Size-dependent predation of the mesopredator Marthasterias glacialis (L.) (Asteroidea)

Asteroids are largely recognized as important predators in all of the world's oceans and for this reason, they play a crucial role in shaping the structure and functioning of benthic ecosystems. The spiny starfish Marthasterias glacialis is generally considered a voracious predator of molluscs, in particular bivalves. Using field observations and carbon (δ13C) and nitrogen (δ15N) stable isotopes, we explored possible changes in diet in relation to size of M. glacialis. Data were collected at Ustica Island (Southern Tyrrhenian Sea) from June 2008 to June 2010. M. glacialis showed a size shift in feeding preferences due to different use of food items: bivalves, Columbella rustica, Euthria cornea and Trochoidea were strongly selected by small starfish whereas the larger ones preferred Arbacia lixula and Paracentrotus lividus. The clear picture that emerged is that large M. glacialis is able to control sea urchin populations and indirectly Mediterranean rocky reef ecosystems.