Horseshoe crab

Horseshoe crabs are special creatures that belong to a group called arthropods. Even though they are called crabs, they are not really crabs or even crustaceans. They are more closely related to spiders, ticks, and scorpions. Their bodies have three main parts: the cephalothorax, abdomen, and telson. The cephalothorax is the largest part and looks a bit like a horseshoe, which is where they get their name.

Today, there are only four kinds of horseshoe crabs left. The Atlantic horseshoe crab lives along the eastern coast of North and Central America. Others live in parts of Asia near mangroves or in coastal waters. Most horseshoe crabs live in the sea, but some can also be found in brackish water, which is a mix of fresh and salt water. They usually stay on the bottom of the water but can swim when they need to.

People sometimes catch horseshoe crabs for their blood, which helps scientists detect harmful bacteria. In some places, they are also used as fishing bait or eaten as food. Sadly, horseshoe crab numbers have been going down because of habitat loss and overharvesting. To protect them, many places have made rules about catching them and started breeding programs to help their populations grow.

Phylogeny and evolution

Horseshoe crabs have been around for a very long time. Fossils of their relatives date back to about 480 million years ago. Modern horseshoe crabs first appeared around 250 million years ago and have changed very little since then, earning them the nickname "living fossils."

Though they look like crabs, horseshoe crabs are actually related to spiders and scorpions. They belong to a group called chelicerates. Today, there are only four species of horseshoe crabs left. One lives along the Atlantic coast of the United States, while the others are found in parts of Asia. Scientists study these ancient creatures to learn about life from millions of years ago.

Anatomy and physiology

Horseshoe crabs are special animals that are not true crabs or even crustaceans. They are more closely related to spiders and scorpions. Their bodies are divided into three main parts: the cephalothorax, abdomen, and telson.

Horseshoe crabs have a hard shell called a carapace that covers their upper body. They also have several pairs of legs, including walking legs and pusher legs that help them move through mud. Some of their legs are specially adapted for mating.

Horseshoe crabs have many types of eyes, including large compound eyes on top of their shell and smaller eyes on the front and sides of their bodies. These eyes help them see both during the day and night, including ultraviolet light.

Like other arthropods, horseshoe crabs have an open circulatory system. They use a copper-based protein to carry oxygen, which makes their blood appear blue. They also have a heart that pumps this blue blood through their bodies.

Horseshoe crabs eat by using special bristles on their legs to break up food before it enters their mouths. Their digestive system includes a foregut, midgut, and hindgut, with most digestion happening in large sacs called hepatopancreases. They excrete waste through their gills and a special opening at the end of their bodies.

Distribution and habitat

Horseshoe crabs are found in certain parts of the world. The three Asian species live in South and Southeast Asia, along the Bay of Bengal and the coasts of Indonesia. One special type, the tri-spine horseshoe crab, can also be found as far north as China, Taiwan, and Southern Japan. In the Americas, the main species lives from Nova Scotia down to the Gulf of Mexico, with another group near the Yucatán Peninsula. These crabs usually live in salt water, but one kind, the mangrove horseshoe crab, is often seen in areas where the water is less salty.

Long ago, during the Early Pleistocene, about 2 million years ago, horseshoe crabs lived as far north as Greenland. At that time, the ocean there was warmer than it is today.

Past adaptation to freshwater

Some ancient horseshoe crabs lived in rivers and swamps instead of the ocean. They made this change more than once throughout history.

Behavior and life history

Horseshoe crabs eat worms, mollusks, crustaceans, and small fish, usually at night on the ocean floor.

They spend most of their time on the bottom but can swim, especially when young or when heading to shore to breed. They swim upside-down using their legs and a tail-like part called a telson to steer.

Baby horseshoe crabs start life as tiny larvae about 1 cm long, looking similar to adults but without all their body parts. As they grow, they add more parts and get bigger with each molt. Females grow to be larger than males. The smallest species is the mangrove horseshoe crab, while the largest can reach nearly 80 cm long.

During breeding season, horseshoe crabs move to shallow coastal waters and lay thousands of eggs in nests on beaches. The males attach to the females, and the eggs hatch after about two weeks, though many are eaten by birds.

Relationship with humans

Consumption

In some parts of East and Southeast Asia, horseshoe crabs are eaten as food. Their meat is white and rubbery, similar to lobsters, and can be eaten raw or cooked. However, it must be prepared properly to avoid getting sick. Only certain types of horseshoe crabs should be eaten, as some can cause illness if not handled correctly.

Use in fisheries

In the United States, horseshoe crabs are often used as bait for catching eels, whelk, or conch. Many crabs are caught each year for this purpose. However, some places like New Jersey have banned catching horseshoe crabs to protect birds that rely on their eggs for food.

Use in medicine

The blood of horseshoe crabs contains special cells that help detect harmful germs in medical testing. These cells are used to check for dangerous substances before medicines are allowed to be used. While most crabs survive the process of having their blood collected, it can affect their health and ability to reproduce. Scientists are working on new ways to test for germs without using horseshoe crab blood, and some companies have started using these new methods.

Conservation status

Horseshoe crabs face many challenges that make it hard for them to survive. Building along shorelines takes away space where they can lay eggs and can block their access to these areas. In places like Malaysia and Indonesia, the number of horseshoe crabs has gone down a lot since 2010 because people catch and eat them. This especially affects female crabs, which are needed for the population to grow.

Horseshoe crabs are also hurt by pollution, being used for fishing bait, and their blood being used in medicine. Some species, like the tri-spine horseshoe crab in Taiwan, have disappeared from certain areas. To help protect them, a breeding center was created in Johor, Malaysia, where baby crabs are raised and released into the ocean every two years.

In Delaware Bay, fewer horseshoe crabs could hurt migratory birds called red knots, which rely on their eggs for food. Efforts are being made to manage crab harvesting to protect both the crabs and the birds. In 2023, harvesting was stopped in South Carolina during part of the year to help the crabs reproduce.