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Interesting Facts about Horseshoe Crabs

[Phylum: Arthropoda] [Subphylum: Chelicerata] [Order: Xiphosura (sword tail)] [Family: Limulidae]

Long before the dinosaurs appeared, horseshoe crabs (Limulidae) have scuttled their way around shallow seas and sandy shores for more than 450 million years.

Yet, despite being mistaken for crabs, they're not actually a crab at all and they're not even a crustacean. They are in fact closely related to spiders, parasitic arachnids, and scorpions.

Horseshoe Crabs: Living Fossils of the Sea!

They rank among some of the most prehistoric of all living animals on planet Earth.

They belong to a separate group of arthropods that are more closely related to spiders and ticks.

Fossil evidence shows that their ancestors appeared over 450 million years ago.

That means horseshoe crabs were around before dinosaurs and before flowering plants.

But it's a species worthy of protection. They are not relics of the past, they are active chelicerates that fulfill an essential component in healthy and modern coastal ecosystems.

Any animal that can survive for nearly half a billion years is a testament to its resilience and adaptation. Hence, a continued protection ensures that this ancient lineage has a chance to endure for generations to come.

Horseshoe Crab Characteristics

They have a hard, domed shell (called a carapace) that provides some protection against crashing waves and their natural predators, such as sharks and sea turtles. Underneath, they have ten (10) legs that they use for crawling around and for digging.

Even though the elongated telson (pointed tail) looks dangerous, they don't actually use it as a weapon. Instead, it's sometimes used as leverage so they can flip back over if something turns them upside down.

The body of a horseshoe crab is divided into three main sections:

Prosoma (the head region): The hard, horseshoe-shaped shell protects their brain and vital organs. It also acts as support for two large compound eyes and several smaller light-sensing organs.
Opisthosoma (the middle part of the body): This section contains the book gills (respiratory organs) and their legs. They also have leg spines (gnathobases) and abdomen spines running along the sides for protection.
Telson (the tail spine): It may look like a nasty stinger, but it is harmless to humans. It actually serves as a kind of lever to help them 'right themselves' if they get flipped over.

Underneath, horseshoe crabs have ten legs for walking, burrowing, and for handling their food. They breathe via their book gills, which are layered structures that resemble the pages of a book.

Fun Fact: Horseshoe crabs can breathe air for short periods using their special layered gills if they are kept moist.

Where Do Horseshoe Crabs Live?

They inhabit the shallow coastal waters along the Atlantic coast and certain parts of Asia, including Indonesia and the Bay of Bengal.

Interesting Facts about Horseshoe Crabs Each spring, triggered by high tides and full moons, they come ashore to lay thousands of tiny green eggs in the sand.

Here's the thing:

Horseshoe crab eggs are a critical food source for migrating shorebirds, especially the red knot (Calidris canutus).

The knot actually times its migratory behaviour to match the horseshoe crab's spawning season.

Biomedical Significance

Beside being a part of a complex food web, including providing nourishment for fish, sea turtles, and other marine animals, horseshoe crabs have an extraordinary role in modern medicine.

Unlike iron-based blood in humans, their copper-based blue blood contains a substance called Limulus Amebocyte Lysate (LAL), which clots in the presence of bacterial endotoxins.

Put another way...

Since the 1970s, LAL has been used to test the safety of vaccines, intravenous drugs, surgical implants, and other sterile medical products.

Recent advances are reducing the need for wild-harvested crabs through the use of synthetic alternatives such as recombinant Factor C (rFC). This helps to lessen some impact on wildlife populations.

Horseshoe Crab Conservation Status

After surviving mass extinctions of some animal kingdoms, horseshoe crabs now face a number of human-driven challenges, most significantly:

Overharvesting (such as for bait in eel and whelk fisheries)
Loss of habitat (mostly due to coastal developments)
Climate change (ocean acidification affects spawning grounds and migration timing)

Interesting Fact: Only four species of horseshoe crabs remain in the world today. One lives in the Atlantic Ocean (particularly in the Gulf of Mexico) and three species are found in Asia, including China and Taiwan. They all belong to the family Limulidae within the class Merostomata.

4 Types of Horseshoe Crab

Atlantic Horseshoe Crab (Limulus polyphemus)

It's often called the American horseshoe crab because it's found along the Atlantic coast of North America, such as areas close to Maine and further south in the Gulf of Mexico.

It is the only horseshoe crab species found in the Americas and the one that's been most studied as part of biomedical and ecological research.

Pro Tip: In 2016, the International Union for Conservation of Nature assessed the Atlantic horseshoe crab (Limulus polyphemus) as "Vulnerable" with a "Decreasing" population trend.

Indo-Pacific Horseshoe Crab (Tachypleus gigas)

Tachypleus gigas is more commonly found in the coastal waters of India, Malaysia, Thailand, and Indonesia. They are one of the smaller horseshoe crabs that prefer to live around muddy or sandy flats.

Mangrove Horseshoe Crab (Carcinoscorpius rotundicauda)

The round-tailed horseshoe crab gets its name from the short, rounded telson. They have a preference for brackish water in mangrove areas and estuaries across some parts of Southeast Asia.

Tri-spine Horseshoe Crab (Tachypleus tridentatus)

It is distributed in Southeast Asia, Japan, and parts of China and it's the largest species of horseshoe crab, sometimes growing up to eighty (80) centimetres long (2.5 feet).

Pro Tip: In 2018, the IUCN listed Tri-spine horseshoe crabs (Tachypleus tridentatus) as "Endangered" with a "Decreasing" population trend.

Related Information and Help Guides

Note: The short video [2:45 minutes] presented by "Deep Marine Scenes" contains even more horseshoe crab facts with some amazing footage of these ancient arthropods.