The Horseshoe Crab - A Prehistoric Marvel

Ranger's Corner - August 2012
A Prehistoric Marvel and Living Fossil - all in one!
Horshoe Crab: Limulus polyphemus
by Cristy Leonard

Mature Horseshoe Crab

Out of the worldwide four species of horseshoe crabs, only one is found in North America. So, what is it? Horseshoe crabs are more closely related to arachnids (spiders, ticks, and scorpions) then they are to crustaceans (crabs, shrimp, and lobsters).

They are sometimes referred to as Living Fossils because they are said to predate dinosaurs. On your visit to the park, you are likely to encounter a Horseshoe Crab, or at least evidence of one. During the nesting season, predominantly in spring and summer, male horseshoe crabs travel parallel to the shoreline on sandy flats and capture females as they pass by. If a male is successful, he attaches himself to a female by using his specialized front claws, and together they crawl to the beach.

The male fertilizes the eggs as the female lays them in a nest in the sand. The majority of this nesting activity takes place during high tides in the three days before and after a new or full moon. After the spawning is complete, the crabs leave and the waves wash sand over the nest. The female can lay between 60,000–120,000 eggs in batches of a few thousand at a time. The eggs take about two weeks to hatch. The larvae molt six times during the first year. The eggs are also an important food source for migratory birds; it’s proven that over 50 percent of the diet of many shorebird species consists of horseshoe crab eggs.

Although their tail looks sharp and uninviting, they are harmless to humans. On the contrary, horseshoe crabs are very beneficial to us! A horseshoe crab's tail, while intimidating, is not a weapon. Instead, the tail is used to plow the crab through the sand and muck, to act as a rudder, and to right the crab when it accidentally flips over. Horseshoe crabs grow by molting and grow by 25 percent with each molt. After 16 molts (usually between 9 and 12 years) they will be fully grown adults. Horseshoe crabs initially molt an average of three or four times a year. Males are sexually mature at their sixteenth molt, which is usually their eighth or ninth year. During their final molt, they develop specialized clasping claws for holding the female during reproduction. Females need at least 17 molts, or one more than the males, so they mature in their tenth year or even later and are, on the average, 30% larger than the males.

Unlike mammals, horseshoe crabs do not have hemoglobin in their blood, but instead use hemocyanin to carry oxygen. Because of the copper found in hemocyanin, their blood is blue. Yep, blue! The medical profession uses an extract from the horseshoe crab's blue, copper-based blood called lysate to test the purity of medicines. Certain properties of the shell have also been used to speed blood clotting and to make absorbable sutures. So basically, if you have ever had a tetanus shot, a flu shot or any kind of shot a horseshoe crab has made it possible for you to not get sick following the shot. Biomedical Companies need a large supply of horseshoe crabs in order to extract their blood. Fortunately they do not have to harm the horseshoe crabs. They harvest the horseshoe crabs, extract about 1/3 of its blood, and then they release the horseshoe crabs back into the wild. Please do not harm a horseshoe crab even though it may look frightening, they are harmless and beneficial to us!