Three NAKED facts
NF#1: Ancient animal The horseshoe crab is a strange, ancient creature that has been around since before the dinosaurs. It is NOT a crab or even a crustacean. It is more closely related to spiders and scorpions. Its interesting body parts are under the shell. It has different kinds of legs: for walking, picking up food, cleaning itself. The flaps near the tail are book gills.
NF#2: They do NOT use their tails to sting people. The sharp tail is not venomous and is not used as a weapon. It is merely used as a lever to right itself if it is overturned. The tail is also used as a rudder when moving underwater.
If a horseshoe loses its tail, it is doomed. So please don't dangle a horseshoe crab by its tail.
NF#3: It is threatened! The horseshoe crab is listed among the threatened animals of Singapore due to habitat loss. It's a pity that something that survived for so many millions of years can be wiped out by humans.
Hints to Naked Hermit Crabs
Horseshoe crabs are sometimes buried in the ground. Look for tell-tale lumps or the tail sticking out.
They are often seen in the pairs. The male is the one on top. Please don't separate them. The male usually clings on very tightly.
If you have to lift up a horseshoe crab, hold the sides. Don't hold it up by the tail.
Be careful of the tail. It may twirl around and may poke a child's eye out if you are not careful. It is best NOT to lift the horseshoe crab and just look at it on the ground/in the water.
If you have to turn it upside down, do it for a short while only and help it to turn back around.
WAY Too Much Information
Horseshoe crabs are fascinating, but ordinary people usually can't take a full dose of horseshoe crab info at one go.
Two kinds of horseshoe crabs are found in Singapore. The Coastal horseshoe crab (Tachypleus gigas) is more often encountered on the Southern shores. It is large (diameter 25cm) tends to be greyish and has a tail with a triangular cross-section. The Mangrove horseshoe crab (Carcinoscopius rotundicauda) is smaller (diameter 15cm), tends to be more brownish and has a tail with a round cross-section.
The horseshoe's exoskeleton, unlike a crab's, does not incorporate calcium and is made of chitin and protein instead. Sometimes, you might come across what appears to be dead horseshoe crabs on the shore. These might just be moults. Moults are lightweight, have transparent eyes and no bad smell.
What does it eat? A harmless creature, the horseshoe crab bulldozes quietly along on the sea bottom feeding on worms, clams and anything edible including dead animals. They may also scrape off algae.
Eating with its legs! The horseshoe crab has no jaws. It has to grind down its food with the rough spiny areas (called gnathobases) near the base of the walking legs. The first pair of legs are tiny with small pincers which pick up and pass titbits into its four pairs of 'food processing' legs. Walking movements grinds up the food and the bits flow into the mouth, which is between the second pair of legs and conveniently faces backwards. So a horseshoe crab can only eat while it walks! In fact, the Class it belongs to is called Merostomata, which means 'thigh mouth'.
Galloping Horses? Horseshoes generally creep slowly over the sea bottom. However, they can move more speedily if they have to. They can use their last pair of legs, called pushers, to lurch forwards. These legs are longer, have spines which flare out when pushed against the sand. These legs are also toothed, and thought to direct water flow over the gills and to clean the gills. Horseshoe crabs can also swim for short distances, using their swimmerettes and gill flaps. They can also 'hop' over the sand slowly by bending their hinged body then pushing forwards against the tail, which is anchored in the sand.
Super gills: Horseshoes breathe well in oxygen-poor water. They have five pairs of flap-like appendages which contain book gills.
Blue blood: Horseshoe crab blood contains copper compounds which carry oxygen, the way iron does in our blood. So horseshoe crab blood is blue when exposed to air! But the horseshoe crab is NOT the only blue-blooded arthropod. Some true crabs and other arthropods also have blue blood.
Reproduction: Horseshoes mate during high spring tides when they can reach the highest part of the beach. The males are smaller and usually hitch a ride on the females using their specially adapted hooked first legs. The female digs a pit near the high water mark and lays her eggs. The males release sperm over the eggs and the nest is covered.
Eggs hatch at the next full moon when the tide is at its highest again. The hatchlings (called trilobite larvae) look like miniature adults but without tails, and are bright green! The larvae burrow into the sand and after a few moults begins to develop tails.
Human uses: Horseshoe crab blood has a substance that is so sensitive to bacteria that purified extracts of the blood are used to test for the presence of bacteria in human medication (e.g., intravenous fluids) and in medical tests. About 200,000 crabs are bled every year for this substance. 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 the basic principles of vision is based on studies of the horseshoe crab's eyes.