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Commonly referred to as living fossils, horseshoe crabs first appeared on this planet approximately 300 million years ago (that’s 100 million years before the first dinosaur.) Over the course of that time, they have remained relatively unchanged. Today, there are four different species of horseshoe crab. Three of these species can be found living in the Indo-Pacific, while the fourth inhabits North America all along the East Coast from Maine to the Gulf of Mexico.

Horseshoe crabs belong to the phylum Arthropoda, which contains three major classes; crustaceans (crabs, lobster and shrimp), arachnids (spiders and scorpions) and insects. Although horseshoe crabs are often referred to as crabs, they are not true crustaceans. They belong to their own class called merostomata and are in fact more closely related to spiders and scorpions than  true crabs.

“Are they dangerous?” is the first question that comes to mind when encountering a horseshoe crab. To everyone’s surprise, the answer is no. “But how can they be harmless with a long, sword-like tail?” A horseshoe crab’s first line of defense is its hard outer shell. Very few predators can penetrate the shell of an adult horseshoe crab. Their underside, however, is extremely vulnerable, as it contains many soft body parts that can easily be preyed upon if flipped upside down. This is when the horseshoe crab’s “dangerous” tail comes into play. When flipped upside down, the horseshoe crab arches its body and uses its tail as a lever to right itself. You should never pick up a horseshoe crab by its tail, because it is delicately attached to their body. If it should break off, it will leave the horseshoe crab with no way to right itself.

Similar to spiders and scorpions, horseshoe crabs have numerous eyes, 10 to be exact. The most noticeable are the two large, well-developed compound eyes located on each side of the front section of the horseshoe crab’s shell. In close proximity of these two eyes are an additional five; two located directly behind the compound eyes, and three located front and center on the shell. Unlike the large compound eyes, these eyes are not as developed and are used to detect light and dark. Two simpler eyes are located on the underside of the horseshoe crab near the mouth. Multiple photoreceptors located along the tail make up the 10th eye.

During the months of May and June, horseshoe crabs will congregate in large numbers along protected back-bay beaches in order to spawn. This gathering coincides with the full and new moon phases. It is during these lunar phases when tides will be at their highest and lowest levels and are referred to as spring tides.

Credit: Chris Paparo
Credit: Chris Paparo

Receptive female horseshoe crabs will release pheromones into the water in the hopes of attracting a mate. Using their keen sense of smell and their well-developed eyesight, male horseshoe crabs will hone in on fertile females and attach to their abdomen with the use of a pair of modified walking legs known as pedipalps. Once secured, he will be dragged to the water’s edge, where she will dig several nests and deposit roughly 4,000 eggs. As she drags him over the nest, he will fertilize the eggs and hopefully their spawning activity will conclude before the tide recedes. Otherwise, the pair will be left high and dry until the next high tide, six hours later.

Over the course of the season, a female horseshoe crab can lay up to 100,000 eggs. These eggs will incubate in the warm, moist sand for approximately two weeks, hatching at the next set of spring tides. The juveniles are carried to the sea by the ebbing tide, where they will spend the next eight to 10 years maturing, before returning to a beach to perform this ancient ritual.

Horseshoe crabs are a keystone species in our local food web. Many shore birds, the red knot in particular, time their migrations to overlap with horseshoe crab spawning. Terrestrial predators, such as gulls and raccoons, take advantage of any horseshoe crabs that become stranded at low tide. Fish and invertebrates will indulge on the seasonal abundance of horseshoe crab eggs and larvae. Loggerhead sea turtles also partake in the feast, munching on adult and juvenile horseshoe crabs.

Besides filling an extremely important ecological niche, horseshoe crabs have played a major role in public health care. Without their “assistance,” many of us might not be here today. Not only is their blood blue in color (due to a copper-based protein used to transport oxygen), it has remarkable antibacterial properties. Blood harvested from horseshoe crabs is used by the biomedical field to detect bacterial contaminations in injectable drugs, vaccines, and medical devices (pacemakers, implants, etc.).

Unfortunately, in recent years there has been a dramatic decline in horseshoe crab populations. Pollution, loss of habitat and over-fishing for the bait industry are all factors. Scientists from Cornell Cooperative Extension and the Peconic Estuary Program have been monitoring local horseshoe crab populations through tagging programs. Data gathered by these researchers will allow fisheries biologists to better manage this species to ensure that it stays a living — and not an extinct — fossil.

Chris paparo

Places to see horseshoe crab spawning

Indian Island Park, Riverhead

Cedar Beach, Southold

Mattituck Inlet, Mattituck

Orient Point State Park, Orient

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