-1 Biological characterisation of a subtidal tunnel in Sa˜o Miguel island (Azores) J. MICAEL, J.M.N. AZEVEDO and A.C. COSTA * CIRN – Centro de Investigac ¸a ˜o de Recursos Naturais and Departamento de Biologia, Universidade dos Ac ¸ ores, Rua Ma ˜e de Deus, 9500 Ponta Delgada, Portugal; *Author for correspondence (e-mail: accosta@notes.uac.pt; phone: +35-1296650466; fax: +35-1296650100) Received 16 August 2004; accepted in revised form 21 January 2005 Key words: Biotic zonation, Sessile organisms, Special area of conservation, Stratification, Submarine lava tunnel Abstract. Biological characterisation of the Santa Clara submarine tunnel in S. Miguel Island (Azores) is made. Spatial description of the cave is provided and zonation and stratification pat- terns of sessile epifauna are described. Profiles were based on sketches drawn in loco. Fauna census was conducted with non-destructive techniques: in loco observations complemented by still images. The tunnel is a 14-m long lava tunnel at 14 m depth. It is obstructed in the north end where a small opening permits light penetration and some water circulation. There are two entrances, two twilight zones and one middle dark zone. The floor of the tunnel is predominantly occupied by sponges, the polychaetes Pomatoceros triqueter and the coral Caryophyllia smithii. On vertical walls there were some encrusting sponges and a few corals such as C. inornata. On the ceiling the major groups found were individuals of C. inornata and Spirobranchus polytrema. Several crustacean species (e.g. Dromia marmorea, Scyllarus arctus and Stenopus spinosus) and fishes with sciaphylic habits (e.g. Apogon imberbis, Conger conger, Phycis phycis and Gaidropsarus guttatus) were also seen. Introduction In the European Union, a legislative instrument in the field of nature conser- vation, the Habitats Directive (Council Directive 92/43/EEC), provides for the creation of a network of special areas of conservation, called Natura 2000. Submarine caves constitute one of the listed habitats (code 8330). The Euro- pean Commission (EC 1999) considers that these habitats are sufficiently represented in the proposed sites for the Macaronesian region. However, their study is scarce and fragmentary. Checklists of species characterising marine cave biotopes under the CORINE Biotopes Classification, do not exist in the Azorean Region. Thus, faunistic and ecological studies are required in order to characterise these habitats in the Region. Submarine caves are distinct from other littoral marine benthic habitats due to sharp gradients of physical parameters such as light, hydrodynamics and sedimentation that are dictated by their geostructural characteristics (Bibiloni et al. 1984). The origin and the geomorphology of the cave determine physical and biological continuum. Variations in light intensity, hydrodynamics and Biodiversity and Conservation (2006) 15:3675–3684 Ó Springer 2005 DOI 10.1007/s10531-005-1537-2