Horseshoe Crab

One of the oldest animals on earth, horseshoe crabs have been roaming the ocean for hundreds of millions of years. They adapted to continents forming and reforming and survived the Ice Ages and cataclysmic events that made the dinosaurs and so many other species go extinct. When humans arrived, they discovered that horseshoe crabs could be used as food and bait to catch other kinds of food, and now they are used for medical research.

Quick Facts

Some fascinating facts about the horseshoe crab include:

• Horseshoe crabs are arthropods, which means they are related to spiders.
• They’re not crabs. They’re not even crustaceans.
• They evolved about 450 million years ago, which makes some people call them living fossils.
• The blood of these animals is sapphire blue when exposed to oxygen.

Classification and Scientific name

Horseshoe crabs belong to the family Limulidae, within the suborder Xiphosurida, and the order Xiphosura. This group includes four genera: Carcinoscorpius, Limulus, Mesolimulus (extinct), and Tachypleus. 

Species

The four living species of horseshoe crabs are:

• Carcinoscorpius rotundicauda (Mangrove horseshoe crab)
• Limulus polyphemus (Atlantic horseshoe crab)
• Tachypleus gigas (Indo-Pacific horseshoe crab)
• Tachypleus tridentatus (Tri-spine horseshoe crab)

Mangrove horseshoe crabs are found in southern Asia. Atlantic horseshoe crabs live along the east coast of the United States and Mexico. The Indo-Pacific horseshoe crab is also found in south and southeastern Asia. And the tri-spine horseshoe crab lives in Southeast and East Asia.

Appearance

Horseshoe crabs have a distinct horseshoe-shaped carapace, or protective covering, which covers their body, dividing it into three parts: the prosoma (head), opisthosoma (middle body), and telson (tail). The telson, which is long and stiff, functions as a rudder during swimming and helps right the animal if it becomes overturned. Despite its menacing appearance, the tail is not used for defense. Horseshoe crabs are equipped with multiple eyes. Some are located on the prosoma. In contrast, others are situated on the telson, helping them navigate their environment and even detect changes in light.

Adult horseshoe crabs vary in size depending on the species. For instance, the male mangrove horseshoe crab typically grows to about 12 inches. The female tri-spine horseshoe crab can reach up to 31.25 inches and weigh as much as 9 pounds.

Horseshoe crabs have many eyes. Their eyes are on and under their heads, and there are even eyes under their tails. This may be why they are sometimes found swimming upside down. If the animal is flipped upside down, a person can see its book gills, which it uses to breathe underwater. These gills get their name because they look like the pages of a book.

Though they lack teeth, they can still eat shellfish whose shells aren’t too thick. This is because they have a mouth filled with bristles in the middle of their five pairs of legs. They use their legs to move their food to their mouth, then chew it by walking.

Distribution, Population, and Habitat

All species of horseshoe crabs prefer shallow water with muddy or sandy bottoms. There are millions of these animals in the world’s oceans, though their numbers are declining in some places. The mangrove and tri-spine horseshoe crabs can be found in salt or brackish water around southeast Asia and north into China and Hong Kong. The Atlantic horseshoe crab is found along North America’s Atlantic coastline and in the Gulf of Mexico. The Indo-Pacific horseshoe crab is found on the coasts of the South China Sea down to the Bay of Bengal.

Predators and Prey

Horseshoe crabs are opportunistic feeders and consume a variety of organisms, including small fish, bivalves, invertebrates, and algae. Their predators include sea turtles, birds, and various fish, such as eels, flounders, and perches, which feed on the adult crabs and their eggs. Shorebirds, in particular, rely on the horseshoe crab eggs as a critical protein source during migration.

These animals also have their share of symbionts, since at one point they will no longer molt. Clams scuds, marine worms, algae, barnacles, oysters, starfish, and other creatures live on and in the horseshoe crab’s shell.

Reproduction and Lifespan

Horseshoe crabs can live for up to 20–30 years. They undergo frequent molts during their early years, sometimes as many as 18 times, before reaching reproductive maturity around age nine. The spawning season is one of the most impressive natural phenomena, occurring in spring during high tide at the new or full moon. Males arrive at the spawning site first and await females. Once a female arrives, she is typically surrounded by multiple males. A single female can carry between 60,000 to 120,000 eggs, which she lays in batches of about 4,000 eggs at each high tide.

Horseshoe Crabs in Medical Research

The horseshoe crab’s blood coagulates in the presence of bacterial toxins. In human hands, this helps to ensure that vaccines and other medicines are free of dangerous chemicals. To harvest the blood, the animal is caught, drained a little, and released back into the water. This method has a mortality rate as high as 30% though.

Population

The Atlantic horseshoe crab is classified as Least Concern by the IUCN, indicating stable populations. However, overexploitation for medical research and bait use poses a local threat in certain areas. In contrast, species like Tachypleus gigas are classified as Vulnerable, with population declines attributed to habitat loss, overharvesting, and pollution.