Starfish, or sea star, is a common marine animal found from seashore to ocean depths; 1600 species are known worldwide. They belong to the invertebrate phylum echinodermata. Starfish are usually radially symmetrical, often with 5 radiating arms joined to a central disc. The mouth is located on the underside and anus on the upper surface of the disc. Another important characteristic is the water-vascular system, an arrangement of canals and tubes that operates the starfish's tube feet by hydraulic pressure. Tube feet occur on undersurface of arms and are the principal means of locomotion and attachment.

Although feeding habits are varied, starfish are often formidable predators which generate frantic escape behaviour in snails and bivalve prey. There are many species on Canada's coasts, ranging in size from tiny (50 mm) Leptasterias hexactis to giant Pycnopodia helianthoides, up to 1 m in diameter and with as many as 24 arms.