The population of Errina aspera (Hydrozoa: Stylasteridae) of the Messina Strait (Mediterranean Sea)

Errina aspera is the only species belonging to the family Stylasteridae known from the Mediterranean Sea. The distribution of this species includes the Messina Strait, the Gibraltar Strait and the Atlantic coast of Morocco. In spite of the taxonomic and biogeographical importance of this species, no studies were conducted on its population structure in the Messina Strait. Here we report a study carried out with the aid of a ROV on a horizontal transect 350 m long at a depth comprised between 83 and 105 m where an extremely dense population of this stylasterid (101.4 +/- 13.3 m(-2)) was recorded. The main components of the benthic community other than E. aspera are the coralline alga Lithothamnion philippii and the white zoanthid Epizoanthus sp. The colonies of E. aspera have a height ranging between 1.2 and 18.8 cm (4.1 +/- 0.1 cm on average); they generally show a bi-dimensional, fan shaped morphology, but three-dimensional, brush-like specimens are also observed. The size-frequency distribution of the colony height is unimodal with a mode in the classes of 3-5 cm and a long tail of rare large colonies suggesting that the current may act as a limiting factor for the colony growth. The association with the gastropod ovulid Pedicularia sicula is confirmed.