Horseshoe crabs have been around on Earth for the last 445 million years, and have changed very little in all that time. There are four different species of horseshoe crabs found worldwide. One species lives along the eastern coast of North and Central America (the American horseshoe crab, Limulus polyphemus), and there are three Indo-Pacific species, Tachypleus gigas, Tachypleus tridentatus and Carcinoscorpius rotundicauda. All four species are similar in terms of ecology, morphology, and serology.

The American horseshoe crab, Limulus polyphemus, is found along shorelines from Maine down to Mexico and is commonly seen off beaches in Florida.

The American horseshoe crab, Limulus polyphemus, seen off Blue Heron Bridge
Photo by Patty Simone

Although they are called "crabs," a quick look at their body structure shows us that they are more closely related to trilobites, rather than crustaceans. The horseshoe crab belongs to its own class of Arthropods called Merostomata, which means "legs attached to the mouth." Despite their intimidating appearance, horseshoe crabs are not dangerous to humans. Most measure from 10-20 inches.

The soft, delicate bodies of horseshoe crabs are protected by a hard outer shell (carapace), shaped like a horseshoe. They have two large compound eyes on top of their shell which have a range of about three feet, and multiple smaller simple ones scattered on top of their carapace (along with 2 ventral eyes). These simple eyes are not true eyes, but photo sensitive spots used to find food and detect predators. Horseshoe crabs can detect UV light!

They have five pairs of legs that they use walking, swimming and moving food into their mouths. Horseshoe crabs can swim upside down, but they usually are found on the ocean floor searching for worms, crustaceans and mollusks to eat. Their menacing spiked tail is not used as a weapon, but functions as a plow to pull the crab through the bottom sediment, and also acts as a rudder to right the crab when it accidentally tips over.

As horseshoe crabs are surrounded by a snug protective exoskeleton, they have to shed (molt) this shell periodically in order to grow. Once they shed the old shell the new soft one hardens in about 12 hours. With each molt horseshoe crabs emerge 25 percent larger. After 16 molts (usually between 9 and 12 years) they are fully grown adults.

Having shells of chitin (like beetle or lobster shells) rather than of calcium carbonate (shells, eggshells) gives them a better advantage to tolerate lower-oxygen water and ocean acidification.

Horseshoe crabs have light baby-blue colored blood! Human blood is red due to the iron based red pigment called hemoglobin. Horseshoe crabs blood contains a copper based substance called hemocyanin, giving it a blue color. Their blood also contains very sensitive chemicals, used to fight off infections, that is also used by humans to detect potential toxins in various medical products. These are used as a fast and effective way by pharmaceutical and medical device manufacturers to test their products for the presence of potentially fatal bacterial substances.

Limulus amebocyte lysate, or LAL, derived from horseshoe crab blood can detect viruses, bacterial toxins and other contaminates. It is used to test the purity of all prescription drugs, vaccines, I.V. fluids, surgical implants and other medical paraphernalia that goes under the skin. LAL can detect contaminants at levels as low as one part per trillion!

The current harvesting techniques is to collect wild specimens (as they do not live long in captivity), drain up to one third of their blood, and then return to the ocean. Seventy to eighty five percent survive the ordeal, which accounts for the mortality of 20,000 to 37,500 horseshoe crabs per year.

Horseshoe crabs usually spend their summers in the sandy shallow coastal waters and their winters offshore in the mud.

Horseshoe crabs have quite elaborate reproduction rituals. Most of their reproduction occurs during the high tides in late May and early June, at the time of the full or new moon. During the springtime, males gather at the shorelines, and the females arrive a week or two later.

Horseshoe crabs preparing to mate at Mosquito Lagoon, Spring 2016

Female horseshoe crabs are on average 25-30% larger than males and they attract the males by releasing a pheromone into the water. Males select a mate, and cling onto her back as she moves towards the beach. At the waters edge, she scoops out a nest, where she lays anywhere from 60,00-12,000 eggs. As she lays her eggs, the male fertilizes them. Moisture from the incoming tides, and the warmth of the sun, allows the eggs to hatch in the two week period.

There is a high level of predation on the horseshoe crab eggs, the emerging larvae, and juvenile horseshoe crabs. Those that do survive spend their first and second summer on the intertidal flats, generally feeding before the daytime low tide and burrowing in the sand for the rest of the day. As they grow larger, they move from this “nursery” area into deeper water.

Horseshoe crabs mating, seen off Blue Heron Bridge
Photo by Jim Peachey

The American horseshoe crab is a very unique and valuable marine invertebrate. Unfortunately like so many other animals, horseshoe crabs have been exploited over the last few decades, which raises concerns for their current population levels. Horseshoe crabs are taken in substantial numbers to provide bait for American eel and conch fisheries. Natural causes of death (such as predation, disease and beach strandings) also impact on populations; beach strandings during spawning cause an estimated 10% of adult deaths, either from prolonged exposure to heat or from seabirds, who eat the overturned crabs.

Many states are now tasked with managing and conserving this unusual and valuable species. Specific conservation efforts are underway to better understand and manage horseshoe crabs throughout the regions where they are found.