Collecting spawn from the field is most appropriate for corals that reproduce through broadcast spawning. Thus, knowledge of local coral populations and their reproductive biology should be determined before collections begin. Identifying which coral species you will obtain larvae from prior to collections will help ensure higher success because it will allow you to determine the timing of mass spawning events.
Selecting Coral Species for Larval Collection
Healthy parent populations are key to success and larger colonies will produce more gametes. For collection, about 5-10 healthy colonies of reproductive size should be located in a site that is logistically feasible for night diving. For most species, colonies 30 cm in diameter or larger will be reproductive, though some colonies that are 20-30 cm in diameter may also spawn.
For some coral species with branching or lobed morphologies (such as Acropora, Pocilliopora, Orbicella), frequent fragmentation may yield parent populations that are highly clonal (or genetically identical) in some locations. Gametes from two parent colonies that are the same clone will not fertilize, so collecting gametes from highly clonal populations will likely yield low fertilization rates. For this reason, knowledge of clonal structure is helpful, but if not available, simply avoid making all collections from a ‘thicket’ or closely clumped branching colonies as these are more likely to be clones. If there are concerns about high clonality, gametes may be collected and mixed from colonies that are at least 10 m apart or from nearby sites if logistical resources allow.
Predicting Coral Spawning
Once a collection site and the target coral species have been identified, the next step is to determine when spawning will occur. Importantly, spawning times often vary between coral species and regions, so using prediction charts or local prior knowledge in your specific location can help better predict spawning. Most broadcast spawners release gametes in the night, although some spawn prior to sunset, and most spawning occurs on a somewhat predictable lunar schedule. Local predictions are often made by experts and may be accessed through the Coral Spawning Research Facebook Group.
There are several factors that influence the timing of mass coral spawning events. First, the rate at which sea surface temperatures increase dictates the month(s) when corals spawn. In the Caribbean, this means that most coral species spawn in the warm months between August and October. Second, the lunar cycle dictates the day when corals spawn. For instance, most coral species spawn for 2-3 days after the full moon. Lastly, the sunset time dictates the time when coral spawn, which usually occurs 30 minutes – 2 hours after sunset. Tips on predicting coral spawning can be found on the Caribbean Coral Spawning Webinar and prediction charts below developed by the Coral Restoration Consortium’s Larval Propagation Working Group.
As coral spawn collections will most often occur at night, it’s highly recommended that all divers are familiar with the site using daytime orientation dives to identify or place temporary landmarks within the sites. It is also recommended that visual tags easily viewed at night (such as subsurface floats, reflective flagging tape, or flashing lights) mark the general reef area where parent colonies are located, but not individual colonies as light can attract predators to gamete bundles. Divers should be prepared with collection equipment, primarily tent collectors with removable jars and extra lids. Collection equipment is listed in the Larval Propagation Shopping List below and the Caribbean Coral Spawning Webinar.
For hermaphroditic species, the appearance of a pink bulge in the polyp mouths, called setting, is a visual cue that a colony is preparing to spawn and that it’s time to deploy a tent collector over the colony. Release of the bundles generally occurs within 10-30 minutes after bundles appear in the polyp mouth. For gonochoric spawners, large syringes are the most common method to collect gametes in situ and should be collected as quickly as possible before gametes are diluted within the water column and are no longer visible.
As gamete bundles are released, the buoyant eggs will cause them to float up into the collector jar. Collector jars should be capped off when approximately 25% full, or within 10 to 20 minutes after spawning starts, to avoid gametes dissolving, being lost in the water column, or poor water quality in the jar. A new jar can be replaced on the collector if the colony is still spawning. Jars of gametes should be returned to boat or shore to proceed with the fertilization step.
Monitoring when corals spawn in your location can help you better refine your predictions for the next year. Click here for a coral spawning monitoring template developed by the CRC Larval Propagation Working Group.
- Know when and where to go look for spawning – timing is crucial for planning the dive and capturing the gametes.
- Be prepared – nighttime diving and boat operations require extra planning and experience.
- Branching and lobed species may have many high clonality within a site that will not fertilize, thus mixing gametes from colonies 10 m apart or some other sites may be needed.