In the kelp forests,
M. gelatinosus is one of the dominant
predators along with the starfish
Stichaster striatus,
Luidia magellanica and
Heliaster helianthus, the fish
Pinguipes chilensis,
Semicossyphus darwini and
Cheilodactylus variegatus, and the
Chilean abalone (
Concholepas concholepas), a gastropod mollusc. In a study in
Tongoy Bay in north central Chile,
M. gelatinosus was found to be a generalist predator in sand and gravel habitats, but showed a marked preference for the
Peruvian scallop (
Argopecten purpuratus) in
seagrass meadows. During the study, the number of scallops declined dramatically when they were heavily harvested commercially over a short period. This caused the starfish to increase its preference for the scallop in both habitats, but it was forced to broaden its diet in the seagrass areas, substituting small
epifauna for the missing scallops. The starfish can feed on all sizes of scallop, from in diameter, but particularly preys on the larger specimens. The local fishermen are aware of the competition they face from the starfish for the scallops. In another study, the main prey was the
Chilean sea urchin (
Loxechinus albus) and
M. gelatinosus often selected and pursued its prey with great care. The sea urchin responded to a foraging starfish at a distance of at least a metre by fleeing. This is an effective response where there is vigorous wave action, as the sea urchin gets swept away by the surging water. This sea urchin appeared to be able to distinguish between a foraging
M. gelatinosus and a non-foraging one, taking evasive action in the first case but not in the second. In fact this sea urchin has even been observed touching the starfish when it is not searching for prey. ==References==