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Coral spawning to begin but scientists anticipate decrease

By: Kenny Ehman

Date Posted: 1999-05-22

Every year during late spring and early summer, corals around the world release billions of tiny bundles of egg and sperm into the sea as part of their reproduction cycle. The spawning is sometimes synchronized along certain reefs, by many different species of coral, to occur on the same night, usually taking place just after a full moon. Although it is still unknown exactly how the corals manage to synchronize this mass spawning, it is believed that there are many factors involved, such as sea temperature, tides, and the position of the moon. In other areas, coral species spawn during different times of each other, lengthening the time period egg and sperm bundles are released from that particular reef. This amazing event also attracts many divers and researchers who wish to view and study coral reproduction.

Here in Okinawa the majority of corals usually begin spawning at the end of May or beginning of June. The spawning can last for several days, giving divers and researchers a good chance to witness this reproductive process. The remaining coral species spawn at different times later in the summer.

The fertilization of the eggs actually takes place in the water, upon which the larvae float to the surface. The coral larvae then begin a journey to their new home, the majority being carried away by the ocean's currents to a particular area. After floating for approximately four to five days, the larvae will begin to settle at the ocean bottom, finding a hard surface to attach themselves to. The larva then grows into a polyp - a living animal which forms the hard calcium skeleton most hard, reef-building corals are known for. Inside the polyp lives a microscopic algae called zooxanthellae. The polyp survives through a symbiotic relationship with the zooxanthellae, depending on it for much of its energy. Eventually the polyp will begin to form a colony through asexual reproduction by either budding or dividing itself. This method of reproduction usually ensures that certain reefs will be populated with young corals as long as the producing site remains healthy.

Some scientists are worried about the effect of last year's coral bleaching on the ability of recovered corals to reproduce. The phenomena, which occurred throughout coral producing oceans of the world, caused zooxanthellae to mysteriously disappear from coral colonies, leaving only a ghostly white color and without their major supply of energy. Many scientists believe unusually high sea temperatures is one of the main causes of bleaching, but other theories are still being examined.

Dr. Robert van Woesik Ph.D. of the University of the Ryukyu's Marine Science Department is one coral researcher who is not expecting a large spawning to take place this year. He believes the recovery process from last year's bleaching may have left many corals depleted of energy needed for reproduction. "Much of the coral has recovered, but we don't know if we are going to find eggs in these corals," said Woesik.

Although it is known any small change in the marine environment can greatly affect a coral reef, scientists have also learned that corals are very resilient. Destroyed areas of reef have been re-populated in the past, and researchers are learning more about corals by studying these patterns.

Ecologist and dive instructor Kensuke Yokoi, who has been photographing and recording data on Okinawa's corals for the past two decades, recently documented what he believes to be a chemical signal from corals given approximately 28 days before spawning occurs. Yokoi explained the signal to be a pheromone release, which visibly clouds the surrounding water of coral colonies, greatly reducing underwater visibility for two to three days.

According to Yokoi, the best time to witness spawning in Okinawa will be approximately between June 1 and May 5 when Acroparidae corals will begin to spawn. This will be followed by the remaining Acroporidae and Faviidae corals spawning between June 27 and July 9. Merulinidae and Sinularia are predicted to spawn from August 2 to August 6.

For more information on dive tours to coral spawning sites, check with your local dive shop.

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