Operation COT: crown of thorns sea stars

A crown of thorns starfish in a bag on its way out of the lagoon.

A purple and orange, slimy, spiky, dangerous eating machine: the crown of thorns starfish (COTS) (Acanthaster planci) eats coral. Normal low numbers of COTS are fine: some corals are eaten, but the reef as a whole remains healthy and the corals can recover. However, if there are too many COTS, the whole system can be damaged, with fewer adult corals left to help the reef recover.

COT starfish on coral (lower right corner shows uneaten coral; upper right shows coral skeleton).

Populations of sea stars and other organisms naturally vary, but human influences can change the rate (how often) and the magnitude (how much) of these variations. COT blooms, or sudden increases in abundance, are thought to naturally occur every 80 years or so, which would give lots of time for reefs to recover in between. Now, COT blooms seem to be happening every 15 years or so, making it harder or impossible for corals to recover (in the face of other challenges, like pollution).

Young starfish eat plankton while they are living in the water, before they settle on the reef as adults. Therefore, large plankton blooms favor the survival of these starfish by providing lots of food. Lots of nutrients can stimulate large plankton blooms. In this way, excess nutrient run-off from land (like agricultural fertilizers, sewage, eroding dirt from construction sites) can cause a population boom of COT starfish.

Normally, other animals keep COTS in check. Some small crabs of the genera Trapezia and Tetralia protect coral from the COT starfish by breaking off the starfish’s spines. Natural predators of COTS include the beautiful triton’s trumpet snail, a species often targeted for collection in the tourist trade. People love to buy and collect the gorgeous shells… but that collection leaves fewer living snails in the environment to fulfill their roles, like controlling COTS populations. Excessive blooms without enough coral protectors and starfish predators cause COTS to go rampant.

This is why divers, fishers and other conservationists now remove COTS when they see them in heavily affected areas.

Expedition leader Shaun free-diving to retrieve a COTS.

The Aitutaki Lagoon Monitoring Project team found five adult COTS in Aitutaki’s lagoon, in the southern region.

COTS are distinguished not only by their spines but also by the number of arms that they have. Most sea stars have 5 arms, but COTS can have 12 or more. They’re also covered in spines, which can give a nasty sting! The ALMP team removed all of the COTS they found, using a variety of methods to avoid getting stung by the spines. Some used a dive knife, gloves, and a bag…

Some used their fins as make-shift shovels…

Chief Science Officer Mareike uses fins to pluck a COTS from coral.

And some used a machete (bush knife), which had been brought along to open coconuts for lunch refreshments.

Two COTS on the boat (machete barely visible behind). Note the yellow tube feet on the right.

The COTS were put into a bucket for transport on the boat.

Scooping the COTS into the bucket.

Once on shore, the COTS were buried. This practice keeps the biomass in the region (through decomposition) but also keeps the coral safe.

Digging with found island tools (dried palm fronds).

As can be said about most environmental issues, the problem of COT starfish is one of balance. Low natural levels of starfish are just fine, even though some coral dies for the benefit of the starfish. Natural blooms are all right, even though we can see changes in the ecosystem. The problem is excessive, unnatural blooms caused by the alteration of the entire ecosystem — when we alter beyond normal levels, we also break or overwhelm the reactions that would have helped things return to ‘normal’. Drastic changes sometimes require drastic actions to help.

Mareike bringing a crown of thorns sea star to the surface.

Local fishing clubs in the Cooks have already taken action: identifying the trumpet snail as a COTS predator and boycotting the collection of the shells as well as retrieving hundreds of COTS during outbreaks. This issue has been presented as a topic of concern by Cook Islanders, and more research and action will help Cook Islanders manage their ecosystems.

Do you study crown of thorns starfish? Let us know in the comments!

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