Luzon sea star

Five cylindrical tapering arms on a small disc, arm span usually 10 to 15 cm, the surface finely granular and slightly swollen-looking, and the colour orange, brick red, brown or a dull mottled green. Symmetry is the exception rather than the rule: most individuals have arms of noticeably different lengths, and a large share are comets — a single long arm with three to five short regenerating arms budding at one end. The confusions are Linckia multifora, which is more slender, smoother and flecked with red and which also produces comets, and Echinaster callosus, which is banded and carries large pale swollen nodules. The comet form is not a species character and cannot be used as one.

Fission is normal reproduction in this animal rather than evidence of an injury. It divides across the disc or sheds an arm, and the fragment regenerates a disc and a new set of arms over some months, which is why local populations can be dense and why an intact five-armed individual is in some places harder to photograph than a comet. It grazes encrusting sponges, biofilm and detritus off hard surfaces.

It is worth looking at for what is on it. The sea star shrimp Zenopontonia soror sits on the disc and along the arms and takes the colour of its host closely enough to be missed at first pass. More interesting, and much more often misread, is Coeloplana astericola: a benthic ctenophore that lives flat on the upper surface of this species, looking like a small ragged patch of tissue until it casts out and reels back a pair of long sticky tentacles. Comb jellies are otherwise an open-water group, and a crawling one on the back of a sea star is one of the odder things in the muck fauna.