We first noticed these little crabs about a year ago during surveys of the Cyrene Reefs that began, as the season dictated, a couple of hours before day broke and the dredges of the port authorities became evident in the waters between these patch reefs and the container ports of Pasir Panjang. There is a gap in the rubble near the northern rim of Terumbu Pandan, where a tongue of sand breaks the lime barrier and invades the meadow at the heart of the flat. Here, the inflatable finds a gentle slope and intertidal explorers enjoy a soft launchpad to begin their wanderings across the shore.
Protoreaster nodosus prowl the edge of the seagrass meadow in rude droves and unrestrained colours. Some are hunched over pieces of rubble, with their extruded innards making short work of encrusted organisms. Others sprawl on the grass, exchanging invisible messages with their mates or awaiting the signal to emit cells of mutual, meotic attraction. As the sky brightens, unfortunate individuals that fail to languish in deeper pools greet the sun with deflated crowns and cringing arms as the source of their strength and rigidity seeps out of their grooves. On the sandier stretches of the flat are slightly less imposing echinoderms: purplish sand dollars, spiny Archaster in pre-coital assemblies, sea urchins with a sartorial bent, and cucumbers with hides in the shapes of stones and snakes. Amid these familiar bodies, we find at times a few rarities – cakes, cushions and oreasterids distinguished by cones on their marginal plates and a striking network of spines and tubercles on their aboral discs. Here, too, are carpet anemones with sticky tentacles and smaller creatures within their folds, as well as free-wheeling polyps with fair names and foul rubs.
As expected during visits on tropical springs, the waters of Cyrene last Tuesday were ruffled by a chill wind and brooding clouds that rushed in from the southwest to battle the eastern light. The storm line just missed the reef, though, and unleashed its wrath instead in a shroud of grey that turned the city invisible but spared the crew who arrived to collect specimens for the southern leg of the Comprehensive Marine Biodiversity Survey of Singapore. Among those who sampled the flat were Len McKenzie of Seagrass-Watch, who was trying to determine why tape seagrasses at Cyrene were suffering severe foliar damage compared to other nearby Enhalus beds, and Gretchen Lambert, a veteran taxonomist specialising in ascidians, dense colonies of which dot parts of the meadow.
The crabs in question betrayed their presence in the rockish strand a little to the left of the landing site, where patches of fine sand and grass intermingle with substrates of coarser material. Slightly more substantial outcrops provided refuge to the odd Eriphia ferox, which must lack for cover and companionship in these isolated ridges far from richer fields of rubble, while tiny broken-back shrimp in glassy shades of green and brown paddle in futile loops around shallow pools until they tire and fade from sight on a strap of turtlegrass. These crustaceans easily command attention for their disarming charm and delicate meanderings, but it became hard to ignore a regular series of lurches that took place in the same torch beam and which could not be handily explained by a rogue wavelet. The movement came from clods of rubble that wobbled or jerked with no visible source of animation. Turned over, each piece of coral rock gave up its true identity as the extended phenotype of a small brown crab with strongly built chelipeds.
Not a few groups of higher crustaceans have evolved the habit of using bits and pieces of their environment, whether inorganic, dead or still living things, as additional layers of armour, the better perhaps to stay ahead in an arms race between those that hide and those that bite, harder. Hermit crabs are arguably the most well-adapted of these decapods, having developed twisted, vermiform abdomens that fit the coils of dead gastropods as well as primary chelae that effectively plug the apertures; these mobile retreats may even have an offensive element in sea anemones that settle on or are transplanted by the crab onto the shell. In the deep sea, where empty shells are scarce, this relationship has become obligate; one genus of actiniarians secretes a chitinous roof over its parapagurid vehicle, which obtains protection in return for transporting its stinging rider to fresh pastures.
Among the true crabs, dromiids are the best-known backpackers. These rotund animals snip and don befitting fragments of sponges or colonial tunicates, which are held by modified legs and continue to grow over the carapace of their bearer. Sea anemones are the preferred armament of Lybia, which wields the polyps like boxing gloves on its claws, and Hepatus box crabs, which carry the cnidarians on their carapace. Certain spider crabs sport hydroids or corallimorphs on their limbs and flanks, while larger majids such as Camposcia retusa turn their entire bodies into a motley array of sponges, polyps, seaweed, bryozoans and other sessile taxa. Another group of brachyurans, the dorippids or porter crabs, enjoys modest fame for their use of dead leaves or other debris as lightweight cloaks; larger members of the family unabashedly press into service hapless creatures such as fire urchins, sea anemones, biscuits stars and even nudibranchs.
The little crabs that use chunks of rubble as roving shelters are in quite a different class altogether. For one, the animal must conduct the business of foraging for food and mates while lifting a rock many times its bulk and weight. The crab grips its home using its final pair of legs and gets about by pushing against the seabed with its robust chelipeds. The hollow occupied by the carapace is also peculiar; each concavity has a wall with distinct impressions, the likely result of sustained rasping by the crab's chelipeds, which have rough, beaded granules on the outer surface. Beyond this sculptured aperture is a passage that leads to a deeper chamber, in which the crab nestles when the rubble is disturbed. When the coast is clear, the crab creeps out of the rock towards the substrate and using deft legwork and leverage from stout claws, flips the slab back over its body in a fast and fluid stroke that recalls the spinning of a top.
Identified as Actumnus setifer by Peter Ng, this crab has long been regarded as an excavator of burrows in live and dead coral, sponges and soft stones. Two closely related species have been observed to carry living pieces of branching and massive corals, in which snug retreats are fashioned and which, when overturned, the crabs assiduously manœuvre to reposition the primary opening downwards. Similar behaviour was reported for A. setifer in 2008 by Australian authors who described how these pilumnids dwell in a "moveable coral shelter" consisting of a downward-facing entrance and feeding chamber, a habitation chamber and various other smaller openings which may aid in respiration and food detection. The crabs seldom, and only swiftly, leave their refuge, except to grab food and perhaps to mate. The conjecture of earlier observers that the arrangement is mutually beneficial is disputed, however, as there is little to suggest that the corals gain in mass and survival from being dragged across the reef or rubbed bare by rough claws.
Local specimens inhabit colonies that are already dead or have perished after persistent bumping against other rocks, and the observed habitat at Cyrene, with its mosaic of fine sediments and sheltered outcrops, appears similar to that of crabs in Moreton Bay. How this mode of carriage emerged, and whether it is exhibited by discrete populations within the species' range or only when conditions permit it, is still unclear. The gap between creating fixed galleries and gallivanting on unobstructed flats is perhaps one that is forded by crabs beholden to a calculus that juggles the availability of environmental resources, the cost of a heavy burden and the risks of life in the holes of exposed reefs.