Lunar periodicity of planula release in the reef-building coral Stylophora pistillata

Stylophora pistillata is a dominant stony coral on reefs in the Red Sea, but the levels and periodicity of planula release in this coral are not well understood. We examined patterns of planula release by colonies under laboratory conditions in outdoor tanks at Eilat during 1998, then tagged and returned the corals to the adjacent reef. In 2003, we re-collected them and re-estimated their planula release patterns. In both years, the number of planula larvae released per night varied widely between colonies, and did not correlate with coral colony volume or diameter. Planula release rate per live coral volume was 5.6-fold lower in 2003 than in 1998, while coral volume increased 5.5-fold during this period. In both years, colonies of S. pistillata released planula larvae on a lunar cycle during the 4 mo of peak planulation (March to June). Peak release was on Night 6 following the full moon each month during 1998 and on Night 2 during 2003, with most planulation occurring between Nights 1 and 12. Collection of released planulae from randomly selected colonies of S. pistillata in the field during both years revealed the same lunar cycle of planula release as under laboratory conditions. In field colonies, the number of planulae released per coral was 3-fold lower in 2003 than in 1998. The observed reduction in planula release between years in the field parallels documented declines in coral recruitment and reef growth at Eilat, and may have resulted, in part, from anthropogenic impacts to local reefs. Added effects of coral aging in tagged experimental colonies may have caused the larger observed reduction in planula release observed under laboratory conditions. The proximate factors that regulate the above lunar periodicity of planulation are thought to include night irradiance patterns and tidal cycles, which also may be linked to gametogenesis cycles. Lunar periodicity ultimately may have developed to enhance the dispersal of released propagules during neap or spring tides, or to maximize fertilization of gametes through tight synchronization of coral reproduction among colonies.