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1 Marine Biological Association of the United Kingdom, Citadel Hill, Plymouth PL1 2PB, UK
2 Institute of Marine Biology of Crete, PO Box 2214, 71003 Iraklio, Crete, Greece
3 Zoological institute and Zoological Museum, University of Hamburg, Martin-Luther-King Platz 3, D-20146 Hamburg, Germany
4 School of Ocean Sciences, University of Wales, Bangor, Menai Bridge, Gwynedd LL59 5EY, UK
Hydrothermal vents, at depths varying from the littoral zone to water depths of 115 m, have been explored around the islands of Milos and Santorini in the Hellenic Volcanic Arc. The biota were surveyed by scuba divers and with a remote operated vehicle as well as by soft-bottom sampling with grabs and corers. No species specific to hydrothermal areas were found at any of the sites investigated. Animals in the immediate vicinity of the vents belonged to opportunistic species, such as the polychaetes Capitella capitata, Microspio sp. and Spio decoratus. The nassariid gastropod Cyclope neritea was the dominant macrofaunal species found in the bacterial mat areas at Milos which overlay hot brine seeps. At all rocky hydrothermal sites deeper than 35 m water depth the echiuran Boniellia cf. viridis was observed. Around the periphery of the seeps the meiofaunal community was dominated by the nematode Onchlaimus camplyoceroides. Few nematodes were found in the centre of the hydrothermal brine areas. At Santorini the bacteria at the venting sites were dominated by iron bacteria, whereas at Milos large globular sulphur bacteria, Achromatium volutans, covered the hydrothermal brine seeps. Unlike deep-sea vents, little of the biomass production was due to symbiotic associations between animals and chemoautolithotrophic bacteria, although a few stilbotnematine nematodes were found at the periphery of the brine seeps at Milos. Macrofaunal biomass reached a maximum of 80 g m–2 around the vents, compared with
c 500 g m–2 at hydrothermal vents elsewhere. Much of the bacterial biomass appeared to be exported from the sites.
