FRISKY coral are being blamed for the closure of a Sunshine Coast beach today.

Swimmers were evacuated from Caloundra's Bulcock Beach after reports of a substance in the water described as a "chemical spill".

But a later inspection indicated the unusual contaminant may in fact have been coral spawn.

The annual coral spawning season usually happens four to six nights after the full moon in November.

The last full moon was on Tuesday this week and the next is not until late November.

During the spectacular mating event, coral polyps release sperm and eggs en masse for them to mingle and fertilise.

A surf lifesaving spokesman said the beach would not be reopened until authorities had completed tests.

"It looks like it could be coral spawn but they think there might be something else mixed with it so the beach will stay closed," he said.

A decision would be made Sunday whether to open the beach.

Originally published as Coral sex frenzy forces beach to close