This article originally appeared as “Crown of Thorns” in the Subject Spotlight section of the print magazine Scuba Diver Through The Lens (SD Issue 4/2014, TTL No. 11, pp. 12–13), published by Asian Geographic Magazines in association with DPG. Get your subscription to the successor magazine, Scuba Diver Ocean Planet, in the DPG Shop.
The Crown of Thorns starfish, Acanthaster planci, is one of the most maligned creatures in the ocean. But do these echinoderms deserve their reputation?
A benthic (bottom dwelling) species found throughout the Indian and Pacific Oceans, Crown of Thorns starfish are voracious predators of, primarily, scleractinean (hard coral) polyps. They feed by extruding their stomach sac and digesting the soft tissue below, with each adult capable of feeding on an area of up to 250 square centimeters per day.
Extreme population outbreaks of Crown of Thorns starfish, or CoTs, can decimate a reef system, especially in areas where coral colonies are already stressed by other factors such as nutrient run-off and increasing temperatures. These invasions typically last up to five years, but on large and complex reef systems may last for as many as 20 years.
Healthy reefs dominated by fast-growing corals may take 15 to 20 years to recover from a CoTs invasion, but recovery for slow growing corals such as Porites can take up to 500 years. Coral reefs that are stressed by other factors may never fully recover.
As a result, and using a number of controversial techniques, divers will often destroy CoTs that they come across. In some places, organisations have attempted systematic, but ultimately unsuccessful, CoTs eradication programmes, ranging from the contentious practices of injecting CoTs with poison, and the induction of a transmissible disease (methods with potential knock-on effects that are as yet not understood) to the simple removal of CoTs by divers.
Just Doing Their Job
In healthy coral reef ecosystems, CoTs play an important role in contributing to ecological succession; they help ensure coral diversity on a reef, clearing substrate for new larval corals to colonise.
CoTs are also a food source for species such as Napoleon wrasse, white-spotted pufferfish, titan triggerfish, yellow-faced triggerfish, harlequin shrimp, and bristle worms. Giant triton shells, Charonia tritonis, are one of the primary predators of adult CoTs.
Are We To Blame?
Evidence shows that these outbreaks have occurred regularly for many thousands of years. However, it is likely that human activity is partly responsible for the increased frequency of these population explosions, and for exacerbating their impact. It appears that stressed reef systems, including over dived reef sites, are more susceptible to plague-level outbreaks.
It is also speculated that CoTs “plagues” could be linked to increases in water temperature, salinity, and terrestrial nutrient run-off which causes algal blooms that, in turn, radically improve the survival rate for CoTs larvae.
Another significant contributing factor is overfishing, which has removed a significant number of their predators. Compounding the problem is the trade and collection of triton shells, Charonia tritonis, the most important natural predator of adult CoTs, which would have, in the past, helped mitigate the impact of these outbreaks.
It is imperative not to support the unsustainable trade and collection of tritons and other shells – all of which serve essential functions in marine ecosystems.
Crown of Thorns Basics
- CoTs are covered with armour of venomous, thorn-like spines of around five centimetres in length. Cells on the surface tissue of these long thorns contain a chemical that is toxic to humans. Stings can be very painful, and may cause nausea and vomiting, with effects lasting for many hours.
- They have anywhere between seven to 23 regenerative arms lined with muscular, tubed “feet” that propel them over the sea floor.
- It takes two years for CoTs to reach sexual maturity. They reproduce sexually in annual spawning events, during which adult females can release as many as 60 million eggs. Egg production in females increases with size.
- CoTs gametes develop into free-swimming larvae that settle on the seabed after about two to three weeks.
- CoTs can live for up to nine months without feeding.
- They can grow up to a metre in diameter.
- During the day CoTs generally stay in sheltered areas under ledges in the reefs or in the lower reef depths, coming out at night to feed, though they have also occasionally been observed feeding during the day.
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