Biology/Natural History: This species is a voracious subtidal predator, feeding on bivalves, snails, chitons, urchins, other asteroids, sea cucumbers, sand dollars, and crabs (in other words, just about anything it wants!). It will also scavenge dead animals. It may be the largest and fastest seastar in the world. It can move up to 3 meters per minute, and has been known to travel at least 3 km. It has over 15,000 tube feet. Tiny, newly metamorphosed juveniles of this species have only 5 rays but rays are added as the individual grows. Has very prominent spines and (crossed) pedicellariae, plus purple papulae. Loss of rays upon handling seems to be due to autotomy. In Puget Sound this species excavates butter clams (Saxidomus gigantea) by picking up sediment particles over the clam, passing them out to the ends of the rays, and dropping them. Often eat urchins such as Strongylocentrotus purpuratus, whose spines may pierce through from the stomach to the aboral surface. Can evert its stomach but more often swallows its prey whole. Predators include Alaska King crab and some large Cancer crabs. Individuals are agressive toward one another (and to almost any other seastar). Spawns March to July (some also in winter); has fertilizable eggs at least from December to June. May stand on the tips of their rays while spawning. Pelagic larvae metamorphose to benthic, 5-rayed juveniles at 9-10 weeks.

This species has a large, fleshy body with an only loosely articulated skeleton, and relies on fluid pressure to maintain its body form. It appears to rely more heavily on fluid uptake through the surface than on uptake through the madreporite. Its perivisceral fluid is more hyperosmotic than that of several other local species. This may aid in fluid uptake (Ferguson, 1994) and maintaining body form.