Horseshoe crab … a living fossil?

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All belangkas have an arch-shaped upper body and a horseshoe-shaped shell with long, thin tails that are almost stingray-like.

IT was the superb photograph of local people holding two horseshoe crabs in a recent edition of The Borneo Post that jogged my memory of my first encounter with these creatures.

Lying on a sheltered section of the beach at Tanjong Aru, Kota Kinabalu, at high tide and looking at the sunset behind Sapi and Manukan, suddenly I focused on the immediate inshore waters as a huge commotion occurred.

Seawater shot into the air as two creatures wrestled with each other in the shallow water. The scenario repeated itself.

Upon closer inspection I observed two sea creatures whose form seemed not dissimilar from trilobites that I had studied many moons previously on a Geology course at university.

These sea creatures, before my very eyes, also had three sections to their bodies not unlike the trilobite fossils found in shale rocks worldwide and dating back about 450 million years.

I made a quick sketch and that evening showed it to a Malaysian fisherman friend who exclaimed, “You are lucky, they were belangkas or king crabs. They were mating!”

The belangkas or horseshoe ‘crab’ isn’t actually a crab for it is related to arachnids and more akin to scorpions, spiders and cattle ticks.

It is an anthropod and an invertebrate with jointed limbs and a chitinous outer skeleton.

Four species of horseshoe crab exist in our seas, one in the North East United States Atlantic waters (Limulus polyphemus) and three Asian species.

The Japanese version (Tachypleus tridentus) is found along Eastern Asian coasts. The coastal horseshoe crab found in South and Southeast Asian waters is Trachipleus gigas and there is also the Southeast Asian mangrove variety, Carcinoscorpius rotundicauna.

If you see a horseshoe crab stranded upside down, do flip it over.

All three species of Asian crabs are found in Bornean waters. The female crabs are slightly larger than the males.

All species have an arch-shaped upper body and a horseshoe-shaped shell with long, thin tails (almost stingray-like) used as a steering rudder and to lift their bodies by a flipping action in digging into the sand when stranded ashore on their backs thereby gaining composure.

With a thorax (prosoma), an abdomen (opisthorma) and a tail (telson) one can see why these ‘crabs’ are likened to primeval trilobites. Sporting 12 legs, the first pair (chelicerae) function as pincers to feed its underbody mouth.

The next pair are grasping legs and used while mating. The three pairs of pedipalps are used for walking along the seabed and yet another pair is for propulsion through the water.

Finally, near the tail are the branchial legs or ‘book gills’ allowing oxygen to be absorbed yet keeping water out. When beached these gills keep the ‘crab’ alive. Its tubular shaped heart, with a heart rate of 32 beats per minute, may be seen through its outer shell extending almost to its tail.

It possesses 10 eyes, which are mostly used as light sensors or mate finders. Its two lateral compound eyes housing 1,000 inbuilt receptors also provide night vision.

Near the top of its thorax there are two median eyes and one endoparietal eye all used for the detection of ultraviolet light from the sun and the moon’s reflection off the sea surface.

These eyes are particularly sensitive to the lunar cycles spawning peaks at New and Full Moon times when high tides allow the creature higher access to the shore.

Other eyes, such as the ventral eye and the photoreceptors in the tail, assist its brain to detect light and darkness.

In the breeding season horseshoe crabs move to shallower waters on bay head beaches protected from the surf by headlands. Males are first to arrive in search of partners and outnumber females six to one.

The female weighed down by the male clambering on her back proceeds to dig a hollow in the sand then lays a cluster of up to 4,000 eggs, which are quickly fertilised by the male and covered over.

Carefully following the lunar tidal pattern, the female will return to the same beach to lay up to five clusters of eggs each time. In one breeding season she will lay as many as 100,000 eggs.

These will take two to four weeks to hatch and within three weeks of hatching the youngsters begin the first moult of their exoskeletons. Such small shells may be found amongst flotsam and jetsam at high tide mark.

Male belangkas take nine years to reach sexual maturity and females 11 years during which they shed their shells with 20 moultings. Feeding mostly on worms, clams and algae, they are seabed scavengers enjoying any edible food there.

For centuries the belangkas has been seen as a delicacy but more recently prized in haute cuisine restaurants in New England (USA), Mexico, Japan, Thailand and Vietnam.

The USA has recently placed a partial embargo on fishing for horseshoe crabs there together with an annual import limit on Asian species.

The International Union for Conservation of Nature (IUCN) Red List details the belangkas as a species which is over exploited and with the reclamation of mangrove swamps and tourist development in sheltered bays slowly their habitats are fast disappearing with the species on the verge of extinction.

Whilst the human treat looms threateningly in their ultimate survival, they are prone to natural seabird predators when stranded upside down on a beach as well as turtles and sharks, which can easily penetrate their chitinous exoskeletons.

As with turtles, breeding sanctuaries could be established on known breeding sites. With a long lineage and true blue blood, when exposed to oxygen, the belangkas has provided bio-medical researchers with an incredible bacteria-fighting substance.

We need them but currently they need us more to survive as another species on Earth.

Should you see one upside down stranded on a beach, gently flick it over. It won’t nip you and one more belangkas would have been saved.

The Malaysian Nature Society
Established in 1940, the Malaysian Nature Society is the oldest scientific and non-governmental organisation in Malaysia. Our mission is to promote the study, appreciation conservation and protection of Malaysia’s nature heritage. Our 5,000-strong membership, spread across 12 branches nationwide, come from all walks of life, bound by a comment interest in nature. For further information on membership or our activities in Kuching contact us at mnskuchinggmail.com. For information on our activities in Miri contact Musa Musbah ([email protected]). You can also visit www.mns.org.my,
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