Anthozoa from a subtidal hydrothermal area of Milos Island (Aegean Sea), with notes on the construction potential of the scleractinian coral Madracis pharensis

Eleven species of Anthozoa (two Octocorallia and nine Hexa-corallia) were found on subtidal (2 to 90 m depth) hard substrata off Palaeochori Bay (SE Milos Island), an area characterised by submarine hydrothermalism. For each species, depth distribution and habitat were recorded. Their ecology at Milos was consistent with previous knowledge in other areas of the Mediterranean Sea. The corallites of Caryophyllia inornata (and perhaps other species) were comparatively small-sized, thus apparently conforming to the so-called 'eastern nanism' (i.e., high occurrence of dwarf individuals in the Aegean as compared to the western Mediterranean) for many Mediterranean benthic species. Madracis pharensis was very common, especially in sites close to vents, locally covering up to 30% of the primary rock with a calcareous crust. Skeletal bulk density (1.95-2.10 g-cm -3 ) was higher, and porosity (29%) lower, than in typical constructional corals. Layering of distinct generations of colonies suggests the significant accretion of a persistent biogenic substratum. It cannot be excluded that the construction potential of the species is enhanced by the hydrothermal activity: impulses of heat from vents maintain water temperature warmer during winter, and emission of Ca and CO 2 may favour the biological deposition of carbonates and/or the activity of symbiotic zooxanthellae.