Changes in coral reproduction following slight changes in temperature
Selina Ward
Centre for Marine Studies, The University of Queensland
Queensland, Australia
The effects of elevated temperature on coral reproduction and physiology were examined at Heron Island, Great Barrier Reef during manipulative experiments using a temperature control system in aquaculture tanks and by monitoring corals that had bleached in the field. During the 1998 Great Barrier Reef mass bleaching event extensive bleaching of corals occurred at Heron Island Reef. In March 1998, 200 corals were labelled and sampled on the reef flat at Heron Island. These were both bleached and normal colored corals of many species. Bleached colonies of all sampled species had lower densities of symbiotic dinoflagellates and lower chlorophyll a concentrations per algal cell and per cm coral tissue than normal colored colonies. In all sampled species there were significantly fewer eggs present in the bleached than unbleached colonies. In many cases there were no eggs present at all in the bleached colonies. In most species the eggs were significantly smaller in the bleached than the normal colored colonies and in all species there were significantly fewer polyps containing eggs or testes in the bleached than the unbleached colonies. The percentage of tissue made up by lipids in the bleached colonies was significantly lower than that of the normal colored colonies. Levels of MAAs (mycosporine like amino acids) were reduced in bleached colonies of some species.
In the manipulative experiments heated tanks were kept 2℃ warmer than control tanks and this treatment had severe effects on coral reproduction. Heated colonies spawned a month earlier than the control colonies. Rates of fertilization of coral gametes were reduced in the heat treatments and larval size and development were altered. In separate experiments using a range of temperatures from room temperature to 34℃, fertilization rates were extremely low at the high temperatures and the number of irregular embryos formed was significantly higher at all temperatures above 28℃ than at room temperature. Settlement of larvae was also significantly reduced above 28℃. This research demonstrates that small changes in temperature of seawater have a dramatic effect on coral reproduction.
