FWS Focus

Overview

Characteristics
Overview

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.

Scientific Name

Limulus polyphemus
Common Name
Atlantic horseshoe crab
American horseshoe crab
Kingdom

Location in Taxonomic Tree

Genus

Identification Numbers

TSN:

Characteristics

Characteristic category

Life Cycle

Characteristics
Life Span

Horseshoe crabs are fairly long lived animals, with individuals living more than 20 years.

Reproduction

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.

Life Cycle

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.

Characteristic category

Similar Species

Characteristics
Similar Species

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.

Characteristic category

Physical Characteristics

Characteristics
Size & Shape

The horseshoe crab has primitive body structure structure
Something temporarily or permanently constructed, built, or placed; and constructed of natural or manufactured parts including, but not limited to, a building, shed, cabin, porch, bridge, walkway, stair steps, sign, landing, platform, dock, rack, fence, telecommunication device, antennae, fish cleaning table, satellite dish/mount, or well head.

Learn more about structure
. 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.

Weight

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.

Color & Pattern

Horseshoe crabs are generally a dull olive green or brown in coloration when viewed from above. The underside is more brown in appearance.

Characteristic category

Habitat

Characteristics
Habitat

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.

Marine

Of or relating to the sea.

Characteristic category

Food

Characteristics
Food

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. 

Characteristic category

Behavior

Characteristics
Behavior

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.

Geography

Characteristics
Import/Export

Horseshoe crab blood plays a vital role in human medicine. The straw-colored, copper-based blood turns blue when exposed to high concentrations of oxygen. Horseshoe crab blood contains primitive large blood cells called amoebocytes. A clotting agent called Limulus Amoebocyte Lysate is derived from the amoebocytes of the horseshoe crab. When the clotting agent comes in contact with bacterial toxins, a clotting reaction occurs. Pharmaceutical companies test the sterility of vaccines, drugs, prosthetics and other medical devices using Limulus Amoebocyte Lysate.

Horseshoe crabs are also harvested for bait in conch and American eel fisheries on the Atlantic Coast. Horseshoe crabs suffered a substantial increase in harvest in the 1990s that spurred the need for management on a coast-wide scale. In 1998, the Atlantic States Marine Fisheries Commission, representing 15 states from Maine to Florida, developed a horseshoe crab management plan, that along with its subsequent addenda, established mandatory state-by-state harvest quotas and created the 1,500-square-mile Carl N. Shuster Jr. Horseshoe Crab Sanctuary, off the mouth of Delaware Bay. Active management, as well as innovative bait conservation techniques, have successfully reduced commercial horseshoe crab landings in recent years.

Range

Horseshoe crab populations range from the Gulf of Maine to Florida. The species range extends into the Gulf of Mexico from Florida west into Louisiana and south to the Yucatán Peninsula. The species is considered to be absent from Texas to Tabasco, México.

Genetic information suggests that the distribution of the American horseshoe crab is comprised of multiple population units divided among large geographic regions. These regions are:

  • Gulf of Maine (USA) - including embayments from Great Bay estuary in New Hampshire and north into Maine
  • Mid-Atlantic (USA) - including all embayments south of New Hampshire to and including North Carolina
  • Southeast (USA) - including embayments in South Carolina and Georgia, with the Georgia population extending into northern Florida
  • Florida Atlantic (USA) - including embayments along the Atlantic coast of Florida south of the Georgia population
  • Northeast Gulf of Mexico (USA) - including embayments along the Gulf coast of Florida, Alabama, barrier islands of Mississippi and easternmost barrier island of Louisiana
  • Yucatán Peninsula (México) - including embayments on the western, northern and eastern portions of the peninsula, including the Mexican states of Campeche, Yucatán and Quintana Roo and Mexican portion of the Caribbean Sea.

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