Monday, May 21, 2007

Coral Broadcast Spawning

Corals have two types of reproduction – sexual and asexual. It is the sexual reproduction that results in the formation of a new genetically different offspring and reproduction, producing clones within a colony. Coral have two strategies for sexual reproduction- broadcast and brooding. Brooding species release the male gametes into the water column. When the gametes come in contact with the surface of a colony of the same species they are absorbed into the tissues were fertilization of the female gametes occurs. The fertilized eggs develop into a planulae larva. Once fully developed the planulae leave the parent colony and become planktonic before eventually settling on a suitable substrate and growing, forming a new genetically distinct colony. In contrast, broadcast spawning corals, as shown in this video, release both male and female gametes into the water at the same time. Only viable for a couple of hours, the density and timing of gametes released is critical to maximize the probability of successful fertilization in the water (Oliver & Babcock 1992). Positively buoyant, these gametes float to the surface and can form slicks on the surface of the water. These slicks can be seen from low flying aircraft. Once fertilization has occurred, the planulae may remain planktonic for up to thirty days. Mass spawning of different species can occur during the same day. A timing divergence of a couple hours however, may occur and is vital for reproduction isolation (Knowlton et al. 1997). In the case of the Montastraea species complex, differences in gamete release times appears responsible for the speciation between closely related reproductive sibling species Montastraea annularis, Montastraea franksi, and Montastraea faveolata (Levitan et al. 2004). This video sequence shows the release of gametes from a broadcast spawning event.