Horseshoe crabs are often referred to as "Living Fossils" since the fossil record shows that very similar species existed as far back as 250 million years ago. There are 4 species of horseshoe crab that exist today. 3 of these occur in the waters around India, Japan, and Indonesia. The fourth species, Limulus polyphemus, occurs in the waters along the east coast of North America from Northern Maine to the Yucatan peninsula. 90% of the population lives along the mid-Atlantic coast with the largest concentration along the Delaware Bay. (The species L. polyphemus is assumed throughout the rest of this article.)
The horseshoe crab is not a true crab, it is actually more closely related to the spider. True crabs have 2 pairs of antennae, a pair of mandibles, a pair of claws and four pairs of legs. Horseshoe crabs do not have antennae or mandibles, but do have a pair of small pincers in front for maneuvering food. They also have 5 pairs of legs including 4 pairs with pincers. Horseshoe crabs grind up their food with the base of their legs and push it into their mouths which are located between their legs (image). The horseshoe crab's body is divided into 3 parts: the front dome-shaped prosoma, the middle spine-edged opisthosoma, and the rear spike or telson. The legs are attached to the underside of the prosoma while book gills are attached to the opisthosoma. The telson is used primarily by the crab to flip itself if it gets turned over during spawning (image).
Horseshoe crabs take 9-10 years to reach sexual maturity and can live to be 16-17 years old. They can grow to a length of 2 feet and a weight of 10 pounds. For most of the year they crawl along the bottom of bays and along the continental shelf feeding on marine worms and shellfish.
In late spring and early summer mature horseshoe crabs migrate into shore to spawn.
Along the Delaware Bay, peak spawning events usually occur
with the high lunar tides around the full and new moons during May and June.
The numbers of crabs spawning around each lunar tide varies from year to year.
Peak events usually occur at night or in the low light around dawn or dusk.
Spawning usually occurs along sandy beaches in bays and coves
where there is protection from waves.
Weather conditions and water temperature can effect when and where crabs spawn.
Heavy surf will prevent crabs from spawning.
During spawning, males arrive first and accumulate at the base of the beach
and await the arrival of females.
The females make a more direct route to the beach.
Males are smaller than the females and have a front pair of
"boxing glove" pincers which they use to grasp the female's
opisthosoma (image) as they follow her up sandy beaches to spawn.
A female crab with one or more accompanying males will work their
way up the beach.
The female burrows into the sand to form a nest at a depth of
15-20 cm (~6-8 in).
These nests are usually located between the low and high tide marks on the beach.
The female deposits eggs into the nest and as she pulls away, the male
fertilizes the eggs as he passes over the nest.
Several nests may be laid during a single beach trip and
females will make additional trips on subsequent tides.
Studies in Delaware found that females laid an average of
3,650 eggs per nest and can lay as many as 88,000 eggs per season.
If the density of crabs and nests is high enough, a female crab is likely to dig up a previous nest and its eggs when digging a new nest. This is significant to shorebirds since the disturbed eggs accumulate on the surface of the beach and are readily available as food. Eggs in undisturbed nests are inaccessible to most shorebirds.
The time eggs take to develop in the nest is dependent on
temperature, moisture, and oxygen.
These factors vary considerably based on the level
of the nest relative to the tides.
Most larvae emerge from their eggs after a month or more of development.
After emerging from the egg and escaping the beach, the larvae swim
for around 6 days before settling to the bottom and molting.
The swimming larvae tend to stay in the intertidal areas near
the beaches where they hatched.
The horseshoe crab continues to grow with subsequent molts.
Molts take place as follows: 5 molts in the first year,
2 or 3 molts in the 2nd year, 2 molts in the 3rd year,
and then a single molt per year until maturity is reached at
9-10 years of age.
Juveniles tend to stay in the intertidal flats near where they hatched
for their first two years and then gradually move to deeper waters as they mature.
Once they reach maturity they will migrate back to shore each year to spawn.
Because of the long period required to reach maturity and the long life span after maturity is reached, horseshoe crab populations tend to be fairly stable. The impact of a single or several poor spawning years on the long term population is lessened due to these factors. It takes impacts sustained over long periods to cause large changes in the population. The long period to reach maturity also means that any impact on spawning and reproduction will not become apparent until at least 10 years later.
There is evidence for some amount of spawning site tenacity. In the Delaware Bay, adults tagged during spawning showed a tendancy to return to the same side of the bay though not necessarily to the same beach. Of 55 horseshoe crabs relocated in subsequent spawning seasons, 44 were found on the same side of the bay where they were originally tagged.