Saturday, May 2, 2009

Earth day with GVI Seychelles

For Earth day this year GVI joined up with “The Underwater centre” and some of our partners to take part in a crown of thorns cleanup operation on a local reef. The star fish are infesting several reefs here in Seychelles, their populations can quickly get out of control and large parts of the reefs can be devastated in a very short time. The starfish are weighed and measured, a DNA sample is taken from every fifth one and the data sent to the Cook University in Australia for analyses to better understand these veracious reef predators.

The Crown-of-Thorns is a corallivore, a carnivorous predator that preys on reef coral polyps. They climb onto reef structures, and then extrude their stomach onto the coral. This releases digestive enzymes that allow the starfish to absorb nutrients from the liquefied coral tissue. The Crown-of-Thorns also preys on brittle stars in a similar fashion. They are voracious predators. An individual starfish can consume up to 6m2 of living coral reef per year. During times of food shortage the Crown-of-Thorns can survive on energy reserves for over six months.

The Crown-of-Thorns starfish has gained notoriety as a threat to the coral reef ecosystem, particularly in the Great Barrier Reef off the coast of Australia. Overpopulation of Crown-of-Thorns has been blamed for widespread reef destruction. Birkeland (1985) describes the starfish as one of the most influential species in the diverse biotic communities that make up tropical coral reefs. Some ecologists point out that the starfish has an important and active role in maintaining coral reef biodiversity, driving ecological succession. Before overpopulation became a significant issue, Crown-of-Thorns kept the fast growing coral from overpowering the slower growing coral.

Other factors negatively affecting the reef ecosystem, such as coral bleaching or Black band disease, mean that outbreaks of the Crown-of-Thorns can now cause permanent and devastating damage. Increasing outbreaks are also thought to be caused by possible environmental pollution triggers. Algal blooms caused by agricultural run-off may supply predators of Crown-of-Starfish larvae with plentiful alternative food sources. These explanations may also explain why massive outbreaks seemingly appearing out of nowhere, with no previous indication of an increasing population at the affected site.

The natural defences of the adult starfish make it an unattractive target for other reef predators. Venom and sharp pointy parts aside, the Giant Triton (a mollusc) and the harlequin shrimp attack and feed on Crown-of-Thorns starfish. Some large reef fish, particularly Humphead wrasse, may also prey on the starfish.

Sea star larvae are planktonic, so the major population control of the species comes from planktonic predation of junior species members. In a cruel twist of fate, a large solitary coral polyp of the genus Pseudocorynactis has been observed attacking, and then wholly ingesting a crown-of-thorns starfish of similar size. Decline in predator populations (through overharvesting, habitat destruction) has also been offered as an explanation for increasing outbreaks of Crown-of-Thorns starfish.