About Marthasterias glacialis (Linnaeus, 1758)
Marthasterias glacialis (Linnaeus, 1758) is a fairly large starfish with a small central disc and five slender, tapering arms. Each arm has three longitudinal rows of conical, whitish spines, which usually have purple tips. Each spine is surrounded by a wreath of pedicellariae. The species' background body colour is variable; it may be brownish or greenish-grey, tinged with yellow or red, and sometimes has purple colouring at the arm tips. This starfish can reach 70 cm (28 in) in size, but a more typical adult size is 25 to 30 cm (10 to 12 in). It is sometimes mistaken for the related northern starfish Leptasterias muelleri. Marthasterias glacialis is native to the Eastern Atlantic Ocean, occurring from Iceland and Macaronesia in the North Atlantic, while the related species M. capensis is found from Angola and South Africa in the South Atlantic. It is also native to the Baltic and Mediterranean seas. Around the British Isles, its distribution is limited to the western side of Scotland, Wales, the western part of southern England, and most of Ireland. It occurs from the subtidal zone down to a depth of approximately 200 m (656 ft), and can be found on both sheltered muddy substrates and on rocks. Like other starfish in the family Asteriidae, Marthasterias glacialis is a predatory species that feeds mostly on bivalve molluscs and other invertebrates. Secondary metabolites called saponins found in the starfish's tissues have a dramatic effect on the whelk Buccinum undatum. At low concentrations, these saponins cause the whelk to move away from the starfish; at higher concentrations, they cause convulsions in the mollusc's musculature. The sea urchins Strongylocentrotus droebachiensis and Psammechinus miliaris are also affected by these chemical compounds. S. droebachiensis flees from the starfish, but P. miliaris has toxic pedicellariae and can successfully defend itself. The reproductive biology of this starfish has not been extensively studied, but off the coast of Ireland, adult individuals gather in very shallow water during July and August. A few days after gathering, on warm afternoons, they have been observed arching their bodies and releasing spawn into the sea. A rise in water temperature appears to trigger this spawning event. Male starfish as small as 2.5 cm (1 in) have been observed spawning, while spawning females are at least 9 cm (3.5 in) in diameter.
