One of the most miraculous and important events in any organism’s life is reproduction. Every species does it differently, each evolving their own strategies to fit the physical and biological conditions specific to them. Normally, we are overjoyed to observe this rare event on the coral reef, such as with our yearly coral spawning and larvae culturing project. Or we spend large amounts of time ensuring the act was successful by caring for juvenile sea turtles and giant clams. However there are a few organisms on the reef which we would rather not see reproducing (other than the divers).
On September 12th, 2014 we observed the mass spawning event of several Crown of Thorns Starfish (COTs). These starfish are a major problem around our island, and have reached outbreak levels in some sites over the last 5 years. They are a corallivore, which can consume about 148-238 cm2 of our living coral from the reef each day. In many parts of the world, such as the Great Barrier Reef, they have been conclusively found to be a leading threat to reef health, and they have been trying to control them since the 1970’s. We also remove many of the starfish here around our island as often as we can, collecting 138 individuals from 9 sites in 2014.
In a mass spawning event, these starfish will aggregate along the shallow reefs, and release sperm and eggs into the water. Fully grown COTs are incredibly fecund, producing about 47-53 million eggs per spawning cycle, about 25% of their body weight. Although the animal has been highly studied, still little is known when they spawn, and what physical cues they use to all know when to release the gametes (eggs and sperm) into the water at the exact same time. Based on dissections, there is literature on the season of spawning, but almost no records of the event being observed in the wild.
It is important to know when they spawn, so that reef managers can more effectively time their removal efforts to maximize the benefits. If the COTs numbers or densities can be reduced prior to spawning then the reproductive event will be less successful, and reduce the potential size of future populations that perpetuate an outbreak. For us to have observed this event is quite special, and warranted a short publication in the Journal Marine Biodiversity.
Often, people ask why we would collect COTs if they are a natural part of the reef. Although the COTS are a natural part of the coral reef, their current numbers are anything but natural. When 43-57 million eggs are released from an animal, it is expected that very few of them will survive to adulthood, in fact the number needed sustain the population is about 1 in every 192 million eggs. But, due to anthropogenic effects to coral reefs, mostly nutrient enrichment and over-fishing, the survivorship of the offspring has become much greater. With more nutrients in the water, fewer of the pelagic starfish larvae starve to death before finding a suitable habitat on the reef. Less filter feeders like corals and bivalves means that less of the larvae will become food before getting chance to metamorphose. And less invertebrate feeding fish means that the juvenile clams can easily avoid predation and grow to adults. Once adults, some animals might eat them, but extremely rarely. A higher density of adults on the reefs means that more eggs will be fertilized during spawning, and a vicious cycle thus quickly spins out of control.
You can find out more about the Crown of Thorns Starfish here, and also get trained and involved in our COTs management program by joining one of our 2-4 week Reef Conservation Programs. Also be sure to check out the short publication, entitled “Spawning Observation of Acanthaster planci in the Gulf of Thailand” on the Journal of Marine Biodiversity website.