Sexual reproduction of Pocillopora damicornis (Cnidaria : hexacorallia) at high latitude off Durban, South Africa.

Abstract

Pocillopora damicornis is one of the best-studied and most widespread of corals. Previous studies on its sexual reproduction have yielded a diverse and contradictory view of its reproductive strategy. This coral has long been considered a typical hermaphroditic brooder. However, recent studies have demonstrated that its dispersal may also occur through broadcast spawning of gametes. Since both modes of reproduction have advantages for dispersal, P. damicornis reproductive traits are probably driven by environmental pressures. High-latitude habitats are considered marginal for corals since they fall beyond the “normal” limits of coral development. They provide, therefore, a good opportunity to study reproductive strategies that have evolved under different environmental pressures. The pattern and timing of reproduction were studied in P. damicornis on the high-latitude reefs of Durban, KwaZulu-Natal, South Africa (29°S). Branches were randomly selected from colonies on a monthly basis from October 2007 to April 2008. After fixation and decalcification, the coral tissues were processed for histology and cross-sectioned to reveal the presence and stages of gametes within the polyps. The number and size of gametes were estimated using image analysis. P. damicornis in Durban is hermaphroditic and all polyps contained oocytes and spermaries at the end of the breeding season. Since no brooding of embryos or planulae was found in the 175 polyps analysed during the seven-month study period, this coral is presumed to be a broadcast spawner in South African waters. P. damicornis invested seven to eight months in gametogenesis and oogenesis was initiated one month before spermatogenesis. Gametogenesis was confined to the warmer months of the year, commencing during the increase in seawater temperature at the onset of summer. Gamete development was poorly synchronised between colonies during the initial months of breeding, but mature oocyte stages were dominant in all polyps at the end of the breeding season. Spawning was inferred from the disappearance of gametes in April 2008, and may have occurred during full moon in March 2008. This happened before the drop in temperature at the end of summer. P. damicornis in KwaZulu-Natal exhibited a pattern and timing in its reproduction similar to that found in south Western Australia at 32°S. Keywords: coral; Scleractinia; Pocillopora damicornis; high latitude; sexual reproduction; South Africa.