It is found in various habitats including beaches during very low tides, and at depths down to about 500 m (1,600 ft).

Also known as the vermilion sea star, it is the type species of the genus Mediaster and was first described in 1857 by the American zoologist William Stimpson.

[3] There is a row of conspicuous marginal plates along the edge of the arms on the aboral surface, and the central disc bears many flat-topped ossicles (platelike calcareous structures); these consist of a central group of granules surrounded by a ring of about twenty-five marginal granules.

[4] M. aequalis is native to the western coast of North America, its range extending from Chignik Bay in Alaska southwards to Baja California.

It often inhabits rocky substrates, at depths ranging from the low intertidal to around 500 m (1,600 ft).