The aboral surface contains many small spines (ossicles) that are arranged in a netlike or pentagonal pattern on the central disk.

[3][4] In Pisaster 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.

[4] 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.

[6] The reproductive system consists of a pair of gonads branching into each ray off a circular genital strand which is along the oral inner surface of the central disc.

[5] During maturation of the gametes, the gonads increase in size and can account for up to 40 percent of the sea star's weight.

It can evert its stomach through its mouth and engulf its prey, liquify it with digestive enzymes and ingest the processed food.

Mussels hold their valves together very securely but P. ochraceus can insert part of its everted stomach, or some digestive juices, through the narrow gap that exists where the byssal threads emerge from the shell.

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.

[2] 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.

[7] P. ochraceus can be found from Prince William Sound in Alaska to Point Sal in Santa Barbara County, California.

[13] This sea star can be found in great numbers on mussel beds and on wave-washed rocky shores.