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The cushion star, Asterina phylactica, has the star-shaped body typical of starfish with five equal length arms originating at a central disc. The lower surface of the central disc bears the mouth. A deep furrow runs from the mouth to the end of each arm. In each furrow are two rows of tube feet. Like similar starfish species, Asterina phylactica, has inconspicuous marginal plates, papulae on the all surfaces and a sharp edge between the upper and lower surfaces.
Asterina phylactica has a flattened body only reaching 1.5 cm in diameter with small compact arms. It can be distinguished from Asterina gibbosa and similar species by its much smaller size, having very few ventral mouth plates and no radial thickenings. It has a grey-green upper surface with a conspicuous dark star-shape in the centre. It is pale underneath.Asterina phylactica has an unusual reproductive strategy. Individuals are simultaneous hermaphrodites, and during breeding season will form small aggregations of up to 10 individuals to ensure cross breeding. They then brood the eggs by laying over the top them, remaining with the breeding aggregation until the eggs hatch.