Composition and seasonality of a heterobranch assemblage in a sublittoral, unconsolidated, wave-disturbed community in the Mediterranean Sea.

The habitat preferences of the Mediterranean heterobranchs are generally well known; however, some environments, especially those considered poorly structured or of modest biodiversity, have been scarcely studied. Here we evaluate the annual variations in species composition, seasonality and diet preferences of the sea-slug assemblage in an environment of gravel and coarse sand at Noli Cape (northwestern Ligurian Sea), an unconsolidated wave-disturbed community, between 5 m and 20m depth. The assemblage, studied by means of 64 scuba-diving surveys from January 2015 to December 2016, was found to be extremely rich in terms of biodiversity, reaching 45 recorded species in just 3,000m2 (including six new records for the Ligurian Sea), of a total richness, in the entire basin, of about 150 species. Seasonality is a strong feature of the studied species, showing a high frequency of species appearance in winter; overall, 60% of the recorded sea slugs showed a cold affinity, even though the most common species were chiefly endemic ones. An expected finding, in view of the dominant environment, was that most of the species were typical of hard-bottom communities. Despite the high total species richness, the main feature characterizing the assemblage was the poor representation of sponge-eaters (13%), while the majority of the species were cnidarian-eaters (29%), or omnivorous and true predators (24%). Some common sponge-eaters were absent or very rare, because of the lack of their prey. The species composition of the Noli Cape heterobranch assemblage appears strongly related to the presence of their prey which, in turn, directly depends on the effect of hydrodynamism that can be considered the primary constraint. At the same time, the presence of many boreal species, and the evident seasonality with peaks of biodiversity during cold periods, suggest that temperature is also a significant variable.