These little sea urchins were patchily abundant at Changi, where they wriggled and worried their way on the sand with bits of seaweed entangled on their black spines. The usual (and otherwise highly informative) guidebook turns out a blank, although Temnopleurus toreumaticus is one species noted to exist on this locality. The larger urchin on the right is just slightly bigger than a ping-pong ball. They are quite well-hidden beneath thick clumps of Ulva or Halophila and run the risk of getting stomped by bootied ducks. Fortunately, my sensitive duck is deeply averse to intimate contact with sharp objects and so trod with care.
Sharing the sand were white urchins (Salmacis sphaeroides) with more numerous, shorter spines. The spines never seem to stop moving, pulsating visibly as the urchin glides over the substrate. Using its spines and tube feet, the animals crawl at a steady pace, seeking out edible detritus that is consumed via a mouth on the underside of the test. An interesting feature of some echinoderms is the pedicellariae, which are small organs with a set of valved jaws, neck and stalk that dot the body of the animal. They come in several forms, with some looking like smiley skulls. They are used to remove encrusting organisms such as barnacle nauplii or debris from the surface, emit defensive toxins or simply perform general housekeeping (see this paper on the pedicellariae of sea urchins). It's as if you had little pincers on your skin that could flick away biting insects and touchy ducks or perform micromassages...
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