About 150 spiky sea urchins were swept onto the short beach at Koh Lon, off Rawai, at the weekend.
''In 30 years I've never seen anything like it,'' said island chief Manit Yotarak. ''The marine environment around the island is usually very healthy.''
Experts from the Phuket Marine Biological Centre were unable to offer an instant answer for the phenomenon but feared some kind of disturbance to the sea urchins' natural habitat off Koh Lon.
''We will make further checks but so far we haven't been able to determine any change in the water,'' said centre marine biologist Nipon Pongsawan.
No bad water runoffs are present on Koh Lon but Khun Nipon warned island residents against using chemicals, just in case.
Sea urchins feed mostly on algae.
Koh Lon has been retained as a collection of traditional Muslim houses amid tranquil natural beauty just a short distance from the fully-developed coast of Chalong and Rawai.
Khun Nipon said it was possible that garbage containing chemicals or run-off from construction sites had caused the problem, with Chalong Bay semi-enclosed and sediment locked in.
The reef near Koh Lon had been degenerating more rapidly recently, he said.
''We need more cooperation between public and private sectors to solve these problems. It's not just Chalong Bay,'' he said.
''The problem occurs around the island, which will cause serious damage to Phuket's environment.''