Horseshoe crabs are commonly found on tidal flats or mud flats. These areas are intertidal, meaning they are above water at low tide, and below water at high tide, and they contain soft sediment that make it easy for horseshoe crabs to dig under the muddy sand and hide. The Powder Point Bridge area in Duxbury, Massachusetts is an area much like this, which is the setting for the book, “The Bumpy, Lumpy Horseshoe Crab.” Other marine life that can be found in these areas are clams, limpets, barnacles, moon snails, hermit crabs, sea worms and crabs, to name a few. 

Horseshoe crabs are a member of the Animalia Kingdom, and are invertebrates, meaning they have no backbone. Even though we call them crabs, horseshoe crabs are more closely related to scorpions and spiders. Because horseshoe crabs have jointed legs, they are a member of the group called Arthropods and belong to the Arthropoda Phylum.

The horseshoe crab’s binomial name or scientific name is Limulus polyphemus. A binomial name is the Latin name given to an organism that includes its genus and its species. The horseshoe crab’s genus, Limulus, means odd or askew, which is thought to refer to the way the compound eyes are placed on the horseshoe crab’s body. Polyphemus, which is the species, means one-eyed giant, and refers to the simple eyes located on the front of the horseshoe crab’s shell. The horseshoe crab also belongs to the order of Xiphosurida, which means sword-tailed animals. The horseshoe crab’s subphylum is Cheilcerata, which includes animals without jaws.

Finally, the horseshoe crab’s class is Merostomata, meaning mouth surrounded by legs. It’s amazing how well the scientific names of horseshoe crabs describe its physical body perfectly.

Salt water can vary in its level of salinity significantly depending on where you are along the Atlantic coast. Although horseshoe crabs are very tolerant of different salt levels in their environment, they seem to be influenced by the rise and fall of salinity in the water. When choosing a beach for spawning, horseshoe crabs tend to prefer salt levels that are above 20 parts per thousand and below 30 parts per thousand. Anything beyond that range will significantly slow down the development of an embryo. Common ocean salinity is 35-36 parts per thousand. Horseshoe crabs can’t survive in water where salinity levels drop to lower than 4 parts per thousand. Young horseshoe crabs as well as adults like to live in salty water that‘s between 8 parts per thousand to 36 parts per thousand.

During the winter in warmer climates, horseshoe crabs continue an active life. But in northern climates where the water gets cold, horseshoe crabs move to deeper water or dig under the mud and rest. As days get longer in the spring, horseshoe crabs move back closer to the shore to spawn. May and June are the most common times to see horseshoe crabs laying eggs on the beach, especially during a full or new moon. And although it’s more common to see horseshoe crabs laying eggs at night, it’s not unusual to see horseshoe crabs spawning during the day as well. Of course, the darker it is, the safer the horseshoe crab and her eggs are from predators. Male horseshoe crabs usually arrive at the beaches a few weeks before the females. When the females arrive, they release a chemical called pheromone into the water that will attract a mate. The horseshoe crab’s compound eyes are another important tool that horseshoe crabs use to find a mate. When a male has found a female, the male holds onto the female’s shell with its pedipalps or hooks, clinging to the female’s opisthosoma, or middle section of her body. They both make their way to the beach, where the female digs a nest with her pusher legs. Her nest will be about 4-6 inches deep, and is located somewhere between the low and high tide mark. Then, the female horseshoe crab lays five to seven clumps, with about 2,000 to 4,000 eggs in each clump. When finished, the female horseshoe crab will have laid about 20,000 eggs. Horseshoe crabs will lay eggs several times during a spawning season, with a total of about 90,000 eggs being laid by each spawning female each year. Yet only about ten of these eggs can ever hope to hatch and survive long enough to become an adult horseshoe crab.

Eggs just laid are about 1/16 of an inch in diameter. By the sixth day, the horseshoe crab larvae will have molted once. Larvae have no tail, but they do have tiny legs. A yolk surrounds the larvae, which is their only food until they hatch. It takes four weeks to a couple of months for a horseshoe crab egg to hatch, depending on the warmth of the sun. The horseshoe crab molts four times while still in the egg. When it hatches, the baby horseshoe crab digs its way out of the sand, still lacking a tail and functioning compound eyes. At twenty-one days old the young horseshoe crab molts and gains a tail, basically looking like an adult horseshoe crab that’s only one-quarter of an inch wide. When a male horseshoe crab reaches adulthood some eight or nine years later, he will measure 7-9 inches wide across the prosoma, or first section of his shell, and about 13-16 inches long. Female horseshoe crabs are about 30% larger than males when full grown, and measure about 9-12 inches across, and 16-20 inches in length. Because it takes eight to ten years for a horseshoe crab to reach adulthood, it’s easy to see how over-fishing could greatly impact the horseshoe crab population.

Most scientists agree that horseshoe crabs can live to be about twenty years old. However, most horseshoe crabs don’t ever reach that ripe, old age because sea gulls and loggerhead turtles enjoy eating them, and shorebirds such as Dunlins, Sandpipers, Red Knots, and Ruddy Turnstones like to eat horseshoe crab eggs. It takes eight or nine years for a male horseshoe crab to become an adult, while it takes female horseshoe crabs about ten years to reach reproductive maturity. Currently, humans are a threat to the horseshoe crab population, because they are using horseshoe crabs for bait to catch eel and conch.

In order to grow, a horseshoe crab needs to shed its shell. The way the horseshoe crab accomplishes this is amazing. First, a new, much softer shell will form underneath the horseshoe crab’s old, hard shell. When the horseshoe crab is ready to molt, it sucks lots of water in through its gills, which bloats the inside of the old shell and causes it to split along the exuviation suture. The exuviation suture is located on the underside of the prosoma, or the first section of the horseshoe crab’s shell. The horseshoe crab crawls out of its old shell through the u-shaped opening wearing a brand new softer shell. It takes about 24 hours for this new shell to harden, and the new shell is about one-quarter to one-third bigger than the old shell the horseshoe crab has just abandoned. Eventually, the old shell washes up on shore, and a lot of people think they’ve found a dead horseshoe crab. But really they’ve found a molted shell, and the horseshoe crab is alive and well, swimming around with a bigger, roomier shell on. A dead horseshoe crab doesn’t have the opening in the top of its shell along the exuviation suture like a molted shell does. Molted shells can be taken home, rinsed out, and kept as souvenirs. They won’t develop a rotting odor, because there isn’t an animal inside anymore. Lobsters and crabs grow by molting also, much like the horseshoe crab does. However, unlike horseshoe crabs, lobsters and crabs eat their shells while they’re waiting for their own, new shell to get hard. That’s why you don’t find the shore scattered with molted lobster and crab shells, like you do molted horseshoe crab shells. From the time a horseshoe crab emerges from its egg, until about five or six years old, a horseshoe crab will molt about three or four times each year. From the ages of six to nine or ten, horseshoe crabs molt about once a year. After a horseshoe crab has reached nine or ten years old, it has its finished shell, and probably won’t molt again.

Just like in The Bumpy, Lumpy Horseshoe Crab, some horseshoe crabs found in the wild have many varieties of living sea creatures attached to their shells. Most of these creatures are harmless like barnacles, periwinkles, limpets, blue mussels, crabs and a variety of seaweeds. However, fungi, bacteria, and especially flatworms can be a little more menacing to the horseshoe crab. Fungi and bacteria can work their way into scratches in the horseshoe crab’s shell, leaving the horseshoe crab vulnerable to deadly bacteria. If flatworms are present, they glide along the mouth of the horseshoe crab, feeding on leftover food. Then the flatworms lay their eggs by cementing them to the horseshoe crab’s gills, causing the gills to harden and crack, and again exposing the horseshoe crab’s system to harmful bacteria.

Horseshoe crabs have a remarkable way of eating. They capture food like soft clams and sea worms with their first set of legs called chelicerae. These legs push the food back towards the bristles or gnathobases that surround the horseshoe crab’s mouth. Food gets caught in these bristles, and as the horseshoe crab crawls, the movement of its walking legs causes the food to scrape back and forth within the bristles. This action breaks apart the horseshoe crab’s food. The horseshoe crab doesn’t have a jaw, so it’s impossible for it to chew its food like we do. The horseshoe crab does have a gizzard though, that helps to grind the horseshoe crab’s food further once it passes through its mouth.