Marine biologists say the most likely cause of the unusual occurance is red algae and the recent warm spell - although mass suicide has not been ruled as a cause.

BBC: News | Wales | Sea urchins may have committed suicide

The problem started innocuously enough: Caulerpa taxifolia, a type of red algae not native to the Mediterranean shore, was spotted off the shores of the grand Oceanographic Museum of Monaco.

ECONOMIST: Small things often hold big secrets

Unlike other algae blooms, red tides are not caused by pollution, the wildlife service said.

CNN: Algae bloom kills record number of manatees