. Organic matter content was partly associated with the distribution of some opportunistic deposit-feeding taxa, whereas granulometry or geographical distance among caves showed no detectable effects. Finally, the lack of detectable environmental drivers together with the high species turnover revealed by β-diversity analysis, suggest that marine caves may function as isolated ecological islands, where community structure would be partly shaped by stochastic processes during larval settlement. However, further research is required to better understand the processes underlying community assembly in marine caves.