Identifying predators of the SW Atlantic Patagonian scallop Zygochlamys patagonica using stable isotopes

In this work, we evaluated the food web structure associated with a Patagonian scallop Zygochlamys patagonica fishing bed using stable isotopes as an indirect technique. Scallops showed δ13C isotope signatures between −17.9‰ and −18.8‰ in muscle and −20.9‰ and −22.6‰ in gonads, which may indicate additional fractionation during metabolic transfers among organs. The isotopic signatures also suggest that larger scallops feed on organisms higher in the food web. Coexisting species, such as sponges, the ascidia Paramolgula gregaria, the anemone Actinostola crassicornis, colonial tunicates and Anthozoa, as well as the epibiont polychaete Idanthyrsus armatus, showed much lighter values of C than scallops, which indicates that they are using different food sources. Isotopic signatures suggest that species whose diet includes scallops are the gastropods Fusitriton magellanicus magellanicus, Odontocymbiola magellanica,Adelomelon ancilla and the sun starfish Labidiaster radiosus. The starfish Calyptraster sp. appears to be a top predator in this area, most likely preying on the gastropods previously mentioned but not directly on scallops. Our results suggest that there are at least three consumer trophic levels. Therefore, any strategy toward managing predators in this system should take into account potential trophic cascade effects given that extracting starfishes could enhance gastropods with an undesirable negative effect on the target species, the Patagonian scallop.