Comparing demersal megafaunal species diversity along the depth gradient within the South Aegean and Cretan Seas (Eastern Mediterranean)

Knowledge on biodiversity patterns of demersal megafaunal species in the Mediterranean and particularly in its eastern basin is still very scarce. In the present study, fine-scale diversity patterns in relation to depth were analyzed for three major megafaunal groups (fish, cephalopods and crustaceans) in three subareas of the eastern Mediterranean (Crete, Cyclades and Dodecanese islands). The analysis was based on data from the Mediterranean International Trawl Survey conducted during 2005–2014 and the relationship between depth and two different diversity measures (species richness and Shannon-Weaver) was examined using Generalized Additive Modeling (GAM) techniques. Species richness of fish decreased with depth in two of the three subareas (Cyclades, Dodecanese), while the opposite was true for crustaceans in all subareas. Cephalopods had higher species richness at intermediate depths, near the shelf break. Significant differences among subareas were found, with Crete showing a distinct species richness-depth pattern, which was more obvious for fish and cephalopods. The differences among subareas were also highlighted based on the occurrence of alien species of Indo-Pacific origin, which were more frequent in Crete. Our results suggested that the importance of depth-related factors in structuring communities was higher for cephalopods and less important for fish, and that Crete showed a distinct diversity-depth relationship, a fact that can be attributed to its specific geographical and oceanographic characteristics. These results support the current GFCM/FAO’s characterization of Crete as a unique geographic subarea. The findings of the study contribute to understanding the causes of underlying diversity patterns and would assist various environmental management actions, particularly those related to the establishment of marine-protected areas.