Invader of the Week: 8 Reasons Why the Crown-of-Thorns Starfish Reigns the Reef

31 July 2014 by Lucy Anderson, posted in Invader of the week

Crown-of-thorns starfish on a coral reef

Crown of thorns starfish on a coral reef. Credit: Shutterstock

Despite only occupying 1% of the marine environment, coral reefs are the rainforests of the ocean and home to a quarter of all marine life. They also provide vital food, income and storm protection to millions of people in coastal communities across the globe. Yet one voracious species poses an enormous threat to these fragile habitats. For this week's 'Invader of the Week', I look at how the crown-of-thorns starfish (COTS) has become an invasive predator in its own backyard. 

1. A nemesis of coral reefs around the globe

These spiny savages have been responsible for 42% of the Great Barrier Reef's coral decline over the last 30 years with huge repercussions for other reef dwelling species. In other parts of their range, which extends eastwards from Mauritius to the west coast of the USA, they've caused coral cover to decline by up to 90%.

2. They devour vast areas of coral in spectacular fashion

First, they evert their stomach lining onto the reef, maximising the surface area of coral they can ingest. Second, they release digestive enzymes, dissolving the coral into a polyp purée ripe for external consumption. Only brittle coral skeletons are left in their wake.

3. They're no ordinary starfish

Unlike other species of starfish, these pin cushions of the ocean grow up to a metre in diameter, develop up to 23 legs and are covered by distinctive venomous spikes up to 5cm long. A stark warning to any would-be predator.

The venomous spines of a crown-of-thorns starfish. Credit: Shutterstock

4. They're prolific pro-creators

Females can produce up to 60 million eggs per year! Although only a tiny fraction of larvae would typically survive, larval survival increases dramatically when phytoplankton, their food source, becomes more abundant. The production of phytoplankton peaks after extreme weather events like monsoons and flooding, when agricultural fertilizers, sewerage and other pollutants wash into the ocean. When these heavy rainfall events coincide with crown-of-thorns (COT) spawning season, larvae can grow, develop and survive at a much higher rate, leading to an outbreak.

5. What about natural predators? Oh wait.

The few natural predators of the COTS include the magnificent Napoleon wrasse and enormous triton snail. Where these species are present, COTS numbers have generally stayed in check. But overfishing, coupled with a slow rate of reproduction, have caused Napoleon wrasse populations to plummet by 50% over the past 30 years. They’re now endangered. Likewise, tritons continue to be harvested to meet international demand for their shells. Cue COTS population boom.

6. But check out this crustacean defence!

The oh-so-feisty rusty guard crab and Pacific snapping shrimp pair up to protect their coral homes from enemy attack. In a beautiful example of symbiosis, the crack team of crustaceans receive protection from their own predators thanks to the coral's mucus. Everyone's a winner.

7. Lethal injection: the great white hope?

Researchers at James Cook University in Australia have developed a lethal mixture that kills COTS within 24 hours. What’s more, it can be delivered by a single lethal injection (the previous method required 20 of them). The injection works in two ways: it causes an allergic reaction in the COTS, and promotes the rapid growth of COTS naturally occurring Vibrio bacteria, causing them to launch and attack on their host. Most importantly, it leaves other marine creatures unharmed.

Over the past two years, it's successfully eliminated 250,000 COTs from the Great Barrier Reef, making it four times as efficient as previous control measures. It was recently described as “The single best hope we’ve had in dealing with the crown of thorns” by Australia’s Environment Minister. Of course, reducing agricultural run off is another vital control measure (more on both of these in this great video by the Great Barrier Reef Marine Park Authority).

8. Hulk-like powers of regeneration

Other control measures include removing COTS from the reef and burying them on the shore (a somewhat tricky manoeuvre for a creature with venomous spikes) or “sectioning” them (a politer way to describe “chopping them up”). The latter is less effective as the sections of starfish can survive if enough of the central disk of their body remains intact!

For more on coral reefs, check out Amy McDermott's blog ReefsRising


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