Mangrove horseshoe crab

IDENTIFYING FEATURES

Horseshoe crabs are more closely related to spiders and scorpions than to crabs. There are 4 species of horseshoe crab found around the world. Fossils of horseshoe crabs over 400 million years old show that the species has not changed much. Those alive today look almost identical to those that were around millions of years ago. They could be called living fossils.

The body of a horseshoe crab consists of three parts: the prosoma, the opisthosoma and the telson. The prosoma is the dome-shaped part at the front. The spined middle is called the opisthosoma, and the rear extension that looks like a spike is the telson, which is commonly described as the 'tail'. Contrary to popular belief, the telson is used by the animal to turn itself right side up when overturned. The mangrove horseshoe crab grows up to 40 cm in length (including the tail), and its whole body is protected by a hard, dark brown carapace. Horseshoe crabs have two compound eyes. In addition, they have two median eyes, two rudimentary lateral eyes, and an endoparietal eye on their carapace and two ventral eyes located on the underside by the mouth. Scientists believe the two ventral eyes aid in the orientation of the horseshoe crab when swimming.

Each individual has six pairs of appendages. The first pair, the chelicerae, is used to place food in its mouth. The next pair of legs are the pedipalps, which are used for walking. The rest of the legs are used for locomotion and are known as pusher legs. Located behind their legs are book gills. These gills are used for propulsion to swim and to exchange respiratory gases. They are also called “walking museums” because of the amount of barnacles, worms and sponges attached to the shells.

Scientific Name: Carcinoscorpius rotundicauda
Common Names: Mangrove Horseshoe Crab
Family:  Limulidae
Size: Up to 40 cm

DISTRIBUTION

The mangrove horseshoe crab is found in the Indo-West Pacific region where the climate is tropical or subtropical. It is found in India, Indonesia, Malaysia, the Philippines, Singapore, Thailand and Hong Kong.

HABITAT

This species of horseshoe crabs can be found in shallow waters with soft, sandy bottoms or extensive mud flats. The mangrove horseshoe crab spends most of its life close to or at the bottom of a body of their brackish, swampy water habitat, such as mangroves.

DIET

The Mangrove horseshoe crab feeds mainly on insect larvae, small fish, aquatic worms, small crabs and thin-shelled bivalves which live in the soft sea or mangrove sandy or muddy bottom. Lacking jaws, it grinds up its food with bristles on its legs and places the food in its mouth using its chelicerae. The crop can expand to fit the ingested food, while the gizzard grinds the food into a 'pulp'.

REPRODUCTION

In the spring, horseshoe crabs migrate from the deeper water to the shallow, muddy areas. Nesting usually follows the cycle of the high tide. During the mating period, the males will follow and cling to the backs of their potential mates using modified prosomal appendages for long periods of time before egg-laying starts. In addition, the female does not choose her mate. Males find their female mates with the use of visual and chemo-receptive signals. Once a mate is found, the female digs a hole and lays the 60 000 to 120 000 eggs while the male externally fertilizes them. Once the eggs are laid, the male and female head back to the ocean, and the eggs develop on their own. The eggs are large, and after a couple of weeks, they into miniature versions of the adults.

Depending on the water temperature, juveniles grow about 33% bigger each time they moult, and it takes the juveniles about five moults to grow from 2 centimetres to adult size. It takes the hatchlings 11 years to reach sexual maturity and the adults can live up to 25 years.

IMPORTANCE TO HUMANS

Thousands of the horseshoe crabs are caught by local fishers. In some areas, the crab is cooked, and the yellowish roe (eggs) is eaten. However, some reports show that the crabs are toxic.

Horseshoe crabs are prized for their blue blood which contains a chemical called Limulus amoebocyte lysate (LAL). The blood of the crab is important in the biomedical world as a purified version can help detect bacterial toxins, important in disease detection, as well as ensuring the cleanliness of equipment. Lysate is an extract used in cancer research and is an indicator for spinal meningitis. The LAL test is one of the most important medical products derived from a marine organism to benefit humans. 

About 200 000 crabs are bled every year for their blue blood. About 20% of a horseshoe's blood is extracted and in the US, laws require that the animal be returned to the sea. But about 10% die in the process. A team from the National University of Singapore's Department of Zoology has cloned a substance to replace wild-extracted horseshoe blood. Horseshoe crabs have also contributed in other ways to human health. Much of our understanding of vision is based on studies of the horseshoe crab's eyes.

PREDATORS

These animals have few natural predators due to their tough exoskeleton, but a number of shorebirds, sea turtles, invertebrates, and fish feed on the eggs and larvae.

IUCN

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