Come dusk, crabs with the grip and grace of arboreal spiders emerge from the cracks and drainage vents pockmarking the low embankment between Sembawang Beach and the scrubby remains of a village that held out for just a little longer than other traditional settlements in the north. Equipped with flattened carapaces that cast little shadow and patterns that grade into the dirty green-grey of a wall besieged by barnacles and dense blobs, the subtle grazers can easily be overlooked as one casually scans the scene for signs of movement. Upon discovery, the crabs easily manœuvre into crevices or holes, using latigrade motions to sidestep invasive forces or letting go when cornered to land on a ledge.
It was thus a little unexpected to find, among the purple-clawed grapsids that dominate such habitats, something a little different. Greenish eyes and a mottled brown shell that was rather less trapezoidal than the others were what gave the game away. Probably a stray from nearby swamps or a settler with the wiles to survive beyond its usual niche, the crab is a nocturnal forager more often spotted on trees in woodlands at risk and mangroves near the end of the road. Algae and leaves form their usual diet, but the crabs are not above tearing apart ants and caterpillars they come across on the trunk route.
Selatium was a monotypic genus in the family Sesarmidae, but a recent revision has split the crabs into two species and added a couple of close relatives to boot. The newly described crab, Selatium elongatum, has a carapace that is "longer than wide", as opposed to the "wider than long" shape of its sister species. Found in mangroves between Madagascar to Sulawesi (though not in Singapore), Selatium elongatum lacks the serrations that line the side margins of brockii. The claws of the males are also markedly larger than their mates', a trait that supports their reported territoriality. Both species have dome-shaped tubercles on their claws which produce scratchy sounds when rubbed against facial ridges.
Lithos is Greek for 'rock' and aptly describes a new genus of crabs confined to "boulders in the supralittoral zone". One species, the pretty purplish Lithoselatium pulchrum, occurs in southern Taiwan, while the other, Lithoselatium kusu, is known only from Singapore's southern shores and a moult from Sabah. Specimens have been found at the rocky and rubbly parts of Sentosa, Labrador Park, Pulau Kusu (its type locality), Pulau Semakau and Raffles Lighthouse, suggesting that novel creatures still lurk under the noses of intertidal adventurers. A third species, S. tantichodoki, was later described based on animals collected from the Gulf of Thailand.
Male mangrove tree-dwelling crabs that cross paths or covet a nearby female engage in swift and decisive contests for dominance. These encounters were the subject of field and experimental work by Ben Godsall, an unfortunate researcher who wondered if and how individual crabs determined whether they were a match for each other. Two animals, an adult and a sub-adult, served as focal subjects pitted against competitors of similar, smaller or greater size in the presence of food or a nubile object. The arena was a plastic tank with an angled wooden platform and two boxes in which the gladiators are placed. A typology of moves was observed: the crabs lunged at each other, made weak jabs, moved their chelae rapidly, grappled and blocked opponents like players in a rugby scrum.
Aside from physical moves, the crabs stridulated as well as tapped the wood with their claws, moves that Godsall suggested helped convey information about an individual's strength and stamina based on the number and regularity of repeated sounds. Oddly, these audio signals were usually performed after a bout, although mid-fight displays by similarly matched animals could result in either reengagement or retreat by the weaker combatant. Not surprisingly, the bigger beast usually won. But smaller fighters could also triumph if they secured a position of superiority with their foes on the downside of the platform.
A curious behaviour in the aftermath of a fight was the regurgitating of a fluid by the winning male, who would then smear it on the female using his chelae. This disgusting habit also took place between males on occasion and might serve to convey chemical messages in the stillness of thin air. The matches themselves were subject to many variables requiring further isolation and study: from the effect of population density and habitat zonation to the possible use of dishonest signals in the dark of a moonless night. What has been determined so far, is the finding that crabs of near-equal size fought longer than mismatched rivals. A less clear picture emerged with regard to the sequence of behaviours associated with aggressiveness, which hint at a proclivity for mixed messages and the interplay of dynamics that could enrich the theory of a favourite bedtime game.
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