Feeding behaviour of the asteroid Meyenaster gelatinosus in response to changes in abundance of the scallop Argopecten purpuratus in northern Chile

Between winter 1996 and autumn 1997, the prey spectrum and feeding behaviour of the sea star Meyenaster gelatinosus (MEYEN, 1834) and the seasonal abundance of the dominant benthic macrofauna were analysed in: (1) a shore-fringing seagrass meadow and (2) an adjacent sand-gravel benthic system of Tongoy Bay (central-north Chile). Of 524 sea stars collected, 130 (25%) had food in their stomachs or were feeding on the scallop Argopecten purpuratus (LAMARCK, 1819), snails Calytraea trochiformis (BORN, 1778), Tegula luctuosa (ORBIGNY, 1841), and T eryomphala (JONES, 1844), and small epifauna constituted by the polyplacophora Chiton spp., gastropods such as Nassarius gayi (KIENER, 1835), Nucula pisum (SOWERBY, 1835), Turritela cingulata (SOWERBY, 1825) and Mitrella unifasciata (SOWERBY, 1832). The scallop A. purpuratus suffered an intensive commercial harvest during the study period which decreased significantly its abundance in both habitats. Before harvesting, A. purpuratus constituted the principal prey for M. gelatinosus in the seagrass habitat, whereas all prey types were equally distributed in the sea star's diet in the sand-gravel habitat. After harvesting, the frequency of occurrence of the scallop in the stomachs of the sea star in the seagrass habitat decreased and was substituted by small epifauna. Furthermore, in the seagrass habitat after harvesting, the breadth of the feeding-niche of the sea star increased, indicating that the sea star became more of a generalist feeder. In the sand-gravel habitat after harvesting, the diet composition remained unaltered, but M. gelatinosus showed an inverse tendency, specialising on the scallop. Additionally, M. gelatinosus increased its feeding preferences on A. purpuratus in both habitats (especially in sand-gravel) when the abundance of the scallop declined. M. gelatinosus feeds on a wide size-range (4 to 14 cm) of the scallop with a marked dominance of larger size classes. Our results confirm, in part, the observations of local fishermen of Puerto Aldea regarding the possible negative impact of M. gelatinosus on the standing stock of the commercially exploited scallop.