Horseshoe Crabs

Horseshoe crab

A horseshoe crab

Special to The Star
Published: Thursday, July 11, 2013 at 09:41 AM.

By Tom Baird

Slow moving, chocolate brown horseshoe crabs are well known to our residents. Whether snorkeling or wading in the shallows of St. Joe Bay or along our beaches, horseshoe crabs are a common sight for visitors and locals alike. Elsewhere in Florida, tourists with little experience of the marine environment often react to horseshoe crabs nearby with fear. Often assuming the tail has a stinger, there is a lot of general misinformation shared around. In truth, although few creatures are uglier, few are less harmful.

The horseshoe crab has been around a long time, at least 450 million years. Although their clan was once more numerous in past seas, and even in recent times, their distribution is now restricted to the northwestern Atlantic coast from southern Canada around the Gulf of Mexico to Yucatan. All other members of the group are found along Asian coasts from Japan and Korea south through the East Indies and the Philippines.

The horseshoe crab is not actually a true crab. They are more closely related to spiders and share many characteristics with spiders. If you have picked one up, you have noticed that the crab has four eyes: two tiny ones on the forward part of the “shield” and one large compound eye on each side of the shield. Its scientific name is Limulus polyphemus. The name “Limulus” means side-glance or look. The hard body armor or exoskeleton is made of chitin, the same tough, resistant material that forms the body of insects, shrimps, blue crabs, etc. Chitin is heavy, so only in the buoyant sea could the horseshoe crab reach such a large size. The spiny tail is attached to the rest of the body in ball and socket fashion. The tail is used for righting the crab when overturned and possibly as a deterrent to being eaten. There is no poison gland in the tail, although it would hurt to step on a horseshoe crab barefooted.

If you see a horseshoe crab near the shore, do not be afraid to pick it up or touch it. The pincer claws are very weak and can do no harm. The mouth is in the center underside and looks fierce but is also very weak. The last pair of legs tipped with spikes act like “ski poles” in locomotion over the bottom. Bristles at the base of the legs grind food (worms and soft shelled clams) as the crab moves.

Spawning occurs at high tides during May and June, so you have likely seen many horseshoe crabs moving in tandem in shallow water in the last weeks. The female crawls up on a calm beach or sandy marsh edge between high and low tide marks to lay eggs in shallow depressions that she makes by her burrowing movements. The male crab hitches a ride on the female and fertilizes the eggs as they are deposited in each nest. Often one or more of the pair is stranded by the receding tide and the beachcomber finds the large shell the next day. Tiny crabs are hatched within two to four weeks. The waves of the next full moon tide (spring tide) wash the eggs into the bay or Gulf, and the eggs break as they are tumbled against the said.

By the third year of life the horseshoe crab has shed its hard outer shell 11 times and is about 6 inches long. Horseshoe crabs migrate to deeper water until they reach the adult stage. Between ages 9 and 11, the horseshoe crab matures and travels back to shallow water for the remainder of its life. Once it reaches the adult stage it does not shed. Females can grow to a length of 30 inches. Males are smaller.



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