Hydrobiologia 426: 113–121, 2000. G. Liebezeit, S. Dittmann & I. Kröncke (eds), Life at Interfaces and Under Extreme Conditions. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 113 First survey of sessile communities on subtidal rocks in an area with hydrothermal vents: Milos Island, Aegean Sea Silvia Cocito 1,∗ , C. Nike Bianchi 1 , Carla Morri 2 & Andrea Peirano 1 1 Marine Environment Research Centre, ENEA Santa Teresa, P.O. Box 316, I-19100 La Spezia, Italy ( ∗ author for correspondence) 2 DIPTERIS, Universit` a di Genova, Corso Europa 26, I-16132 Genova, Italy Key words: hard substrate epibenthos, community zonation, shallow hydrothermal vents, bioconstruction, Aegean Sea Abstract The major epibenthic communities on subtidal rocks of Palaeochori Bay and the marine tract on the southern coast of Milos Island (Greece) were described down to 44 m depth. Six sites were investigated by snorkelling and SCUBA diving. Samples, photographs and video images were also taken to integrate information. Three out of the six sites were close to hydrothermal vents, a common feature in the area. In total, nine major epibenthic communit- ies were found, most of which were characterised by a diverse algal growth down to the maximum depth explored. Macrobenthic cover was severely reduced only in the close proximity of vents, a white flocculent bacterial mat covering the rock at the point from which fluid escaped. Large-scale effects of vents on the epibenthic communities were not detected. However, the abundance of species with warm-water affinity was recognisable in both algal and animal dominated communities, which may be related to higher winter temperature in the vent area. Epifaunal communities under overhangs were composed of distinct groups of suspension feeders at vent as compared to non-vent sites: this might indicate differences in trophic conditions. Mounds of the bioconstructional coralline alga Mesophyllum lichenoides were conspicuous only at vent sites, thus suggesting enhanced biodeposition of carbonates due to vent activity. Introduction The discovery of deep-water hydrothermal vents in the last decades revealed unusual benthic communit- ies dependent on chemosynthesis (Tunnicliffe, 1991). Communities were more recently found also around vents in shallow-water and continental shelf areas, which showed analogies with their deep-sea counter- parts but also contrasting patterns (Fricke et al., 1989; Tarasov et al., 1990; Dando et al., 1991; Giménez & Marín, 1991; Hashimoto et al. 1993; Kamenev et al., 1993; Acunto et al., 1996). In the Aegean Sea, hydrothermal vents occur along the so-called Hellenic Volcanic Arc, an area with strong volcanic activity (Fytikas et al., 1984). Most published studies on benthic communities around vents in this area deal with soft-bottoms (Diapoulis et al., 1994a; Dando et al., 1995a; Thiermann et al., 1997), whereas epibenthic communities on hard substrates have received little attention. In this paper, we describe the sessile epibenthic communities on subtidal rocks at Milos, an island in the Hellenic Volcanic Arc where hydrothermal activit- ies have been more intensively studied (Dando et al., 1995b). Previous studies on hard-bottom benthos at Milos took into account algal communities only, and the effect of vents was not investigated (Coppejans, 1974; Diapoulis et al., 1994b). Materials and methods Sessile epibenthic communities were investigated in June 1996 in Palaeochori Bay and in the near off-shore tract on the southern coast of Milos (Bianchi et al., 1997). Palaeochori Bay is a sandy bay approximately 1.4 km wide. The bottom of the bay is covered by sea-