A middling tide in the Tebrau Strait early this year was enough to permit a cursory survey of the seawall that supports the pier at Sembawang. The lower reaches of the concrete blocks and reinforced piles were a minefield of stinging beads and cruel plates that threaten to poke, prick and poison the epidermal cells of careless beach apes. Sea slaters, spidery crabs and little littoral snails, however, easily navigate this barrage of vertical encrustations and hypersaline pits, and graze on the filaments that coat the proceedings of an intertidal hour.
A host of tinier creatures – worms, grubs, beachfleas, slugs and polyps that challenge the naked eye – find refuge and risk amid the algal strands or the recesses that form when barnacles jostle for space and create molehills with elbow room for the epibenthic infauna of artificial slopes. Just below the high water mark, the cirripedes settle in loose aggregations that leave parts of the original surface exposed; closer to the sand, they cut loose and wage a war of passive aggression, relying on predators such as drills or a friendly wave to dislodge or chip away at the foundations of headstrong foes.
Bleached patches on this hostile landscape, as well as shells lacking the tell-tale valves of living animals, were the scenes of miniature swarms. Blue-grey bodies less than half a centimetre long, most much smaller, gathered around these crime sites, waving stubby antennae, circling on stout legs and inviting suspicion as to whether the assemblies were merely a clean-up crew or had a hand in the empty footprint of a late barnacle. Described from specimens collected on a sand bank off Billiton, an island east of Sumatra now known as Belitung, Pseudanurida billtonensis is a marine springtail that ranges across Southeast Asia and as far north as Okinawa. In 1971, the genus was revised by a local living legend who named a new Malayan species, P. yini, after his wife and established that only five species were valid.
One species, P. sawayana, is restricted to freshwater and brackish habitats, but the other four inhabit coral reefs, sandy flats and other coastal zones, with P. billitonensis occupying the lowest levels of the shore. The collembolans feed "on stranded, dead or dying soft-bodied animals," using needle-like maxillae that may help to quicken the latter process. Murphy reports that the hexapods can be collected by disturbing the sediment with a water-filled funnel, thereby releasing pockets of buried air "of which a surprising amount occurs between tides and which supports a distinctive fauna of aerophile microarthropods."
Though not visible in the field, the springtails possess a long furcula which provides a sudden and swift means of escaping predators. Another defensive tactic is the rupturing of thin cuticles on the head and parts of the abdomen, resulting in an outflow of haemolymph that presumably repels, or repulses, enemies. These strategies, crude as they are, appear more than adequate to allow the population at Sembawang to teem with impunity and welcome each fall of the tide with mobs of hairy little halophiles on the mortal remains of disinterred barnacles.
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