The Assateague
Naturalist

Picture Picture
Picture

Picture

Picture

The Horseshoe Crab (Limulus polyphemus)
Although horseshoe crabs look dangerous, they are not. And they are really not crabs at all; they are distant relatives of the spider and are probably descended from the ancient order Eurypterida.

Like the Atlantic moon snail, the lightning whelk, the knobbed whelk, and the channeled whelk, they feed on clams; they also include worms and other invertebrates in their diet. The horseshoe crab places a clam near its mouth (third photo down on left) in the center of its underside where its legs are attached and grinds and crushes with the burr-like sections of the legs.

The first four of the five pairs of legs are used for walking, while the last pair, located near the gills (lowest photo on left side), have leaf-like flaps that are used for pushing. (The small pincers on the last pair are also used for cleaning the gills in the abdomen.) Males can be distinguished by the first pair of legs which are heavier than those of the female.

The spike-like tail serves as a rudder, and, if the crab is flipped upside down, it may bend its abdomen at the point where it joins the main shell (carapace) and dig into the sand with the tail to support itself while it turns over.

The horseshoe crab has two pairs of eyes. The first--visible in the upper photo--are compound, and the second pair, located in the middle of the top front side are simple.

Often you'll find slipper shells attached to the underside of the crab.

In the early summer, the female comes ashore to lay eggs - followed by one or more males (sometimes the first will hitch a ride on her spike, and other males will latch on to him forming a chain). She lays the eggs in pits in the sand (200-300 per pit) near the high-water mark where they are fertilized by the males.

While dead horseshoe crabs are often found along the Assateague beach, live ones may be easily discovered by wading in the waters of Toms Cove (map). If you don't see any moving slowly along the bottom in the shallow water, look for mounds in the sand with the horseshoe-crab shape. The upper right photo shows a crab digging in at the edge of the cove.

These creatures have not changed much in the last 350 to 400 million years.

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