Sand star

A flat, stiff, five-armed star with straight tapering arms, 8 to 20 cm across, the whole margin edged by two rows of squared plates, the lower of which carries a fringe of spines that reads along the arm as a comb. The upper surface is covered in paxillae — small pillars each topped with a tuft of granules — which give it a matte, velvety texture rather than the glazed look of Linckia or Fromia. Colour is sand grey, fawn, olive or purple-brown, often with a darker stripe down each arm. Luidia is the near miss: flatter still, more flexible, and in this region usually with more than five arms. Species are separated on plate and spine counts and this entry is pitched at the genus; Astropecten polyacanthus is the name most often attached to Indo-Pacific specimens, frequently without the counts having been made.

Everything about it is built for sediment rather than rock. The tube feet are pointed and have no suckers, which is useless for gripping but effective for pushing through sand, so the animal burrows: much of the day is spent just under the surface, showing as no more than a faint five-pointed swelling, and it ploughs a shallow trail when it moves. It swallows prey whole — small bivalves, snails, crustaceans, brittle stars — taking them into the stomach intact and ejecting the empty shells afterwards, rather than everting the stomach over the food as the reef sea stars do.

On open sand after dark it is often one of the commonest large animals present, and the trail is usually seen before the star at the end of it. Emptied bivalve shells lying together in a small group on otherwise clean sand are frequently the leavings of one.