Depth profile of Aplysina ssp.: morphological, histological and biochemical aspects and their role in species distinction

In the Mediterranean Sea, two species of the genus Aplysina Nardo, 1834 (Demospongiae, Verongida, Aplysinidae) co-occur, both described by Schmidt in 1862. The first one was named Aplysina aerophoba (formerly Verongia aerophoba) due to the change of its bright yellow pigment uranidine to dark blue when exposed to air. The second one was named Aplysina cavernicola (formerly Verongia cavernicola) due to its preferred shaded habitats, e.g., caves. The definition for species differentiation was mainly based on ecological and morphological characteristics. However, as both species display a high degree of phenotypic variability, distinction solely based on morphological attributes is not suitable in all cases. In addition, differences can be found in the species-specific microbial composition of symbionts and metabolite pattern. For the first time, we report an investigation on both Aplysina species along one single transect on an underwater slope in the Adriatic Sea, and the successful species distinction using solely the different biochemical composition of metabolites.