The production of new coral larvae is crucial to the maintenance of coral populations for two reasons. First, during this life phase corals can disperse and colonize new locations, gaining access to new sites that potentially have better environmental conditions for coral growth and survival. Second, sexually produced larvae are the only means to add new genetic individuals to a population, creating a diversity of traits that allow corals to adapt or respond to environmental conditions in different ways. In contrast, fragmentation produces clones and does not increase genetic diversity.
There are two main reproductive modes used by corals to produce larvae. Most large and abundant reef-building species are broadcast spawners, which release eggs and sperm into the water column. The subsequent processes of fertilization and larval development occur at the surface of the ocean. Other species of corals are brooders, which undergo internal fertilization and release their offspring as larvae, which is a relatively advanced stage of development because they are ready to settle soon after release. Broadcast spawners are the species that undergo synchronized mass spawning events that can provide opportunities to collect large volumes of gametes for larval propagation. Brooded larvae can also be collected for propagation, but these species tend to have smaller parent colonies yielding smaller volumes of larvae, which are good for research but less likely targets for large-scale restoration.
Information on reproductive modes of common Caribbean corals can be found on the Caribbean Coral Spawning Webinar and the Coral Restoration Consortium’s Larval Propagation Working Group Page.
In addition to the two reproductive modes, corals can either be hermaphroditic or gonochoric. Hermaphroditic species spawn both eggs and sperm together from each polyp. The eggs and sperm are spawned together in a bundle that protects them from dilution for a few minutes while they float up to the water surface, allowing them to be more easily collected.
Gonochoric species have colonies with separate sexes (male colonies produce sperm and female colonies produce eggs). Collecting gametes from these species is a greater challenge because gametes rapidly dilute in the water column, so collection must be done quickly. In addition, you must ensure that both sexes are represented during collections, so parent colonies must be identified prior to collection. The best strategy is to go to a site with many colonies of the species and watch to determine if colonies are spawning egg or sperm cells. If you only have access to a few colonies, take small samples prior to spawning to determine the sex of the colony.
The first task is to determine the goals of your restoration project and the coral species you will target for sexual propagation. This information will allow you to identify whether the species is a broadcast spawner, brooder, hermaphroditic, or gonochoric, and identify reef sites with sufficient parent colonies and the methods needed for collecting coral spawn.