The diet of Pycnopodia helianthoides varies with geographic location and the availability of prey, such as sea urchins, hermit crabs, sea cucumbers, clams and sand dollars. In the San Juan Islands, its main diet is the clam Saxidomus but on the exposed west coast, one study showed sea urchins as the main prey. P. helianthoides captures clams by excavating the gravel piece by piece with its arms and tube feet until it exposes the clam, then engulfing it with the everted stomach. The Sunflower Star swallows sea urchins whole. In food preference tests, it was equally attracted to Purple and Red sea urchins (Strongylocentrotus purpuratus and S. fransiscanus), but chose Purple Sea Urchins 90 per cent of the time. A study in Barkley Sound found 72 per cent of Sunflower Stars in the act of feeding. Of these, most took snails and clams, followed by crustaceans, and only 4.6 per cent ate Purple Sea Urchins. Larger Sunflower Stars have been found in areas of intermediate exposure on soft substrates, with juveniles more common in protected waters. In the winter in Gabriola Passage, 16.6 per cent were feeding and 80 per cent of those were eating barnacles. In Prince William Sound, Alaska, P. helianthoides feeds on molluscs, but in Glacier Bay, it commonly eats Green Sea Urchins (S. droebachiensis) and Purple Sea Urchins. Both urchins show strong escape responses. Much of P. helianthoides' nutrient reserves are stored in the body wall as well as in the pyloric caeca.
P. helianthoides can outrun most benthic species, travelling up to 160 cm per minute. To defend against this predator, some species have developed specific escape responses. The abalone Haliotis accelerates and simultaneously whips its shell back and forth to break the grasp of the sea star's tube feet. The snails Tegula brunnea and Calliostoma ligatum show typical flight responses as well as shell twisting. The swimming scallops Chlamys spp., the California Sea Cucumber (Parastichopus californicus), the nudibranch Dendronotus iris and the anemone Stomphia didemon swim away when touched by this sea star.
F. helianthoides breeds from March to July by broadcast fertilization. Eggs (120 micrometres in diameter) develop into swimming larvae that may remain in the plankton for up to ten weeks feeding on single-celled plants. After the larva has settled to the bottom and metamorphosed into a young sea star, it feeds initially on the thin layer of single-celled plants coating the bottom. Near the Auke Bay Lab, Richard Carlson tagged two specimens and later saw them 3 km away.
The Sunflower Star readily drops off (autotomizes) its arms when disturbed or irritated as a result of the rapid change in the tensile strength of the catch connective tissue, described in the introduction. Autotomy is triggered by a chemical that is released by injured tissues. Presumably it allows the Sunflower Star to escape from a predator holding onto its arm. Juices from the injured sea star cause an alarm response (increased movement in the direction of the current) in nearby Sunflower Stars.
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