In more temperate climes, mudflats in lagoons where both the surf and slopes are gentle are the scenes of nocturnal orgies between creatures who survived the ravages of deep time and still hound the continents in their slow tango of drift and confederation. Having outlasted the trilobites, ammonites, mosasaurs and virtually all the fishes of Devonian seas, horseshoe crabs offer a case for morphological conservatism, in the retention of a bauplan that protects, propels and pole-vaults the animals as they pursue a level of existence with little risk of revolution.
Shielded against all but the toothiest of predators, the four surviving members of Xiphosurida continue to dine on lowly fare – clams, worms and other benthic fauna they can simply drag out and dig into with little fuss. A stubborn refusal to upgrade their diets to more finicky grub and crass insensitivity to variations in temperature and salinity have probably helped the order sidestep Mesozoic calamities as well as the compulsion to upgrade to distal branches on the tree of life. External stasis, however, is often confused with evolutionary inertia, and the genotypes of modern horseshoe crabs are almost certainly different beasts from those of their ancestors. Phenotypically, they also diverge in form and habit; extinct limulines prowled ancient coastlines as well as the swamps and river systems of lost continents. Today, only Carcinoscorpius rotundicauda, the round-tailed crab scorpion, wanders into mangroves and estuarine waterways to mate and feed.
Their name is a misnomer, for horseshoe crabs, lacking antennae and bearing book gills and anterior chelicerae, claim closer kinship with spiders and scorpions than shrimp and scampi. A lesser known link between xiphosurids and their terrestrial analogues is the way they glow under ultraviolet light, a trait that may help the former identify conspecifics under moonglow and allow the latter to pinpoint refuges in the hostile terrains of deserts and rainforests. Another hidden talent is the hypersensitivity of limuline blood, which flows a royal blue, to gram negative bacteria, thereby triggering a rapid and robust immune response. This discovery gave horseshoe crabs a new quiver in their passive efforts to show humanity that they are more valuable alive than dead, and though disruptive to local population structures, now provides a viable source of diagnostic agents for fishermen who catch, bleed and throw the crabs back into the sea.
Another dubious respite for Limulus, which roams the northwest Atlantic and was once harvested in the millions to feed hogs and cornfields, came when naturalists concerned with the future of cute and cuddly things realised that the survival of red knots, a charismatic shorebird, depended on a pitstop in Delaware, where the waders feast on horseshoe crab eggs before the final leg of their annual migration. Legal protection for Limulus and its breeding grounds thus came into force as a dividend of threatened flocks, a rare inkling that species do not exist in isolation but thrive and tumble on each others' fortunes.
Halfway across the world, the horseshoe crabs of the Indo-Pacific still lack the privilege of their first-world counterparts. Tachypleus tridentatus, a species with a solid hold on Japanese folklore, has a tenuous grip on its final stronghold, a flat no bigger than a baseball field in Imari Bay and increasingly hemmed in by dikes and dystopian developments. Though nominally secure, populations in the Seto Inland Sea may have breached a tipping point that will soon consign the crabs to the realm of legend. They fare no better in the southern reaches of their range, as sandflats by a fragrant harbour give way to an insatiable appetite for earth.
Notions of parks that indulge purely human natures, as well as malignant neglect, threaten local horseshoe crabs, which encounter regular barrages of fatal mesh on the island's northern shores. What little is known about their reproductive ecology suggests that the smaller, more rotund species relies on muddy expanses with access to mangrove channels. Unlike their cousins in colder seas, Carcinoscorpius (and probably Tachypleus gigas as well) do not engage in spawning aggregations, and isolated pairs can sometimes be seen coupling in the tidal brooks of Sungei Buloh Wetland Reserve. These vulnerable habitats also harbour high densities of juveniles, a hint that the preservation of vulnerable coastal swamps is required to ensure the survival of these artifacts as well as the winter visitors who share their flats and perhaps peck at the roe and roly-poly, trilobite-like larvae of benthic tanks. Without such extreme measures, the little crab scorpion may well suffer the fate of its offshore kin, which no longer multiplies on native beaches and remains abundant only in the minds of those who recall a time of richer pickings and receding baselines.
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