Feeding behaviour of Ophidiaster ophidianus (Lmk.) (Asteroidea) in Mediterranean rocky reefs

The thermophilic Atlanto-Mediterranean purple starfish, Ophidiaster ophidianus, is a species protected under the EU’s Habitats Directive which is expanding its distribution in the northern Mediterranean areas as a consequence of global worming. The ecology of the species is largely unknown, specially for the diet. To cover this gap, the feeding habits of O. ophidianus have been studied in two different Mediterranean rocky reef areas: the southern Tyrrhenian Sea (Ustica Island, sampled in 2009) and the eastern Adriatic Sea (Molunat, sampled in 2014) by both field observations and ?13C and ?15N stable isotope analysis (SIA). SCUBA divers collected data by turning adult starfish downside-up and collecting photographs of the prey. Prey items of fifty individuals were identified to the lowest possible taxonomic level in each location by analysing the images taken. No significant differences were detected in the field, O. ophidianus fed mainly on encrusting coralline algae (Lithophyllum spp.), keratose sponges (Ircinia sp.), other encrusting organisms and sedimentary organic matter (SOM). Isotopic signatures significantly differed between the two areas: ?15N was significantly higher in O. ophidianus collected at Molunat than at Ustica. Mixing models showed that O. ophidianus collected at Ustica fed on Lithophyllum spp. (68.1%), Ircinia sp. (15%), SOM (16.3%) and bryozoans (Schizoporellidae, 3.7%), whereas at Molunat the starfish fed on Lithophyllum spp. (79.9%), Peyssonnelia spp. (7.9%), keratose sponges (Scalarispongia scalaris, 7.4%) and SOM (4.8%). Coupling field survey with SIA we argued that O. ophidianus feeds as a grazers, selecting algae and few other animal organisms in Mediterranean rocky reefs.