Ochre Sea Star

This sea star has five stout rays that range in length from 10 to 25 centimeters (4 to 10 in). The rays are arranged around an ill-defined central disk that needs to be a bit attached to a severed arm to grow a new sea star off of it. While most individuals are purple, they can be orange, orange-ochre, yellow, reddish, or brown. The aboral surface contains many small spines (ossicles) that are arranged in a netlike or pentagonal pattern on the central disk. The ossicles are no higher than 2 mm. In Pisaster ochraceus the tube feet have suckers on their distal ends which allow them to attach to the rocky substrate and live in heavily wave-swept areas. P. ochraceus has a simple nervous system and does not have a brain. A nerve ring connects and relays impulses between the star's radial nerves.

Pisaster ochraceus has been described as a keystone species. Experiments by zoologist Robert T. Paine in the 1960s demonstrated that a loss of only a few individual P. ochraceus seastars had a profound impact on mussel bed population, thereby reducing the health of the intertidal environment. With only a few natural predators (sea otters and seagulls) it is suggested that the principal threats to P. ochraceus are human collectors and casual tidepool visitors. Pisaster ochraceus has not been evaluated by the International Union for Conservation of Nature (IUCN).

An outbreak of sea star wasting disease starting in 2013 heavily reduced P. ochraceus populations. Their population is currently recovering. While epizootics are natural disturbances of population, climate change and human interference in starfish habitat exacerbate the impact.

At the larval stage, Pisaster ochraceus are filter feeders and their diet consists of plankton. As an adult, P. ochraceus feeds on mussels such as Mytilus californianus and Mytilus trossulus. They also feed on chitons, limpets, snails, barnacles, echinoids, and even decapod crustacea.

Members of Pisaster are dioecious but there is no sexual dimorphism and sexes can be separated only by the presence of eggs or sperm in the gonads. They reproduce by broadcast spawning, which in the Puget Sound population occurs around May to July. There is no parental investment beyond spawning. Fertilization occurs in the water column and Pisaster ochraceous develops through several larval stages.