Written By Timothy Bateman
Every year the Great Barrier Reef, among many other tropical coral reefs, witnesses an event of epic proportion. Mass coral spawning is a sight to behold and entices us to wonder how all of the individual colonies across hundreds of miles of reef can coordinate their biological clocks to synchronize the release of eggs and sperm into the water to ensure fertilization and maximize reproductive success. The key to this phenomenon is that corals entrain their circadian rhythm to similar biological stimuli. In other words, corals use similar natural environmental cues to tell them when it is time to release their eggs and sperm for reproduction. For many years the exact cues that signaled this event remained a mystery, but a number of studies have unearthed just how corals are able to pull off the timing necessary to harmonize reproductive efforts.
The types of environmental cues that entrain coral reproduction can be categorized as ultimate and proximate factors. Ultimate factors are those that inherently affect coral reproductive success and include large scale environmental processes such as wind speed, tidal phase, and diurnal (day-night) cycles. Proximate factors are those that synchronize and provide the final cues for corals to undertake their reproductive spawning.
One of the most prominent ultimate factors affecting coral spawning is wind speed. While wind speed might not occur to most as an immediately important factor for corals, it does indeed significantly affect coral spawning. Wind speed affects ocean currents that can ultimately allow coral eggs and sperm to form surface slicks that increase fertilization success. If wind speeds are too high, ocean currents can carry eggs offshore before being fertilized.
The performance of coral gametes is directly affected by water temperature making it a crucial ultimate factor in the timing of spawning. Unlike wind speed, temperature affects all stages of coral reproduction, including the development of eggs and sperm inside the coral parent. Thus, corals typically begin oogenesis, or egg production which takes longer, during the cooler periods of the year. While spermatogenesis, or sperm production which takes less time, begins closer to spawning during warmer periods. The final maturation of sex cells, also called gametes, is synchronized as temperatures rise shortly before mass spawning, and most corals spawn as temperatures approach the annual maximum.
Light acts as both an ultimate and proximate cue because the length of day acts as a temporal signal, while the energy provided by sunlight is a critical energy source for corals during spawning. Work by Dr. Van Woesik and colleagues showed that insolation (the amount of solar radiation) was actually a better predictor of coral spawning than temperature.
Specifically, moonlight is a key proximate factor that can entrain coral spawning within minutes for certain species. Dr. Levy and colleagues have shown through numerous studies that corals can detect blue light, including moonlight, through specialized light sensors known as cryptochromes and opsins. Moonlight is relevant to spawning because it informs the coral of the tidal cycle which can affect the water currents which dictates where released eggs and sperm will travel in the water column. By correlating the release of eggs and sperm with oceanographic factors such as tides and currents, corals can maximize the potential for fertilization and increase their reproductive fitness.
Similarly, sunset is a cue for the spawning of many coral species. While a few species spawn during the day, the vast majority spawn at night, often shortly after sunset. By spawning after sunset, not only do corals receive a light cue to synchronize across species, but spawning at night also reduces the chance that gametes will become food for visual predators.
As simple as corals seem, their reproductive cycle is, in fact, much more complicated than previously thought. The synchronization of spawning is entrained by a suite of subtle cues that facilitate the timing and thereby reproductive success of broadcast spawning corals. And there is much more still to learn about the reproduction of reef corals worldwide!
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