Unveiling the diet of the thermophilic starfish Ophidiaster ophidianus (Echinodermata: Asteroidea) combining visual observation and stable isotopes analysis

The starfish Ophidiaster ophidianus is an Atlanto-Mediterranean species protected under the EU’s Habitat Directive. Despite the wide distribution and the current range of expansion of this thermophilic species in the northern Mediterranean Sea, nothing is known about its diet. Using field observations and δ13C and δ15N Stable Isotopes Analysis (SIA), the feeding habits of O. ophidianus were explored in two Mediterranean rocky reef areas located in the southern Tyrrhenian (Ustica Island, Italy) and the eastern Adriatic Sea (Molunat, Croatia). According to field observations, O. ophidianus preys mainly on crustose coralline algae (CCA) and the keratose sponge Ircinia variabilis in both areas. SIA confirmed the role of CCA in the diet of O. ophidianus but not that of the sponge and highlighted a not negligible role played by other sessile invertebrates, such us small gastropods and small sedentary worms. It was concluded that O. ophidianus is a facultative herbivore in Mediterranean rocky reefs.