The presence of round but untidy imprints on the fine silt of Chek Jawa, Cyrene Reef and secondhand coasts on Singapore's eastern rim is usually a reliable sign that a sand dollar lies just beneath the grains. Discs of living velvet that lack the fearsome spikes to their spheroid kin, these fossorial echinoids carve shallow trails under the surface of coastal flats, displacing a thin layer of sediment as they sift the deposits for organic particles and minute members of the benthic community.
The most common intertidal species in local waters is a uniformly purplish creature with just faint markings on its aboral surface to indicate its affinity to a phylum of pentaradial symmetry. The five-fold pattern, which bears modified tube feet used for gas exchange, is more apparent in another sand dollar with plusher hues and more often encountered in the trawl zone. Facing little risk of exposure to the air, this clypeasteroid has a body that does not taper at the edges, unlike the former, which readily goes undercover when submerged bars rise to become sun-baked beaches.
Having short spines that serve merely to ambulate and anchor their bodies to the substrate, sand dollars probably rely on their infaunal habits to escape the attention of most predators. Triggerfish, however, are known to use jets of water to blow the covers of hidden burrowers before chewing them to bits. Pile perches and skates, a family of rays, also regard clypeasteroids as meals worth every penny of pharyngeal effort, despite what would seem an unrewardingly high ratio of test to flesh.
While adrift in planktonic currents, sand dollar larvae engage in asexual reproduction to gain safety in numbers. It's a strategy that continues post-metamorphosis, as the animals can occur in high densities in suitable habitats, which are unfortunately few and increasingly far between in a country that sees natural shores as barren, buildable wastelands. Where they still survive, the urchins face the rapacity of men of untempered desire or suffer the force of feet that easily shatter a brittle shell.
Other than fish with grinding teeth, another natural threat to the innocuous echinoderms is a family of gastropods that emerge at night to feed on prickly prey. With a foot tough enough to withstand and engulf a crown of thorns, these snails force their way through the test with acidic drool before consuming the edible tissue with the tank threads of a rasping tongue. A mere sand dollar, if the snail is able to sniff it out and snare it before the urchin can flee, probably offers far easier (though slim) pickings for cassids such as grey bonnet snails. Last Sunday, we arrived at Cyrene Reef early enough to come across one which was probably hoping to start (or end) the day with a breakfast of sea biscuit.
A rather more surprising adversary revealed itself when we later, prompted in equal part by curiosity and macabre pity, unearthed a knobbly sea star that had half-embedded itself in the sediment. The asteroid was far from immotile, revealing the goo of its innards when overturned and a good-sized sand dollar in its everted grip. Most other oreasterids on the reef appear content to suck the biotic layers that form on coral rubble or graze on the epifauna of seagrass beds. But it would seem that the bold red beasts are not above digging in to make a meal out of a distant cousin when the chance presents itself to cash in with a dollar in hand and no chips to lose.