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Evolution:
Horseshoe crabs have existed for more than 400 million years. There are no subspecies of the Atlantic Horseshoe crab. Their closest living relatives are spiders and scorpions. Their closest ancestor is probably the trilobite. The oldest know horseshoe crab is the Lunataspis aurora that lived 445 million years ago. It also know as a living fossil, because it has not changed much during the 445 million years.
Description:
Horseshoe crabs have a hard, rounded, brownish-green exoskeleton, a spike-like tail and five pairs of jointed legs and they have 10 eyes, 2 in the front and 8 runnig across both sides of their body, which makes them sentisive to light. Their widely spaced eyes look like bumps on the top of their shell. Their gills have folds of membranes that look like the leaves of a book. The Atlantic Horseshoe crab's lifespan is 25 years. They weigh 2lbs and 2 ounces. Their width is 7 1/4 in and they can grow up to 24in in length including the tail. female horseshoe crabs tend to be larger than males and grow their first of walking legs before males too. Which have a hooklike structure that resembles a boxing glove.
Habitat:
Horseshoe crabs utilize different habitats depending on their stage of development. The eggs are laid on coastal beaches in late spring and summer. After hatching, the juvenile horseshoe crabs can be found offshore on the sandy ocean floor of tidal flats. Adult horseshoe crabs feed deeper in the ocean until they return to the beach to spawn. Horseshoe crabs tend to live in water tempertures ranging from 23F -95F. Animals in their habitat include shorebirds, migratory birds, turtles, fish, small clams, crustaceans, and worms. Plants in their habitat are algae, seagrasses and kelp. The average annual rainfall depth is 1034 mm for the North Atlantic ocean.
Distribution:
The atlantic horseshoe crab is found in North America along the Atlantic and Gulf coasts from Maine to Mexico. Atlantic horseshoe crabs are not invasive to North America.
Diet:
Atlantic horseshoe crabs eat small clams, crustaceans, worms and algae. A horseshoe crab picks up food with appendages located in front of its mouth. Since it has no mandible or teeth, the horseshoe crab crushes food between its legs before passing it to the mouth. It spends most of its time rooting through bottom sediment looking for food.
Social Structure:
Atlantic horseshoe crabs are solitary animals. Horseshoe crabs are known to gather in large nesting aggregations, or groups, on beaches particularly in the mid-Atlantic states such as Delaware, New Jersey and Maryland in the spring and summer, where their populations are largest.
Reproduction:
How often do they mate?
Horseshoe crabs spawn multiple times per season and per tide
How many mates do they have
Atlantic horseshoe crabs have multiple mates throughout the mating season.
Time of year for mating
May and June is their breeding season, when hundreds of Horseshoe Crabs come ashore during full and new moons to breed.
Mating rituals
During the late spring and early summer, adult horseshoe crabs travel from deep ocean waters to beaches along the East and Gulf coasts to breed. The males arrive first and wait for the females. When the females come to shore, they release natural chemicals called pheromones that attract the males and send a signal that it's time to mate. Horseshoe crabs perform a mass mating ritual at high tide on a full or new moon in the summertime. Thousands flock to bays all along the coast to fertilize and lay their eggs.
Number of offspring produces at a time
approximately 3,650 to 4,000 eggs in a cluster
Any unique characteristics of their mating
the smaller male crab hooks himself to the top of the larger female's shell by using his specialized front claws, and together they crawl to the beach. The male fertilizes the eggs as the female lays them in a nest in the sand.
Development:
Stages of development
If the egg survives, the larval horseshoe crab will hatch from the egg after about two weeks or more. The larva looks like a tiny version of an adult horseshoe crab, but without a tail. Larval horseshoe crabs travel into the ocean water and settle on the sandy bottom of tidal flats for a year or more. As they develop, they will move into deeper waters and begin to eat more adult food. Over the next 10 years or so, the juvenile horseshoe crabs will molt and grow. At around 10 years of age, horseshoe crabs reach adulthood. They are ready to start breeding and will migrate to coastal beaches in the spring. A horseshoe crab can live for more than 20 years.
How old before sexual maturity
It takes at least 9 to 12 years for a horseshoe crab to reach sexual maturity.
Activities and environment of stages
A crab will molt at least 6 times in their first year of life and about 18 times before they reach sexual maturity.
Difference between males and females
Female horseshoe crabs tend to be larger than males and grow their first of walking legs before males too.
IUCN Rating:
What is the rating?
near threated
Population
estimate of 2.3 to 4.5 million
Population Trend
The adult female crab population increased when compared with last year. But over all the Horseshoe crab populations have crashed in recent decades because of overharvesting and habitat loss..
Competition
Inter-species – within the species
The Atlantic horseshoe crab, has intense male–male competition (high operational sex ratios and multi-male groups around nesting pairs) and very high female nesting densities.
Intra-species – with other species
They do not compete with other species.
What do they compete over
They do not compete with other animals for food or space. They do have commensalism interactions with smaller organisms, such as sponges, bivalves, and many snails are often found just living on their backs and undersides.
Affect of invasive species
There is no evidence that they are affected by any invasive species.
Causes of death:
Predation
Sharks, other fishes, sea turtles and birds all prey on adult and young horseshoe crabs. Many migrating shorebirds feed on horseshoe crab eggs during their spawning season. Humans are also a predator of horseshoe crabs, harvesting them for bait.
Sexual Competition
Horseshoe crab mating behaviour leads to competition between males, but not between females.
Disease
Horseshoe crabs can be impacted by various pathogens including algae, fungi, cyanobacteria
Other causes
Horseshoe crabs commonly get overturned by high wave action during spawning and may not be able to right themselves.
Impact of Humans:
What impacts have occurred
Groins and bulkheads may adversely impact horseshoe crab spawning habitat. Bulkheads can block access to intertidal spawning beaches, while groins and seawalls intensify local shoreline erosion and prevent natural beach migration.
Potential Future impacts
Shoreline development and subsequent habitat degradation is likely an important threat to horseshoe crabs.
How long have humans been interacting with species
from the mid- to late 1800s documents the use of horseshoe crabs for fertilizer and to supplement livestock feed. By the 1870s and for almost a century thereafter, well over a million crabs were harvested annually from Delaware Bay, in support of a regionally significant ''cancerine'' (fertilizer) industry.
Efforts being made to minimize impacts
Starting from 1998, the Atlantic States Marine Fisheries Commission implemented an “Interstate Fishery Management Plan” to preserve the declining Atlantic horseshoe crab population, including regulations requiring states to identify horseshoe crab habitats, threats to the species as a whole, and the methods needed to protect horseshoe crabs from threats of overharvesting and habitat degradation (Schrading et al. 1998). The most significant conservation response has been seen with general fishing harvest regulations for bait along with a smaller effort for biomedical purposes as well. In 2006, bait harvesting was limited to only 100,000 male crabs with a delay in the harvest season to allow for more reproduction in the spring months.
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