Pycnopodia helianthoides is the largest sea star. It has more arms (15 to 24) than any other species and is softer than most. It is also probably the heaviest known sea star, weighing in at about 5 kg. The colour varies from reddish-orange to yellow, violet brown, purplish or slatey purple - the colour can depend on how much of the underlying skin is exposed when the papulae or pedicellariae expand. The oral surface is usually lighter, with yellow to orange tube feet. The arms are up to 40 cm long and the arm-to-disc ratio is 2.5 to 3.5. The aboral surface is soft and flexible, because the calcareous plates are not connected to one another. The plates near the centre of the disc and the base of arms sometimes bear stubby spines surrounded by a wreath of crossed pedicellariae and occasionallanceolate pedicellariae. The number of spines on the arms decline distally and the clumps of pedicellariae are closer together, with up to 75 papulae scattered among them. A slight furrow devoid of papulae occurs from where the arms join to the aboral surface. The superomarginals make up the first row of prominent spines on the side of each arm. Below these are the inferomarginal plates, bearing two rows of spines that overshadow the insignificant adambulacrals with one small spine. Three adambulacral plates are about equal to one inferomarginal. At the base of each adambulacral spine is a cluster of small straight pedicellariae that are usually hidden by the large numerous tube feet. Ten to fifteen adambulacrals form an adoral carina. The mouth plates each have two apical spines and one suboral spine, and clusters of straight pedicellariae.
Similar SpeciesA juvenile Pycnopodia helianthoides might be mistaken for a Crossaster papposus, but the details of their aboral surfaces readily distinguish the two. Deeper than 200 metres, it can be confused with Rathbunaster californicus.