Just about any child’s first memory of life on the quiet water beach starts with the discovery of those curious, large, and fabulous creatures. Horseshoe crabs even excite adults! They are often found dead and in fragments, the large shell looking something like a military helmet.They also are often a parent’s first encounter with explaining sex to their children. “What are those two crabs doing, Daddy?” “Welllllll..darling….”

Horseshoe crabs are often the first thing found on the beach. But, their numbers are declining and NYS Dept. of Environmental Conservation still allows taking large numbers for bait.

Here are the important bits from the top, taken from the horseshoe crab web page.

The Top View of a Horseshoe Crab.

The large shield, or prosoma has two prominent compound eyes on either side that can probably form a crude image. But if you look carefully you will find two more eyes near each of the compound eyes and two more in the front mid-top of the prosoma. These eyes can detect the presence of light but not much more. They are probably important in synchronizing crab activities with day-night cycles.

When you find a live horseshoe crab, just pick them up! (Please don’t dig them out, more on this below!). Underneath you will see that the upper shield part of the body protects a complicated body below.

 

The Underside of a Horseshoe Crab.

First, look at that tail (=telson). It can move around in many directions, which is quite different from most arthropod limbs (yes, horseshoe crabs are arthropods, and are related to the branch of the phylum allied with spiders and scorpions). If the crab is flipped by a wave, the tail can flip the crab back to a right-side-up position. Also look at the 4 pairs of walking limbs. In front of these are the pedipalps and chelicerae. The tips of the pedipalps are graspers in the males, which allow them to hold on to females during copulation. In the back are the book gills, which are used for gathering oxygen from the water. Finally, you can see some slipper shells attached for the ride on the inside of the shell. Often, large horseshoe crabs are cruise ships for a variety of sessile creatures, including barnacles and two species of slipper shells.

Reproduction

Horseshoe crabs prefer to reproduce at night on new and full moon high tides. A male grasps a female and they move onto the beach in the intertidal zone. The female digs a shallow burrow, lays her eggs, as much as several thousand, and the male fertilizes the eggs. These eggs eventually hatch as larvae which look like miniature crabs. They swim for about 6 days and settle on a sandy habitat, they feed on small bottom creatures and grow by molting, which involves shedding the shell, filling it with water and growing a new larger shell.

Horseshoe crabs as bait.

Fishers commonly dig up horseshoe crabs for bait, and these can be found commercially in the late summer. Unfortunately, in recent years horseshoe crabs have been on the decline and maybe it is a good idea to discourage such a practice, even if it once did little harm to the overall population. This seems no longer to be true and horseshoe crabs are becoming less common over their entire geographic range, which spans the east coast of the U.S. Catching horseshoe crabs for bait is illegal in Connecticut. Why not in NY, given that they are in great decline? A law was passed in the NY State Legislature, but it has not yet been signed by Gov. Hochul as of October 2024. You can write to her and ask her to sign the bill into law.

Benefit to Birds and Fish.

Horseshoe crab eggs are an important food source for a number of birds that use the West Meadow sand flats, including sandpipers. Areas where crabs have declined have been related to declines in visits by shorebirds. Killifish and silversides also feast on the eggs.

Horseshoe crabs as medical wonders.

Scientists at Johns Hopkins University and the Marine Biological Laboratory discovered that horseshoe crabs have LAL (Limulus amebocyte lysate) in their blood. When endotoxin, a toxic component of infectious bacteria, is injected into Horseshoe crab blood, extensive clotting follows. This reaction was soon adapted to produce a powerful assay of the presence of infectious bacteria in humans and in hospitals. Purified LAL is injected with a test sample and a gel is formed if bacterial endotoxin is present. This allows the diagnosis of common urinary tract infections and even spinal meningitis. It is believed that the crabs evolved LAL because of their own constant exposure to infection from the marine environment.

Are Horseshoe Crabs Ancient?

Many believe that the common horseshoe crab has been around for over 400 million years. This is not true. There are four species of Limulus alive in the world today and there were horseshoe crabs in existence over 400 million years ago. But it is very unlikely that the species living then are the same as the ones today. Many species have probably come and gone since the beginning of horseshoe crabs.

Last updated October 19, 2024