Sand sifting starfish

Appearance

The upper surface of the comb star is a dark purplish colour, while the underside is orange. On the upper surface paxillae, little pillars with flattened summits, are cream, grey or brown, the colours sometimes making a chevron pattern. Along the edges of the five arms there is a fringe of long, sharp marginal spines, usually with brown bases and pale tips. The arms are fairly broad and have a maximum length of 9 centimetres . The tube feet are pointed rather than having suckers, an arrangement that is more suitable for digging. ''Astropecten polyacanthus'' can be confused with ''Archaster spp.'' which look similar because both have developed features to enable them to dig through sand through convergent evolution. ''Archaster'' has spines that are flat and blunt and on its upper surface has parallel, radial rows of plates while ''Astropecten polyacanthus'' does not.

Distribution

The comb star is found in shallow tropical and sub-tropical seas throughout the Indo-Pacific region from the Red Sea and Zanzibar to Hawaii, and from Japan to Australia and New Zealand. In Australia the range extends from Cape Naturaliste in the west, round the north coast to Sydney in the east. It is often found on silty sand bottoms in harbours and estuaries. It is found at depths down to about 185 metres .

Behavior

The comb star spends much of its time buried in the silty seabed. It feeds on detritus and bivalve and gastropod molluscs which it swallows whole. It also sometimes engulfs pebbles and digests the biofilm and small invertebrates adhering to the surface.

''Astropecten polyacanthus'' contains the potent neurotoxin, tetrodotoxin, also known as TTX which has no known antidote. In a case of paralytic poisoning in Japan it was found that the victim had eaten a trumpet shell, ''Charonia lampas'', which had acquired the toxin through its food chain, thus implicating ''Astropecten polyacanthus''. In a study that followed this incident, most of the 54 comb stars assayed contained TTX, with one individual having a toxicity score of 520 mouse units per gram.