At the end of a nameless track in Kranji Industrial Estate near the Causeway bridge between Singapore and Malaysia lies a roughly 1-km stretch of mudflats. The sea meets small coastal tributaries at several points, creating a greyish sticky mixture of mostly silt and sand. All types of trash and flotsam can be found amidst the scum..
In these mudflats which look completely dull and devoid of life, a species that has existed and barely changed for 400 million years thrive.
The humble Mangrove Horseshoe crab, Carcinoscorpius rotundicauda; Not a crab but more closely-related to spiders and scorpions, it is recognizable via its distinctive armored carapace. Mostly armored head followed by a spiny tail, it seems.
Its blood contains a substance valuable to medical science as a reliable test for bacterial toxins.
There are only four species left in the world, and two of them reside in on this tiny island. I still can't quite wrap my mind around this statistical anomaly.
On a blazing Saturday afternoon, eleven volunteers gathered near the stinky mud to hear Dr Lesley Cartwright-Taylor from the Nature Society explain the afternoon's activity.
"Alright everyone, thank you for coming. What we're going to do is quite simple really. We're going to catch the crabs, bring them to this table to be measured, and release them afterwards."
"Now they are incredibly docile creatures so don't be afraid to handle them. Never pick them up by their tails 'cos they might come off. And a horseshoe crab without a tail will die quickly as it wouldn't have an apparatus to swim or to flip itself over. Just gently pick them up by their sides."
Off we went trudging into the mud, wearing wellies, booties and whatever water footwear we had. Mine were low-heeled sneakers made of a single piece of pure rubber, affectionately dubbed, "the kampung adidas." Apparently rubber tappers in Malaysia wear them for work. They worked well on the mudflats, allowing me to squelch noisily out into areas where the crabs might lie.
I was soon mired in ankle-deep mud though, of the viscous variety that seeps into one's shoes, making extrication of each step from the sticky ooze harder by the minute. Taking any step soon became a strange pantomime--first you put your weight on one foot and strain to free the other from the mud. When your trailing foot finally comes free in a loud sucking release, your momentum suddenly shifts towards your planted foot so you pivot desperately to retain your balance and try not to fall face-first into the muck. And of course your 'stable' foot sinks yet further into the quagmire.
Iqbal, my trainer, had an exceptional eye for the crabs. They generally buried themselves, leaving only trails or little raised sections on the mudflats' surface. He taught me to bend low and look for slightly-rounded sections in the surrounding mud; to detect the slightest of movements that the younger ones will make to swim away from us; to poke a finger into a suspicious mound to feel for the distinctive armored carapace and little spikes.
I was elated at finding my first adult crab. My finger dipped into the mud and encountered something spiky. *poke*poke*, I tapped, hoping for a response. Tracing the line of spikes, a long tail sticking out from a flat, rounded shell was quickly discovered. I hooked my thumb and pinky round the sides and happily extricated it from the mud.
In my hand, a creature that walked the Earth long before the dinosaurs did, and which survived mass extinctions and planetary catastrophes that killed the terrible lizards. The horseshoe crab bent over in some sort of defensive manueveur, waving its five pairs of legs in protest. I was awed. They will probably survive humans as well, if we let them.
It was the mating season, for we captured many coupling pairs - the male on top, clasping the female with his bulbous front appendages and generally hanging on until the act is consumated. One volunteer even discovered a threesome. Young horseshoe crabs nowadays... *tsk tsk*.
As the afternoon wore by, the tide began to creep in and our backs ached from the constant crouching. I wanted to take pictures of the process but was quickly thwarted by the challenge of keeping my hands clean in order to use the camera. Darn.
Eventually, after several catch-and-release cycles, Lesley called a halt to the proceedings and declared that we were done for the day.
The tally: 234 crabs, with the largest(invariably female) measured at around 14cm across, and lots of juveniles. Apparently the total was similar to that of the previous attempt, so that was good.
When they were put back onto the mud, the crabs didn't really showed any urgency to get away from the strange bipedal creatures who had rudely disrupted their afternoon naps, dumped them into a bucket, and unceremoniously laid tape measures on them. Some were even separated from their other halves. Still, they seemed to glide away nonchalantly, not bothered in the least.
Lesley posed the next question. "if only there's a good way of tagging them to record their movements."
"People have done that in North America, so we've been in touch with those guys to find out how they did it. Hopefully we can use similar equipment."
"So little is known about the crabs' habits in this part of the world."
How would you put a transmitter on a crab? Away from the sensory organs at the front probably, so as not to disrupt its normal activities. The tag has to be light and disposable because as soon as the crab moults, it's lost forever. So it can't record and save data either. The folks who did it successfully used radio tags and manually oriented receiving antennas over the bay, in trial-and-error fashion. That worked well it seemed.
Hopefully answers will emerge in due time. We have until the next full moon to find out. I returned with newfound respect for the horsecrab crab.
Scientists say that it is precisely the ability to survive in this kind of brackish boundary between the land and the sea that contributes to their longevity. Charles Darwin wrote of differing rates of evolution--speciation, morphological changes. Apparently organisms who have no/little need to change(to escape predation, to acclimatize to new conditions) shouldn't change that much. The humble horseshoe crab certainly seems to illustrate this point.
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