Threatened by stony tissue coral loss disease, the Diploria labyrinthiformis (grooved brain coral) off the coast of Honduras recently showcased its rare broadcast spawning—a fleeting event only observed for less than three minutes a handful of times each year, resembling pink snowfall scattered throughout the water column. After being treated with antibiotics, this moment offers a glimmer of hope for the species’ survival.

Diploria labyrinthiformis, nicknamed DLAB by researchers, is an important reef-building coral species in the Caribbean. These corals are hermaphroditic, producing male and female reproductive structures and releasing both eggs and sperm into the water column through broadcast spawning. These mass reproduction events usually occur once a year. Polyps form egg-sperm bundles in response to environmental cues. Within a very small window of time, colonies of the same species release millions of of these buoyant gametes synchronously, which then float to the surface, ready for fertilization. This method maximizes reproductive contact, increasing the likelihood of finding and fertilizing a matching bundle, while ensuring optimal environmental conditions for the dispersal, development, and recruitment of coral larvae. DLAB is an ideal coral species to monitor reproduction, as their mass spawning events occur during daytime and several times a year.

How are all of these separate colonies able to time this magnificent mass gamete release across the reef?

Synchronous coral spawning is regulated at three different time scales, month, day, and hour. The month is based on the rate of gamete maturation, which is largely determined by seawater temperature. Scientists have come to determine DLAB spawning to occur between the months of April and October, depending on location. It is expected that spawning day each month is determined by corals sensing environmental changes regulated by the lunar cycle, such as tidal changes and moonlight. Spawning hour is then determined by sunset time.

Throughout May and June, predicted to be the peak DLAB spawning months in Utila, we set out every evening two hours before sunset to monitor DLAB colonies with Bay Islands Conservation Association (BICA). DLAB spawning was predicted to occur 8-12 days following the full moon, 0-75 minutes before sunset, and can be as short as 1-5 minutes. Conveniently, butterflyfish also serve as predictors of spawning, as they often feed on coral gamete bundles. Large aggregations suddenly swarming a colony, preparing to feast, is a good indication that this beautiful moment is near. Extremely close bonds were formed with the individual coral colonies that we observed for hours every evening, deeply wishing for reef prosperity to create more beautiful and resilient coral babies.

Over the course of monitoring, we observed 4 colonies spawn in May between 5:10 - 5:14 pm, 10-11 days following the full moon. Some colonies were particularly special to observe, as they had been previously treated for Stony Tissue Coral Loss Disease (STCLD), demonstrating hopeful recovery following antibiotic delivery. Witnessing this incredibly timed natural event was completely unreal- resembling an underwater snowstorm, pink and orange gametes exploding through the water column.

However, there are many disruptions to coral spawning interfering with the natural cues utilized to ensure synchronicity, such as artificial light, pollution, coastal developments, and warming temperatures. If a factor such as light pollution causes some colonies to spawn 1-3 days closer to the full moon compared to unlit reefs, this could drastically decrease fertilization probability and survival. Given the reliance on delicate and reliable environmental conditions, these are very serious threats to coral reef health, easily preventing a spawning event from successfully taking place.