Featured Creature: Horseshoe Crab

Photo from National Wildlife Federation

What creature far older than dinosaurs has survived on earth for 480 million years, loves going to the beach, has enough vision for a Nobel Prize, is very much ‘for the birds’ and deeply treasured for its blue-blooded ‘aristocratic’ character?

The horseshoe crab, or limulus polyphemus, of course!

Photo from National Wildlife Federation

So how old are these creatures?

Horseshoe crabs are “living fossils” dating back almost as far as we can see, basically unchanged from their earliest times. Their fossil relatives are recognized as far back as the Ordovician Period (485.4 million to 443.8 million years ago), and forms similar to modern-day horseshoe crabs date back to the Jurassic Period (201.3 million to 145 million years ago). Horseshoe crab fossils, similar in form to the ones you see in estuaries today, can be found in these ancient rocks.

Horseshoe crabs are ‘benthic’ or bottom dwellers, moving over sand propelled by their six pairs of legs. But horseshoe crabs are not true crabs at all – instead, they’re much more closely related to arthropods such as spiders and scorpions. True crabs have antennae, jaws, and only five pairs of legs. Horseshoe crabs don’t have antennae or jaws and have six pairs of legs. However, two of those legs, called ‘chelicerae’, are not used for movement but for pushing food into the horseshoe crab’s mouth. 

Horseshoe crabs are in a class of their own: Merastomata, which means “mouth surrounded by legs.” They are not poisonous like their scorpion cousins or harmful to humans – as long as you don’t step on their erected tails with your bare feet. (Ouch!)

Photo from American Oceans

Where do we find them?

There are four species of horseshoe crabs found around the world, but only one of them – the most abundant – appears in North America, on the Atlantic coast all the way from Maine to the Yucatan Peninsula in Mexico. The other three species reside in Asian waters. 

There are lots of horseshoe crabs, for example, in Delaware Bay because spawning isn’t very disturbed by wave action there. These horseshoe crabs inhabit sandy estuaries and come out onto beaches to spawn each spring on the big full and new moon tides, to bury their eggs in the sand much like turtles. 

The male has two boxing glove-like claws on its first set of walking legs. These specialized legs are called ‘pedipalps’ and allow the male to hold onto the female during spawning. During the spawn, smaller males attach themselves to the females’ backs, while other males gather round, each trying to help fertilize the eggs being laid in the sand. Young female horseshoe crabs can be identified by the lack of mating scars. So, I guess they all really dig getting into this spawning process…

Photo by Nadine Doerle

What is their reproductive cycle?

The female will lay a cluster of about 4,000 eggs and in one night of spawning can lay several egg clusters, and do this repeatedly for several nights to lay a total of 100,000 eggs during one moon cycle. The eggs are about 1.5 mm (1/16 inch) in diameter and greenish tan in color.

Fertilization of the eggs occurs outside of the body while the female is burying the eggs on the beach. These eggs will hatch within two to four weeks, in time for the next big high tide. The larvae, which look like miniature adults but without tails, will then emerge from the sand and enter the water, spending their first few years on the tidal flats and move further out from the shore as they get older. Adults winter in deep bay waters and then, with the approach of spring, they move en masse toward the beaches to spawn a new generation of eggs.

Horseshoe Crabs Mating (Wikipedia)
Horseshoe Crab Eggs (Wikipedia)

Horseshoe crabs have hard exoskeletal shells, so they have to molt when they grow, crawling out the front of these shells as they struggle to free themselves. They will molt at least six times during their first year of life, and about 18 times in total before they reach sexual maturity after about nine years. Females, being larger than males, will likely have to molt more often. Once they reach maturity, they don’t need to molt again. Adults will likely live for another ten years or so after this, with a total possible life span reaching up to 20 years.

What are their dominant physical characteristics?

Horseshoe crabs have a unique and primitive body structure. They are invertebrates and their exoskeleton has three main parts: the head (prosoma), shaped like a horseshoe; a central part behind the head (episthosoma); and the tail (telson) at the back of that central part. Each part is hinged so that the crab can bend and turn itself over, even on dry land, if it gets rolled onto its back by a wave or some other circumstance.

Horseshoe crabs have several different kinds of visual capabilities. They have two compound eyes at the top front of their main shell which can magnify sunlight ten times, combined with two simple eyes at the front of its shell that can sense UV light from the moon. They also have similarly simple eyes underneath their bodies and on the underside of their tails that are sensitive to light. This may be why horseshoe crabs swim upside down.

These creatures have a set of six gill flaps on the underside of their central part that fold like the pages of a book. These are called book gills, and the first pair of them, the ‘operculum’, helps protect the other gills, covers the opening that the crab uses to release either eggs or sperm, and is used to propel them forward while swimming upside down. As long as these gills stay moist, these creatures can remain out of water for as long as four days! This helps horseshoe crabs survive when they are stranded on a beach, where they can bury themselves in the sand or fold themselves in half to keep from drying out.

Photos from Ecological Research & Development Group Inc.

Much like other parts of this creature, the horseshoe crab’s circulatory system is quite unusual. Horseshoe crabs have a long, tube-like heart that runs the length of their body (except for the tail). It beats about 32 times per minute, pumping the horseshoe crab’s blue blood through arteries and out into the rest of the crab’s body. Their brains rest in the middle of their head section, and nerves run from the brain to the rest of the body, including to the horseshoe crab’s many eyes.

Horseshoe crabs have no teeth. Instead, their mouths are surrounded by spiny mouth bristles at the base of their legs that are used to maneuver food toward the mouth. They eat crustaceans and marine worms of various kinds, such as the clam worms I wrote about in a Featured Creature several weeks ago. Curiously, in order to chew their food without teeth, horseshoe crabs have to simulate walking movements to grind up their food similarly to the way a bird can grind food down by taking in gravel. Whatever our anatomical structures, creatures make do!

What about the horseshoe crab’s interdependent ecological impacts?

The spawning activities of horseshoe crabs support many other bird species that fly up along the Atlantic coast during their spring migration from many thousands of miles south. Those spawned eggs that are not fully buried or get unearthed are a critical source of food for these avian species in order to give them sufficient sustenance to continue their migration northward to Canada for their own reproductive breeding efforts. 

For example, more than half of the total Atlantic flyway population of at-risk shorebirds (such as red knots, ruddy turnstones and semipalmated sandpipers) depend on Delaware Bay’s horseshoe crab eggs as a food supply throughout their spring migration north. These birds can double their weight in two weeks of feeding on these eggs just to recoup all the pounds that they lost during their lengthy journey from South America. Were they unable to take advantage of the horseshoe crabs’ spawn, they would not be able to make it through this arduous Northward journey themselves. This is a good example of an interlinked ecological system where multiple species have evolved together to meet each other’s needs.

Photo from National Wildlife Federation Blog

Why do we care about their ‘aristocratic’ heritage of blue blood?

Human blood is iron-based, while horseshoe crab blood is based on copper. This turns the blood a nice shade of blue when exposed to oxygen, much like a tarnished copper pot. Horseshoe crab blood contains primitive large blood cells called amoebocytes which can be used to produce a critical clotting agent called Limulus Amoebocyte Lysate (LAL), which clots in the presence of any bacterial toxin. Pharmaceutical companies use LAL to test for the sterility of vaccines, drugs, prosthetics and other medical devices, so you should thank the horseshoe crab whenever you get a shot for protecting you from infection! 

A synthetic version of LAL has recently been developed that may thankfully reduce the commercial demand for harvesting blood from this species, though they will likely continue to be used for bait. Horseshoe crabs’ compound eyes have also provided us with a great deal of knowledge about human vision. One notable example includes a study of their vision that was awarded a Nobel Prize in science in 1967, which makes this species very insightful!

Photo from Reddit

How are horseshoe crabs being impacted by humans and other predators?

Human development constitutes the greatest threat to horseshoe crabs, particularly along the beaches where they need to spawn. They can be crushed by recreational vehicles driving on beach fronts, or their young can be harmed by pollution and oil spills from barges and tankers, placing at risk not only these crabs but all those shorebirds and other species that depend upon them at such a critical time in their lives as well. 

Horseshoe crabs have long been harvested for use as fertilizer and bait starting in the late 1800s, but a century later in the 1990s their blood was found to have valuable medicinal properties. By 1998, the Atlantic States Marine Fisheries Commission (ASMFC) developed a Horseshoe Crab Management Plan that established mandatory state-by-state harvest quotas for this species, and created a 1500-square-mile protected sanctuary for them off the mouth of Delaware Bay. 

However, the horseshoe crab population has continued to decline. What was once thought to be 1.2 million spawning female crabs is now down to only around 400,000. Overfishing of this species for bait may be one reason for the lower numbers. There are also many hundreds of thousands of horseshoe crabs taken each year for the extraction of blood for medicinal use, and so the annual harvest of horseshoe crabs for bait and blood is still excessive and at an unsustainable level.

The pharmaceutical industry claims that taking 30 percent of these crabs’ blood and then releasing them back into the ocean only causes a 5-10 percent mortality; the ASMFC estimates it at 15 percent but other more recent studies set the biomedical mortality of this species at as high as 20-30 percent after the harvest of blood and their saltwater release. The International Union for the Conservation of Nature (IUCN) classified American horseshoe crabs as “vulnerable” in 2016; the Japanese version of this species was deemed “endangered” in 2018. Its population in North America appears to have suffered a 90 percent decline since 1990.

The importance of this species for the health of our entire ecosystem – beyond just that of us humans – is very difficult to overstate; the protection of vital spawning areas and horseshoe crab habitats is of critical significance for the sustainability of many other life-forms that depend upon this ancient and venerated denizen of the deep. There is so much that we still have to learn about these interdependent relations, that we ought to approach all these matters with a profound sense of awe, humility and reverence.

By Fred Jennings