Lee Kong Chian Natural History Museum


Photo credit: Wang Luan Keng


Horseshoe crabs are slow-moving arthropods with an outer horseshoe-shaped shell of diameter 150–250 mm. The body is made of three sections: the prosoma (cephalothorax), opisthosoma (abdomen) and telson (tail). Contrary to popular belief, their telson is used not for defence but to help with locomotion and to upright themselves if they have been turned upside down. When turned onto its back, a horseshoe crab reveals a pair of mouthparts (chelicerae) to push food into its mouth, five pairs of walking legs and finally, a pair of ‘pusher’ legs that are also used for walking but unlike the walking legs, do not possess claws.
Horseshoe crabs have book gills located underneath the opisthosoma that are used for breathing and propulsion when swimming. Horseshoe crabs are mainly scavengers, feeding on worms, bivalves, and animal matter, and are more closely related to spiders and scorpions than to crabs. Unlike true crabs, they do not possess antennae or chewing mandibles. Their bristly mouth is located in the centre between their legs, and while they use their hind legs to grind food up, their chelicerae are the pair that are used to place food in their mouth.
Males are often smaller than females, and often cling onto backs of females during mating season, following them until egg-laying occurs. External fertilisation then occurs, with the male spreading sperm over the eggs. After 1–4 weeks, miniature horseshoe crabs emerge from the eggs, and like all arthropods, must moult in order to grow. The juveniles spend their first one to two years feeding on the intertidal flats, and may move out towards deeper water as they grow older.
Through fossil evidence, it has been discovered that horseshoe crabs have not changed in appearance in over 500 million years. Hence, they are often termed ‘living fossils’. 
The horseshoe crab's effective immune system is thought to be one reason why the family has survived for such a long time. Motile cells in the animal’s blood actively engulf foreign particles and release granules and defence molecules to neutralise endotoxins and to immobilise harmful bacteria (e.g., Escherichia coli, Salmonella). This characteristic of horseshoe crab blood made it popular with pharmaceutical companies. These arthropods were harvested from the wild and used in the testing of medical equipment that would come into contact with human blood, e.g., in surgery. The catch-and-release bleeding of wild horseshoe crabs to collect blood severely increased their mortality in the wild, and coupled with habitat destruction, pollution and the encroachment of human development, they have become endangered. However, scientists from the National University of Singapore managed to clone an important enzyme and have created a chemical substitute. 
Two species are recorded in Singapore.

Read more about the Xiphosura order.
Spot any errors? Have any questions? Something to contribute? Email us at dbsthh@nus.edu.sg!
Presented by

Sponsored by