Horseshoe crabs are evolutionary survivors that have remained relatively unchanged in appearance for 350 million years. The horseshoe crab is not actually a true crab, but a member of an ancient group of arthropods, closely related to spiders and scorpions. There are four species of horseshoe crabs around the world and only one in North America. The species in North America is the most abundant in the world and ranges on the Atlantic coast from Maine to the Yucatan Peninsula.
Horseshoe crabs are fairly long lived animals, with individuals living more than 20 years.
In the late spring and early summer, horseshoe crabs arrive on the beaches to lay their eggs. The peak of spawning on the Atlantic coast occurs in Delaware Bay where thousands of crabs will arrive on the sandy beaches in May and June. Delaware Bay provides an excellent spawning area for crabs because the sandy beaches are protected from harsh wave action. The sand and pebble mixture of the beach is perfect for incubating horseshoe crab eggs.
Crabs arrive on the spawning beaches during the high tides of full and new moons when the water rises highest on the beach. When the female is ready to lay her eggs, she crawls up to the high water line on the beach with a male attached to her. The male clasps onto the female’s shell with a modified pair of claws. The female drags him around during the spawning process. In addition to the attached male, several other males may also attempt to fertilize the female’s eggs by arranging themselves on and around the spawning couple during the egg-laying process.
A female may have five or more males attempting to mate with her in a single egg-laying episode. During spawning, the female crab partially buries herself in the sand while she deposits a cluster of about 4,000 tiny green eggs. In an evening of egg laying, a female crab can lay several egg clusters, and she may spawn repeatedly over several nights to lay 100,000 or more eggs.
Horseshoe crabs are characterized by high fecundity, or ability to have a large offspring, as well as high egg and larval mortality and low adult mortality. They breed in late spring on low-energy coastal beaches along the Atlantic and Gulf of Mexico coasts in the United States, laying eggs in nests buried in the sand. Larvae hatch from eggs within two to four weeks, although some larvae may overwinter within nests and hatch out the following spring. Planktonic larvae typically settle within one to two weeks of hatching and begin molting as they grow. Juvenile crabs remain in intertidal flats, usually near breeding beaches. Older individuals move out of intertidal areas to deeper waters.
The American horseshoe crab (Limulus polyphemus) is the only species of horseshoe crab found in North America. Other horseshoe crab species (Carcinoscorpius rotundicauda, Tachypleus gigas and Tachypleus tridentatus) are found in Asia.
The horseshoe crab has primitive body. The body is composed of three parts: the prosoma, or head, the opisthosoma, or central area, and the telson, or tail. The horseshoe crab’s name is derived from the prosoma, resembling the shape of a horse’s shoe. The telson helps the crab to flip itself over if waves on the beach turn it over.
Horseshoe crab size varies along the Atlantic coast, but the largest individuals can be found along the Georgia coast with mature females reaching sizes of 12 inches (30 cm) across the prosoma, and a length of 24 inches (60 cm), including the tail.
Horseshoe crab size varies along the Atlantic coast, but the largest individuals can be found along the Georgia coast with mature females grows to about 11 pounds (4.8 kg). Males are typically 25 to 30% smaller than females.
Horseshoe crabs are generally a dull olive green or brown in coloration when viewed from above. The underside is more brown in appearance.
Habitat requirements change throughout the horseshoe crab life cycle, extending from intertidal beach fronts and tidal flats in coastal embayments for eggs and larvae, to the edge of the continental shelf for adults. Limulus has been described as an ecological generalist by K. Sekiguchi and C.N. Shuster in 2009, that is able to tolerate a wide range of environmental parameters throughout its distribution.
Of or relating to the sea.
The diet of juvenile horseshoe crabs is varied, including particulate organic matter from algal and animal sources. As horseshoe crabs mature, the diet composition shifts to larger prey. H.R. Carmichael and others in 2004 note that horseshoe crabs are known to be important predators of small invertebrates that live in the substrate of marine environments.
Horseshoe crabs are living fossils that are a common sight along the U.S. Atlantic coast. They are commonly observed along beaches in the spring when they come on shore to spawn and their spawning aggregations can be very impressive with thousands of animals coming on shore during high tide events. Females will typically have one male attached in amplexus and also surrounded by several other males.
Although horseshoe crabs look prehistoric and menacing with their multiple legs and long tail that looks like a stinger, they are completely harmless.