Featured Image Credit: iStock. by Getty Images
By: Eva Gruber
Mass spawning by corals represents one of the most prominent examples of synchronized behavior on the planet. How does an organism with no central nervous system coordinate this event with its fellow and potential mates? The answer lies in the timing of different environmental cues. The primary cue is actually ever-so-romantic moonlight.
Corals are sessile animals, fixed to one spot for the duration of their life. Corals can reproduce asexually by budding off little clones of themselves. However, this does not increase genetic diversity, which can threaten populations. So, corals (and many other marine sessile animals) conquer this challenge by engaging in synchronized broadcast spawning.
During synchronized broadcast spawning, corals will simultaneously release their gamete bundles (containing both egg and sperm). These gamete bundles will float to the surface and break apart. There, with current and wave action, cross-fertilization occurs. Viola! Sexual reproduction.
Image Credit: Jim Maragos / U.S. Fish and Wildlife Service
This can happen several times a year, depending on the lunar cycle. Who knew that coral were so romantic?
Broadcast spawning en masse allows corals to mix their genes with corals both near and far, In this way, genetic diversity in increase, therefore increasing the resiliency of the coral population and the reef itself. It is a system of cueing into natural rhythms that has proven advantageous to corals that must rely on variable environmental factors such as currents and chemistry to provide ideal spawning conditions.
Image Credit: Smithsonian Channel via Giphy
In many regions of the world, corals will spawn within the same, incredibly short, time window. On the Great Barrier Reef, the largest coral reef system in the world, 400-500 species of coral spawn simultaneously during the same week. Many species even spawn on the same night in a single 30-60 minute period.
For decades it was a mystery as to how corals synchronized their spawning. It wasn’t even documented in the wild until the 1980s! The spawning event on the Great Barrier Reef is the largest in the world, and serves as an excellent model for understanding exactly how the corals synchronize their spawning.
Image Credit: Department of Environment and Conservation
Researchers from the Australian Institute of Marine Science, along with Israeli colleagues, through field and laboratory experiments, found that Acropora millepora – a coral that makes up part of the Great Barrier Reef – undergoes genetic and molecular changes specifically related to gamete release when exposed to different light treatments.
The researchers identified the gene responsible for triggering the spawning process, known as Cry2 – Cry2 encodes for light-sensing cryptochromes. The scientists found that Cry2 was most active during a full moon. The gene allows the coral to sense blue light and determine what phase of the moon it is, based on the amount and length of light.
The moon has so many important effects on our blue planet as its gravity pulls and pushes on our bodies of water, creating tidal forces.
Image Credit: Home Birth Matters Magazine
Neap tides, when the moon’s gravitational pull is partially cancelled by the Sun’s gravitational pull, create a minimum tidal range and it is during neap tides that Great Barrier Reef corals spawn. A neap tide spawn is advantageous as the minimum tide range decreases the dilution of gametes and allows for them to be more concentrated around each other for maximum fertilization.
Coral reef scientists are still working on unraveling the precise biochemical processes that allow sperm and eggs from the same species to find each other in the spawning soup.
While moonlight seems to be the primary cue for coral spawning, other factors have an impact as well, such as onset of darkness, regionally calm weather, food availability, and ocean chemistry like temperature, salinity, and acidity.
A final note on human impact to coral spawning: The researchers also exposed corals to light treatments mimicking light pollution from coastal cities. They found that light pollution can cause a mismatch between certain cellular signaling processes that prevent spawning from even taking place.
This is a serious issue when coral reefs are disappearing around the world and spawning is the most important key to reef regeneration. It is a tragic thing to be just discovering coral mysteries at a time when the ecosystem is so threatened that it may no longer exist in a few decades time.
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