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.
Horseshoe crabs have been used by humans in many, and sometimes surprising, ways. The tail was once used by Native Americans for spear tips and awls to punch leather. In the 19th century, horseshoe crabs were dried and ground for use as fertilizer or poultry feed supplement. In 1856, 1,200,000 horseshoe crabs were harvested on just a one mile stretch of Delaware beach. As late as the 1920’s and early 1930’s, 4-5 million crabs were harvested annually. Harvesting horseshoe crabs became unprofitable with the use of artificial fertilizer and the reduced crab population.
The horseshoe crab has new value however. The creature has pale blue blood that contains an agent valuable to medicine. The active agent is called Limulus amoebocyte lysate. It can be used to detect extremely small amounts of endotoxins, which are poisons found in a major class of bacteria that cause serious infections in humans. The value of the Limulus lysate test is that it detects smaller amounts of endotoxins quickly and reliably. If you have ever received an injection of a drug or vaccine, the batches were tested with Limulus lysate to ensure no bacterial contamination. To obtain the lysate, horseshoe crabs are bled by inserting a needle into the heart chamber. The donor crabs are released afterwards. Additionally, chitin from horseshoe crabs is used to create a suture material. Chitin-coated suture material reduces healing time by 35 to 50 percent. Horseshoe crabs contribute formidable weapons to medicine’s arsenal against infectious disease.
Meanwhile, the horseshoe crab continues its silent shuffle across warm shallow seas as it has done for millions of years, and with care for our bays and estuaries, it will continue to do so.
Tom Baird has been a fisheries biologist, high school and community college teacher (oceanography and microbiology), director of a science and environmental center, teacher of science and principal in Pinellas County as well as an educational consultant. He retired from the Florida Department of Education and he and his wife divide their time between Tallahassee and Cape San Blas.