The coral reef ‘nibbler’

THE Australian Institute of Marine Science (AIMS) has published numerous papers on the causes of the devastation of coral reefs not only in Australia but in the Indo-Pacific oceanic region.

We know that the major destroyer of coral is climate change, through the process of coral bleaching as our ocean temperatures increase, and through the increasing frequency of tropical cyclones and the force of storm waves.

Add to this further stresses that corals suffer with dynamite and trawl fishing, and ships trailing their anchors across reefs, and the picture is nearly complete. There is but one further factor to consider.

Coral reef nibbler

This corallivore, better known as the crown-of-thorns starfish or COTS (Acanthaster planci), has proliferated on the Great Barrier Reef, where between four and 12 million may be found. My previous article (thesundaypost – March 26, 2017) mentioned this creature’s impact on Queensland’s section of this huge reef.

Found on coral reefs right across the Indo-Pacific region; under the right conditions they can reach plague proportions devastating living hard coral communities. This invertebrate’s domain is vast, from the Red Sea, along the East African coastline and extending across the Indian and Pacific Oceans to the west coast of Central America. I have only once seen this species of marine life whilst I was snorkelling off a reef near Malindi in Kenya.

Corallivores

This carnivorous predator feeds on reef coral polyps by climbing onto a section of live coral and fitting closely over it by spreading its stomach. It then secretes its digestive enzymes to absorb the nutrients from the coral-tissues. One starfish alone can consume up to six square metres of living coral per annum.

In areas where there are seasonal changes in climate, the COTS metabolic rate and speed of movement slows down during the winter months for it depends on the temperature of the seawater.

Covered in long poisonous (even to humans) spines (thorns), these starfish vary in colour from bluish-purple to greyish-red to green. Usually around 25 to 35 centimetres in diameter, some species can, however, grow up to 80 centimetres. Typically, they prefer to feed on hard branching and table corals but they have also been seen devouring foliose (leaf-like) corals and even soft corals, sponges and algae.

Reproduction

COTS normally breed from early to midsummer and congregate in large numbers near the top of a reef before releasing male gametes to fertilise the already released female’s eggs. This act is referred to as synchronised spawning. Normally starfish only emerge at night but during the breeding season, and in such high concentrations, they move by both day and night in search of fresh living coral.

Large female COTS may release as many as 65 million eggs in a single breeding season with high fertility rates. The larvae then settle on certain types of algae, which live in symbiosis with the coral polyps. Often the larvae are carried downstream from the spawning grounds by tides and currents.

Plagues

It has been calculated that a starfish plague or population explosion occurs when there are between140 and 1,000 starfish per hectare of coral reef with plagues more common on reefs closer to land. This is really because of the increased runoff from the land containing a concentration of agricultural fertilizers usually in the form of nitrates.

This leads to an increase in the density of certain species of phytoplankton in the form of ‘blooms’ on which the COTS larvae feed. Last year, further research suggested that terrestrial runoff had little or no impact on larval survival.

Predators

Equipped with long sharp spines on its upper body, this physical defence mechanism is enough to deter would-be predators.

However, these starfish are also equipped with their own form of chemical warfare by releasing saponins, which produce a constant swelling even on human hands. The unpleasant taste of these substances also tend to force fish to immediately release their starfish catch.

There are some predators. The Humphead Wrasse (Cheilinus undulatus), the Stars and Stripes Puffer (Arothron hispidus) and Triggerfish all consume starfish but the biggest enemy is a large gastropod mollusc, Triton’s trumpet, which can tear a starfish into shreds. Other predators include the painted shrimp and a type of polychaete worm. Still yet more marine threats to COTS are under study.

Removing COTS

Manually, this is a very labour intensive task with kill rates of about 120 per hour per diver in areas of high infestations. Until recently, the effective starfish population control was by injecting each starfish with expensive to produce ox-bile or with sodium bisulphate.

In 2015, researchers at James Cook University discovered that a single 20 millilitre dose of injected vinegar had a 100 per cent kill rate with the starfish dying within 48 hours. Since then as many as 350,000 starfish have been exterminated by this very cheap method.

In Australia, the federal government funds COTS eradication programmes, licensing marine tourism operations to kill.

Relatively recent trials, in the Brisbane area, of a starfish-killing marine robot COTSBot, sees this machine administering a lethal injection of ox-bile salts with great success thanks to the pioneering and development work of Queensland University of Technology.

Debate

Much discussion still abounds over the most destructive forces facing coral reefs, causing the live corals to die. Certainly COTS are a factor, but undoubtedly the greatest forces are linked to climate change with an increase in reef damage caused by more frequent El Nino events leading to coral bleaching.

There is ample evidence of more tropical and subtropical storms causing reef collapse and, of course, man’s role in overfishing and releasing chemicals into the seas. All these factors undeniably threaten the fragility of our coral reefs and their associated marine life.

Last May, 70 of the world’s leading marine biologists attended the Great Barrier Reef Marine Park Authority’s summit to establish a blue print on how to respond to threats to this huge reef.

They reviewed various strategies to increase the culling of COTS, expanding monitoring systems, identifying priority sites for coral restoration and developing coral nurseries.

Next time you may be snorkelling over a coral reef in Sabah or Sarawak avoid hitting the coral with your flippers and certainly don’t attempt to touch a COTS or any species of starfish for that matter.