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"These starfish destroy natural reefs," says Niphon Pongsuwan of the Phuket Marine Biological Centre.

Damage to the reefs is evident at Aew island off Phuket, the fifth in the Similan chain and another island off Krabi. In an ecologically sound environment there should be just one crownofthorns starfish for every 100 square metres.

"But in these three locations there are more than 30 per square metre," Niphon says.

In a bid to reduce their damage, centre divers are collecting the starfish from Aew and then destroying them using chemical injections.

Niphon is asking recreational divers to alert nationalpark officials if they encounter the starfish.

"Don't destroy the crownofthorns starfish before you notify officials or else you may face legal action," Niphon says.