THE CONTROLS ON GROWTH FORM OF INTERTIDAL MASSIVE CORALS, PHUKET, SOUTH THAILAND

Massive intertidal corals of the genera Goniastrea, Platygyra and Pori tes from the fringing reefs of Phuket, Thailand, suffer solar damage ( and ensuing partial mortality) on subaerial exposure in the January-Ap ril period most years. As spring low tides fall at dawn (low sun altit ude) and in the afternoon (when the sun altitude may be high) the lesi ons are consistently on the west sides of colonies and asymmetric grow th forms develop. Recovery extends from the surviving polyps and a ske letal epithecal flange disconformably overlies the former area of part ial mortality. As intertidal colonies grow, each new increment of part ial mortality becomes progressively narrower as the incidence of solar damage becomes more frequent. Those corals of one species all at the same elevation on the reef flat develop similar surface morphologies a s they suffer the same irradiation effects. Bioerosion (predominant on the outer reef flat) and the accumulation of fine sediment (predomina nt on the inner reef flat) occur preferentially on the dead (lesioned) areas of skeleton, which eventually result in those colonies at the o uter reef flat being eroded away while those inshore are preserved rec ording the times and extents of partial mortality X-radiography of cor al slabs can help date the major and minor events of partial mortality . The intertidal massive corals of the Phuket fringing reefs show thre e major recent low-water events, one in 1982/83, one in 1986/87 and th e third in 1990/91. These perturbations in growth reflect the unusuall y low tides recorded at the nearby tidal station and probably relate t o El Nino climatic events.