Mesophotic rocks dominated by Diazona violacea: a Mediterranean codified habitat

The football ascidian Diazona violacea was observed in three different sites in the Aeolian Archipelago (Southern Tyrrhenian Sea), from 53 to 116 m depth. A large population of this ascidian was found on rocky habitat, enhancing its three-dimensionality and constituting a peculiar facies. Colonies of D. violacea showed a mean density of 0.87 ± 0.09 colonies m−2 and a maximum of 4 colonies m−2. The number of zooids was used as a proxy of colony size and age (assuming that the higher the number of zooids, the larger and older is the colony), and the population showed a leptokurtic distribution with the dominant presence of colonies with fewer than 100 zooids. Large aggregations of this species proved to play an important role in terms of seabed complexity, influencing the underwater landscape and worth of being recognized as a relevant type of biogenic habitat (although ephemeral) that can characterize mesophotic rocks in the Mediterranean Sea.