Body with a small disc and 5 R/r: 5-9; up to 350 mm in diameter (occasionally up to 700 mm); coloured yellowish, orange, reddish, brownish or greenish.
The spines along the mid-dorsal line of the arms and on the marginal plates are very conspicuous and form regular longitudinal series (the upper marginals with 1, the lower marginals with 2 spines), giving the arms an angular appearance (M. glacialis test).
The ambulacral spines are in a very regular single row, and have no pedicellariae.
Found in a wide range of habitats from fully exposed rockfaces to muddy sites in calm bays. Remarkable size differences are commonly found. Spiny starfish from sheltered waters are larger than those of exposed coasts.
Particularly in the northern parts of the North Sea, Scandinavia. Elsewhere it is found on the western British coasts.