About Marthasterias africana (Müller & Troschel, 1842)
Marthasterias glacialis is a fairly large starfish with a small central disc and five slender, tapering arms. Each arm bears three longitudinal rows of conical, whitish spines that usually have purple tips; each spine is surrounded by a wreath of pedicellariae. Its background body colour is variable, and can be brownish or greenish-grey, tinged with yellow or red, and sometimes has purple colouring at the arm tips. This species can reach a total size of 70 cm (28 in), but most individuals measure 25 to 30 cm (10 to 12 in) across. It is sometimes confused with the northern starfish Leptasterias muelleri. This species is native to the Eastern Atlantic Ocean, ranging from Iceland and Macaronesia in the North Atlantic, while the related M. capensis occurs from Angola to South Africa in the South Atlantic. Marthasterias glacialis is also found in the Baltic and Mediterranean seas. Around the British Isles, its range 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 about 200 m (656 ft) deep, and can be found on both sheltered muddy substrates and on rocky surfaces. Like other members of the starfish family Asteriidae, Marthasterias glacialis is a predator, and feeds mostly on bivalve molluscs and other invertebrates. Secondary metabolites called saponins found in the starfish's tissues have a dramatic effect on the common whelk Buccinum undatum. At low saponin concentrations, the whelk moves away from the vicinity of the starfish, while higher concentrations cause convulsions in the mollusc's musculature. The sea urchins Strongylocentrotus droebachiensis and Psammechinus miliaris are also affected by these chemicals: 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, individuals gather in very shallow water during July and August. A few days after gathering, spawning has been observed when individuals arch their bodies and release gametes into the sea on a warm afternoon. A rise in water temperature appears to trigger this spawning event. Spawning was recorded in male starfish as small as 2.5 cm (1 in) in diameter, while spawning females were at least 9 cm (3.5 in) across.
