No. 32–More on Horseshoe Crabs

Horseshoe crabs use a small pair of pincer legs to direct food to their mouth when scavenge feeding or grazing on algae. Consuming the many other items that make up the main part of their diet is a rather more interesting process involving the use of their legs.

The next time you have the opportunity to view the underside of a horseshoe crab you will observe the first pair of pincer legs followed by four pair of walking legs and a final pair of pusher legs. The walking legs are tipped with relatively weak pincers of no real threat to fingers. The terminus pusher legs is an ingenious construction of four small overlapping leaf like plates that allow the animal to push against even the pluffiest of mud and not become bogged in the process.

The base of each walking leg is covered with spiked hair like projections pointing inward and forming a grove. The base of each pusher leg sports a hard spike called a molar used to crush small clams and crustaceans in a vice like grip. The groove formed between the five pairs of legs is essentially a food processor. As the animal walks, any food items in the grove are ground and moved conveyor style toward the mouth at the top of the grove. It is not a good idea to put one’s finger into the food groove of a live animal.

Though fearsome looking, the long pointed tail is neither an offensive or defensive asset with the animal relying primarily on its heavy armor for protection. The tail is used, however, as a rudimentary rudder as it swims with the aid of its flapping book gills.

Looking something like a boxing glove with a hook, each of the first pair of walking legs on the male horseshoe end with what is called a pedipalp. The pedipalps are used to grasp a female just behind the cephalothorax-abdominal hinge during mating.

Horseshoe crabs collect in large numbers in shallow coastal waters in the spring and summer awaiting the new and full moon tides (spring tides) which facilitate mating. It was such a collection that nearly sank Sam and me off Lands End.

In a slow storming of the beach action, countless horseshoes will cover the sands just above the day’s high tide line. Females with the smaller males in tow excavate shallow depressions into which they deposit their eggs. According to an often sited source, (Ruppert, Fox and Barnes) the female will lay between 2,000 and 30,000 large (2 to 3mm) eggs in a mating session and up to 75,000 in a season. The male fertilizes the eggs which are then covered with sand to await hatching on the spring tides of the following moon.

Named for the ancient creatures they resemble, 1cm long trilobite larva hatch from eggs not consumed over the previous two weeks by shore birds and a host of other creatures which count on the annual feast. Several species of shore birds rely on this rich food source for their annual northern migration.

After a year of molting and growth a carapace width of 4cm is attained and diminutive juvenile horseshoe is every bit the image of the mature animal. Sexual maturity is reached in 9 to 12 years and the animal’s life is thought to be up to 19 years.

While there doesn’t appear to be much to eat on a horseshoe crab, Native Americans found the large muscle bundle at the base of the tail worth the trouble. I would suppose that the abdominal section was cleaved and roasted in a fire with the tail serving as a handle. The carapace of the cephalothorax of large females was used to bail canoes and may have also served as an open container for nuts, berries and such. The tail was used for small game and fish spear points.

Early colonists learned from the Indians that dried and ground horseshoes made an excellent fertilizer, a fact which led to harvesting the creatures on an industrial scale by the mid 1800s. An overall decline in the harvest and the introduction of chemical fertilizers led to a demise of the industry by 1960. The lethal harvest, however, continues to this day with the animals used as bait by the eel and whelk fisheries.

A more benign human use of the ancient creature has been established through biomedical research. Blood is drawn from live animals and processed for use by pharmaceutical companies to test for contamination of intravenous drugs and vaccines. The animals are then returned to the wild little worse for the ware. Considering the contribution of Limulus polyphemus to the health and well being of humanity, it seems only fair to return the favor in some small way. If you happen upon a tide stranded horseshoe on its back on the beach, give it a carry to the surf or at least a helpful flip