When the earliest dinosaurs were first evolving around 230 million years ago, horseshoe crabs had already been scuttling along the ocean floor feeding on marine worms and tiny shellfish for at least 100 to 200 million years; maybe even longer. Today, they still remain relatively unchanged—‘living fossils’ that have survived at least 5 mass extinctions throughout the eons and outlived most of their closest relatives, such as the ancient trilobites, whose famous fossils can be seen in museums throughout the world. Perhaps the term “lucky horseshoes” should really pertain to these amazing little creatures who are one of the oldest species on earth.
As a native of New Jersey, I hold a special place in my heart for these unusual animals. Every summer throughout the months of May and June, they arrive in large numbers along the beaches, most famously in the Delaware Bay, to spawn. I like to walk down the shoreline in the early morning to look for them; every now and then helping to right those that had been flipped upside-down by the surf. Since it is currently the peak of their breeding season, I figured I’d write a little bit about them for anyone who has never had the chance to see one in person.
First, I should point out that horseshoe crabs are not really crabs or even considered ‘crustaceans,’ at all. They are actually more closely related evolutionarily to spiders and scorpions, than crabs, lobsters, or shrimp. Millions of years ago, there were many different kinds of horseshoe crabs, though today only four species remain. Limulus polyphemus, is the species that we see most commonly on the eastern shores of North America and in the Gulf of Mexico, but other species can be found from the shores of India to Sumatra, Java, the Philippines, and China when they come ashore to breed.
All horseshoe crab species have the same general shape, with bodies made up of three sections covered by hard armor-like plating; some can even grow up to 3 feet long. The first and largest section resembles a semi-circle or horseshoe-shape, called the prosoma (I like to call this the head). Looking down at the animal from above, this section contains two large compound eyes on each side, but there are also 5 other rudimentary ‘eyes’ that are much smaller and harder to see. Some are sensitive to visible light, while others are sensitive to the ultraviolet range. It is believed that horseshoe crabs see very well at night, and also pick up on contrast better than we can, but this has not been verified.
If you turn the crab over to see the bottom of the prosoma, there are five pairs of legs—the pair closest to the tail of the animal are modified ‘pusher’ legs that they use to propel themselves with, and to clean their gills. Each leg, save for the pusher legs, have pincers at the end. In females, all these pincers look the same, but in males, their forward-most legs, called the pedipalps, are rounded with a little hook on the end that many researchers refer to as ‘boxing gloves.’ These help them hold onto the female’s shell during mating. Females also tend to be much larger than males, which is another way to tell them apart.
There are an additional two tiny legs in front of the others called chelicerae, which help push food towards the mouth, which is located between the five sets of main legs. The mouth structure is called a gnathobase, is lined with tiny hair-like spines, and the crab can only swallow food (always whole since it doesn’t chew) if its legs are moving. Spiders, the crab’s distant relatives, also have chelicerae, except they are usually in the form of pointed fang-like appendages that some use to grasp food. They are also often hollow, and/or connected to venom glands. But don’t worry–horseshoe crabs aren’t poisonous at all. They even have two additional light receptors or ‘eyes’ near their mouths, which are believed to help them orient while swimming.
The central section of a horseshoe crab’s body is called the opisthosoma. This is lined with movable spines on each side, and contains the musculature to move the tail as well as the ‘book gills’ underneath. These not only exchange respiratory gasses to allow the crab to breathe, but can also move like a series of fins and can help them swim. In fact, as long as the book gills remain wet, the crab can breathe—hence why they can come out of the water and survive by sitting on the wet sand. They don’t swim often though, usually only if necessary to escape predators like sharks, or to help move in rough surf. When they do, they usually swim upside-down.
The last part is called the telson, which is a fancy name for the tail. Many people think it is a poisonous spine, but this is a common misconception. The horseshoe crab just uses its tail to turn itself right-side up if it gets flipped over. Horseshoe crabs also cannot walk or swim backwards—if one gets cornered or stuck, it must use its tail to flip itself over and swim away. The series of bumps along the top and side of the tail are additional light receptors. Being primarily nocturnal animals, these are very important to help it synchronize with the day and night cycles.
Horseshoe crabs are also relatively long-lived, and it is believed they can survive up to 20 years or more. They grow in stages and molt their shells for about 10 years or so, until they mature. Once fully grown, they stop molting and often gain hitchhikers like mussels or barnacles that begin to grow on them. Each spring, during the high tides of the new and full moon, the males line the shores waiting for the females. Once they arrive, groups of males surround each larger female trying to grab on with their pedipalp. Once one does, he is dragged behind her as she lays up to 20 clutches of eggs in shallow holes that she digs, each clutch containing about 4,000 tiny pastel-green eggs. He fertilizes them as they are laid, and they hatch within 3-4 weeks.
These eggs attract huge numbers of shorebirds every year who gorge themselves with them to bulk up for their long migrations. One of the most famous of these birds has one of the longest migration routes known, and it is called the Red Knot. These small birds fly from the very tip of South America in Tierra del Fuego making a pit stop in Brazil, and then fly nonstop to the Delaware Bay to gorge themselves on eggs to prepare for the last leg of their journey—a nonstop flight to their arctic breeding grounds. Their total journey is about 9,300 miles!
Because horseshoe crabs have been hunted excessively in the last century for fertilizer (their bodies were ground up because they are high in nitrogen), and used as bait, the decline in crab numbers has also caused a decline in the numbers of these birds.The crabs have also been harvested in large numbers since the 1960s and 70s for use in the medical industry. Not only are their eyes being studied, but their blue blood is copper-based and contains an interesting substance called limulus amebocyte lysate (LAL) which can be extracted and used to detect bacterial toxins.
I suppose by writing this post, I just wanted to raise awareness of these amazing and fascinating creatures, as well as urge people to protect them. They have survived for so long, through such extreme conditions, that I find the fact that humans have done so much damage to their populations in such a short amount of time very disheartening. We are making advances though. Crabs are no longer killed for blood collection, but once they are captured, only about 30% of their blood is taken before they are returned to the ocean. Their blood volume rebounds within a week or so, while research has shown that it takes about 2-3 months for their blood cell count to return completely back to normal. In the past, moratoriums have also been placed on the harvest of horseshoe crabs in certain areas.
Finally, I can’t describe how many times I see kids at the beach picking the crabs up by their tails. This can seriously harm the crabs and parents must urge their kids not to do this. In addition, if there are shorebirds feeding on the beach, please do not disturb them. They need to eat as much as they can so they can make it to the arctic.
To learn more about horseshoe crabs and the shorebirds that depend on them, feel free to check out the following links: