Sea urchins under threat

The purple sea urchin (Paracentrotus lividus) is an edible species that until a few years ago was found in abundance in shallow waters off the coast of the French island of Corsica. Today, however, it has almost completely disappeared. Overfishing has considerably reduced stocks (28 tonnes caught in 2012 as compared to 470 tonnes in 1975).

During the spawning period, a team of scientific divers from the Stella Mare lab search the waters north of the island once a fortnight in order to identify and collect samples of sea urchins.

On each dive, the scientists bring back several dozen sea urchins that are measured to ensure that only adults are selected.

Back at the Stella Mare lab, the researchers examine the sea urchins, in order to improve their understanding of natural stocks and monitor their development over time.

The sea urchins are measured and weighed in order to draw up population statistics. Although fishing is only permitted from mid-December to mid-April, the number of sea urchins continues to decline.

The gonads (reproductive systems) are analysed with the aim of elucidating and regulating the sea urchin's development cycle. The scientists were pleasantly surprised to discover that there are two breeding seasons – and not just one – per year.

Mikael Demolliens takes an active part in the sea urchin reproduction programme, even coming to the lab in the middle of the night to keep a close eye on the sea urchin nursery after an alarm goes off.

These sea urchins are only a few weeks old and 4 to 8 mm in diameter. The expertise of the scientists at Stella Mare has enabled them to successfully reproduce sea urchins from the larval to the adult stage.

The researchers aim to use the encouraging results obtained in the laboratory to improve methods for the conservation, management and development of the resource all around the coasts of Corsica, with the hope that the seabed will soon be teeming with sea urchins once again.