Orthasterias

Lore & Background

The rainbow star is a large starfish, growing to a diameter of about 50 centimetres (20 in) with an arm length of 21 centimetres (8.3 in). It usually has five slender tapering arms and the aboral (upper) surface is pink or red with irregular patches or bands of darker red, orange or grey. The surface is covered with sharp white or mauve spines, each surrounded by a ring of pedicellariae, tiny pincer-like organs.

The rainbow star is found in northern parts of the Pacific Ocean with its range extending from California to Alaska at depths down to about 250 metres (820 ft). It also occurs in mid-ocean on knolls and seamounts. It is an uncommon species and is usually found on soft bottoms of mud or sand, or on kelp or rock surfaces.

The rainbow star is a predator and feeds on a range of invertebrates including gastropod molluscs, limpets, bivalves, brachiopods, chitons, barnacles and tunicates. In Alaska, it especially favours the ribbed clam Humilaria kennerleyi. It can dig up clams buried in the substrate and force the valves apart with the suction provided by its tube feet. It then everts part of its stomach, thrusting a fold inside the bivalve and excreting digestive enzymes onto the tissues. When these have liquefied sufficiently, the stomach engulfs them and is returned to its normal position inside the starfish. The rainbow star is sometimes attacked by a voracious predator, the morning sun star (Solaster dawsoni). It attempts to defend itself by winding its arms round the attacker and nipping it with its thousands of pedicellariae.

Reader's Guide

The rainbow star is notable as the sole species in the genus Orthasterias, making it a distinct lineage within the family Asteriidae. Its large size—reaching a diameter of about 50 centimetres—and striking coloration of pink or red with bands of darker red, orange, or grey, along with sharp white or mauve spines, make it a visually distinctive sea star in its North Pacific habitat. Ecologically, it is an important predator of a wide variety of invertebrates, including gastropod molluscs, limpets, bivalves, brachiopods, chitons, barnacles, and tunicates, with a particular preference for the ribbed clam Humilaria kennerleyi in Alaskan waters. Its feeding method—digging up buried clams, using tube feet to force valves apart, and everting its stomach to digest prey externally—illustrates a classic starfish feeding strategy. The species also exhibits a defensive behavior against the morning sun star (Solaster dawsoni), wrapping its arms around the attacker and nipping with its thousands of pedicellariae. Its occurrence on soft bottoms of mud or sand, as well as on kelp or rock surfaces, and its presence on mid-ocean knolls and seamounts, indicate a versatile but uncommon distribution from California to Alaska at depths up to about 250 metres.