Sergio Hanquet / Purple sea urchin (Sphaerechinus granularis). Tenerife, Canary Islands.

Sergio Hanquet / Purple sea urchin (Sphaerechinus granularis). Tenerife, Canary Islands.

Pascal Kobeh / Sea urchin (Sphaerechinus granularis) Azores, Atlantic ocean.

Olga Kamenskaya / Purple sea urchin (Strongylocentrotus purpuratus) off the coast of Svalbard, Norway. September.

Nick Upton / Purple sea urchin (Sphaerechinus granularis) shell on sand, Greece.

Sue Daly / Close-up of a Common sea urchin (Echinus esculentus) showing spines and tube feet, Sark, British Channel Islands, UK, 2013.

Sue Daly / Common sea urchin (Echinus esculentus) attached to a wall, with female Cuckoo wrasse (Labrus mixtus) swimming in the background, Sark, British Channel Islands, UK, 2013.

Richard Herrmann / Red Sea Urchin (Strongylocentrotus franciscanus) and Purple Sea Urchin (Strongylocentrotus purpuratus) feeding on Southern Sea Palm (Eisenia arborea) creating urchin barren, Santa Cruz Island, Channel...

Franco Banfi / Purple sea urchin (Strongylocentrotus purpuratus) Little Strytan dive site, Eyjafjordur nearby to Akureyri, northern Iceland, North Atlantic Ocean.

Norbert Wu / Purple Sea Urchin (Strongylocentrotus purpuratus) group feeding on Giant Kelp (Macrocystis pyrifera), creating urchin barren, California

Norbert Wu / Brittle Star (Ophiothrix spiculata) group and Purple Sea Urchins (Strongylocentrotus purpuratus), California

MYN / Dimitris Poursanidis / Purple sea urchin (Paracentrotus lividus) beached, Crete, meetyourneighbours.net project