Abstract
Abereiddy Quarry is shown in Plate I. It lies at 51° 56′N, 5°13′W (Ordnance Survey Reference SM 795315), 6 km northeast of St Davids in Pembrokeshire (Text fig. 1). The quarry, a disused slate working, is surrounded by high cliffs except to the west where there is a channel open to the sea. In 1902–3, operations ceased and the quarry began to fill with fresh water and sea spray. During 1932–3 the west wall was breached to allow access to the sea so that the basin could be used as a harbour. Abereiddy is now a deep, fully saline marine basin which, at low water, is 66 m across from north to south with a surface area of 0.356 ha and a maximum depth of 24 m. In this situation it has been possible to observe sublittoral rocky sea-bed communities in an extremely sheltered area through a summer thermocline and associated oxycline. Abereiddy Quarry has previously been investigated by Bailey, Nelson-Smith & Knight-Jones (1967), who published details of the vertical distribution ofSpirorbisspp. The biology of serpulid worms in Abereiddy Quarry has been the subject of continuing investigations carried out from University College Swansea (E. W. Knight-Jones, personal communication).