People here ought to be some sympathy for hermit crabs, in particular naked ones. The poor things are in constant danger of outgrowing their homes (and not because they are trying to pack a family of five plus parents under one roof) and upgrading is more a matter of life and death than a parlay for greater luxury. Like fellow feeders of detritus such as fiddler crabs, hermits probably have their 'tongues' at the tips of their toes, the better to sense edible bits-and-pieces on the substrate that can be plucked out by a pair of handy chelae. It's a constant race to eat and grow as large as possible in order to outpace the strength and mouths of the many predators who relish the soft abdomen of small Anomurans and who possess the bite or crunch to smash undersized armour.
Alas, most hermits are too much of a wuss to dispatch living molluscs. Empty shells come at a premium, as wave action against rock and rubble renders the remains of many gastropods holey inhospitable or high and dry on the upper zones of the beach, where many of the best specimens end up in homes rather than as homes. Some populations of hermits that face a shortage of suitable shells are known to resort to inter- or intraspecific robbery, with burlier individuals evicting weaker crabs who must make do with damaged goods or perish from exposure or evisceration.
But when real estate is in good supply, the crabs simply plough along until they smell the sweet odour of dying snail, whereupon all crabs in the neighbourhood will hasten to the scene of the crime. It might be a whelk breached by a crab or a small conch in the fatal embrace of a volute. In a talk that focused extensively on making sense and more of marine sex some months ago, Dr. Dan Rittschoff gave an account of what happens when a large predatory snail (probably the Atlantic triton, Charonia variegata) devours a smaller mollusc. While the hulking beast sucks up its "snail shake", the death throes and disintegrating flesh of its prey release molecules that draw a crew of hermits in the hope of new housing. "The molecules contain enough information to tell hermit crabs what species of snail is dying, and the crabs that occupy that kind of shell will show up."
What occurs thereafter is a comedy of crabby manners. "They run around and fondle each other," recounts Dr. Dan. "One crab will run up and grab another crab's shell and feel it, and then it will run and touch another shell, and everybody's doing the same thing." After about 45 minutes, the triton is done with its dinner and ejects a clean shell for the waiting crabs. "They all line up according to size" and perform what Dr. Dan calls a "crab switch", whereby the fella at the head of the queue pops out from his old digs and plunges his squishy bits into the new pad. The guy behind takes over the first crab's hand-me-down and this goes on down the line until the last hermit gets a grip for his growing girth.
In the wild, the crabs would disperse before some big bad wolffish crashes their party. But in captivity, Dr. Dan found that they are sort of like teenagers who buy a pair of new sneakers that look cute but discover that they don't quite fit after an hour. So when he sets up a new predation site for the happy triton, the same hermits come rushing back to sample the new posting on the market and repeat their orderly switch.
Some of the tiniest hermits we find on Changi dwell in the shells of button snails. Somewhat larger ones live in the coils of whelks and often bear the burden of a hitchhiking sea anemone. In a nod to Daphne Fautin, Dr. Dan suspects that these actinarians are specific to hermit crab shells. Some hermits even hoard anemones, hosting up to a dozen sets of stinging tentacles on their abode. The cnidarians are no free riders, for their nematocysts serve to ward off the arms of stone crabs or octopi who might fancy a little kani.
Shell switching is apparently not an issue for such crabs, for they are able to pry their load free and stick the stem tentacle first on the new shell, whereby the anemone holds tight and does a somersault onto its new base. Unfortunately, there's a little twist in this tale, as Dr. Dan reveals. It seems that when predators are palpably absent, the hermits find no need for flower power. "It will take those anemones off, cos they are a pain in the ass, and it will eat some, cos they are good to eat." But when the scent of an octopus is released into the water, the little urns will "run around and pick the anemones all back up again – real fast. Against one foe, however, the hermits' natural inclination ill-serves them. For wily box crabs have jumbo can openers on their claws that can pry apart unwieldy shells, and according to Dr. Dan, some use anemones as bait to attract desperate hermits in search of homeland security.
Instead of juicy morsels, some hermits we encounter hold in their claws a fellow crab less than half their size. These are likely to be alpha males hanging onto their bootycall. While battling off other suitors, they wait for days before the lady finally sheds her skinny inhibitions so that the guy can deposit long suffering packages of sperm into the depths of her secret orifices.
As they grow, larger specimens amass a motley assembly of creatures that share the comfort of a mobile home. There are tiny porcelain crabs that filter-feed from the safety of reident anemones. Tube worms form rude encrustations that the crab cares little for, stinging hydroids sprout with abandon and flat slipper snails settle in waves of gender-bending recruits (i.e. protandric hermaphrodites) that line-up in an ascending series of sexual attachments. Marine ecologist Ed Ricketts calls hermit crabs the "clowns of the tide pools" and this holds true on the western shores of the Pacific, where some hermits wander the reefs with gaudy make-up. Even their plainer kin on the pale sand at Changi gambol with unaffected clumsiness over the grains and seagrass, little live wires of protruding legs and waving feelers that never seem to tire of tiny brawls over tasty remains or the tantalizing treat of a tango for two.
"They all line up according to size"
I would love to witness that.
Posted by: Aydin | 07 August 2008 at 11:43 PM