Awesome Animal - Horseshoe Crab

Perhaps recently you have read news articles about the importance of horseshoe crabs in the development of the COVID-19 vaccines. That's pretty interesting, but the horseshoe crab was an awesome animal way before its blood was used in medical research. In fact, these creatures have been awesome animals for nearly 450 million years!

What the heck is a Horseshoe Crab?

Well, first, they aren't crabs at all (and obviously they aren't horseshoes). In fact, they aren't even crustaceans. Instead, they are related to spiders. Recent studies have placed them into the class Arachnida. Horseshoe crabs get their name from their horseshoe-shaped carapace (the largest part of their exoskeleton). They grow to about two feet long (61 cm), weigh up to ten pounds (4.5 kg), and live up to 40 years.

There are four living species of horseshoe crabs, and they live in shallow coastal waters through much of Asia and the Pacific and on the Atlantic coast of North America and the Gulf of Mexico.

Amazing Facts about the Horseshoe Crab

Okay, I know you want to ask, why isn't the horseshoe crab a crab? It kind of looks like a crab. It lives in oceans. It has an exoskeleton and crawls around in the water on jointed legs. Therefore it seems very crabby.

Still, not a crab. Crabs, along with lobsters, crayfish, and others, are in the class Crustacea. Crustaceans are in the huge phylum Arthropoda. Horseshoe crabs are arthropods, but they are not crustaceans. They are in their own class of arthropods called Merostomata (which means "legs attached to the mouth").

Enough with the fancy names already—why aren't these things considered crabs? Structurally, they are quite different from crabs. For example, horseshoe crabs do not have antennae. Also, they do not have mandibles (mouth parts for chewing food). Instead, they have a pair of small appendages called chelicerae, which help them shove food into their mouths. You know what other animals have chelicerae? Spiders.

Here's what the underside of a horseshoe crab looks like. Can you spot the two chelicerae in front of the five pairs of legs?

By the way, it's not a good idea to hold a live horseshoe crab by the tale like that, as this can harm the animal.

Horseshoe crabs are often called "living fossils." This is because they have existed for a really long time without changing much at all. The've been around, basically in the same form, since long before dinosaurs existed. The oldest fossils of horseshoe crabs are from a species that lived about 450 million years ago, and those looked very much like the horseshoe crabs living today.

Scientists believe the closest relatives of horseshoe crabs are the sea scorpions, which originated about 467 million years ago. Sea scorpions are extinct now, but they were impressive creatures. Some were among the largest arthropods that have ever lived. See the size comparison diagram below, showing six of the largest known species of sea scorpions.

Interesting thought:  Why do some types of creatures exist for hundreds of millions of years without many changes to their basic form? There can be several reasons for this, but perhaps the easiest to explain is that some creatures have a body form that is already perfectly suited to the creature's niche (its specific environment, food source, etc.). Sharks are an example. And, of course, horseshoe crabs. Basically... if it ain't broken, don't fix it.

A horseshoe crab is basically an armored tank. Its entire body is covered by a protective shell. It walks around on the subsurface sand or mud on its five pairs of walking legs, and it preys on numerous types of sea worms, crustaceans, and clams.

A horseshoe crab gets its oxygen from the water using a series of book gills between the legs and the tail (see the book gills in the red outline below). These book gills get their name from the book-like arrangement of gills. Each one of the ten gills contains hundreds of layered folds called lamellae. So, it's kind of like ten books, and each book has hundreds of pages. This increases the surface area for absorbing oxygen.

Horseshoe crabs have amazing eyes. They have two prominent compound eyes on top of their shell. Compound eyes are made up of numerous smaller eyes, called ommatidia. Most arthropods have these (think of a fly's eye). Each of the horseshoe crab's compound eyes have about 1,000 of these ommatidia. But wait, there's more! In addition to the two compound eyes, these creatures have a pair of median eyes, a pair of lateral eyes, a pair of ventral eyes near the mouth (which is between the five pairs of legs), and they also have a group of specialized photoreceptor eyes on the tail. They have a plethora of eyes!

Although they cannot see fine details all that well, horseshoe crabs are amazing at seeing color and light levels. In fact, their cones and rods are the largest of any animal—100 times larger than those in humans. Amazingly, during the night their eyes are a million times more sensitive to light than during the day!

Below you can see one of the compound eyes.

Check out this video about horseshoe crabs.

Horseshoe crabs have been used in medical research for decades, particularly their blue blood. In fact, every pharmaceutical company in the world relies on horseshoe crabs. Why? Because their blood contains a substance with the tongue-twister name limulus amebocyte lysate. This substance is important because it detects endotoxin. What is endotoxin? It's a contaminant that can can kill you if it happens to get into vaccines, or any injectable drug.

It's impossible to understate the importance of this substance. A gallon of the stuff is worth $60,000.

Here's the bad news. Pharmaceutical companies catch half a million horseshoe crabs each year (the harvesting is regulated). They remove a portion of each crab's blood, then they release the crab. However, many of the released crabs are weakened, and they die. This practice, along with fishermen overharvesting horseshoe crabs to use as bait, is resulting in the crab's decline.

As you can imagine, the intense efforts to develop COVID-19 vaccines has resulted in more crab blood needed.

Here's the good news. A synthetic alternative to this substance has been developed. It has been approved for use in Europe, and some US companies have also started using it.

The synthetic alternative is slowly catching on, so maybe eventually we will no longer need to harvest horseshoe crabs for this purpose.

So, the Horseshoe Crab deserves a place in the S.A.H.O.F.
(Splendid Animal Hall of Fame).
FUN FACT: The adjective splendid originated in the 1620s. It probably came from the Latin word splendidus, which means "bright, shining, glittering." Today, splendid is used to describe just about anything good, and it has a broad range of meanings, including gorgeous; magnificent; sumptuous; grand; superb; distinguished or glorious in name, reputation, or victory; strikingly admirable; possessing great talents; excellent; fine; very good; and brilliant in appearance. I guess it's an all-around useful word for praising things.

So, splendid is another way to say awesome!

Photo Credits:
Horseshoe crab #1 - Pos, Robert, USFWS, via PIXNIO
Holding a horseshoe crab - DepositPhotos
Underneath side of horseshoe crab - DepositPhotos
Sea scorpion size diagram - Wikimedia Commons
Horseshoe crab compound eye - DepositPhotos