Horseshoe Crabs

Alex Re (00:01): Hello, welcome to On Wildlife. I'm your host Alex Re. On this podcast, we bring the wild to you. We take you on a journey into the life of a different animal every week, and I guarantee you, you're going to come out of here knowing more about your favorite animal than you did before. This week's animal has been around for an inconceivably long time and they look like something from another world with tons of cool adaptations that help them survive in their environment. But even though they've been successful for so many years, they need our help because we're causing their demise. So join me as we dive into the ocean to talk about horseshoe crabs.

(00:58): Horseshoe crabs are marine arthropods. This means that they live in oceans. Arthropods are animals that have an exoskeleton, a segmented body, and jointed limbs. Three major classes of arthropods are insects, crustaceans and arachni. Their name comes from the fact that their body is kind of shaped like a horseshoe, and while their name has the word crab in it, they're actually more closely related to spiders and scorpions. Horseshoe crabs are in a class called Meris, which only includes them and some extinct scorpion species. And just like scorpions today, horseshoe crabs are also able to glow under UV light. Now, even though they share an ancestor with scorpions and spiders, their closest relative is the trilobyte, which lived during the Paleozoic era and went extinct over 500 million years ago. These animals looked like large underwater cockroaches and horseshoe crabs themselves have been on earth for around 300 million years.

(02:02): Well before dinosaurs and flowering plants. They've survived all of the mass extinction events that they've been present for. So these really are resilient animals. There are four horseshoe crab species. One species inhabits the Atlantic ocean along north and central America, and the other three species live in the Pacific Ocean off the southern and eastern coasts of Asia. These four species are not very different anatomically only minor differences in the length of their spines and the shape of their tails. Horseshoe crabs prefer living in waters less than 30 meters in depth. However, this can change depending on their stage in life or the time of year. The furthest down in the ocean they've been found is around 200 meters, but they'll go to much more shallow waters in order to breed. They also show sexual dimorphism, which means that males and females look different each other.

(03:00): Females grow up to 19 inches long on average while the males only reach around 15 inches. Horseshoe crabs bodies are divided into three separate sections, the proma or head, the abdomen, and the sson or tail. Now let's talk about that tail for a second. Contrary to popular belief, horseshoe crabs are not venomous and their tail is not used for self-defense. People probably assume this because stingrays tales are venomous, but because of the way their bodies are shaped, they can get stuck laying upside down. The main purpose of the tail is to help flip themselves back onto their legs. If they're on their back, the head of the horseshoe crab is encased in a large hard shell that protects everything inside of it like its internal organs. It also protects the largest two eyes that they have. They have 10 eyes in total, but they're not as complex as ours.

(03:59): These two eyes are the most complex. They're called compound eyes, and they're made up of thousands of light sensing structures. These eyes can help them see light and color. Horseshoe crabs are the only animals in their sub phylum that have compound eyes. Five of their eyes are found on top of their shells. They're not compound eyes, but they can sense light including UV light. The rest of their eyes are found on the top of their tail. All of these eyes come in handy when figuring out if it's day or night locating food, looking out for predators and finding a mate. Another interesting fact about horseshoe crabs is that they have blue blood just like octopuses. This is because while human blood has hemoglobin, which is iron based horseshoe crab, blood contains hemo sayan in which is copper-based. This blue blood is incredibly valuable because of its ability to help blood clot extremely fast in the presence of endotoxins. Endotoxins are toxic compounds found in certain bacteria that are released when their cells break break down. Scientists use their blood to help test and create medical equipment, vaccines, antibiotics, and more. Now we're going to talk about how humans impact horseshoe crab populations later on in the episode. But for now, let's take our first break.

(05:35): This week I want to give a shout out to Jack from New Jersey. His favorite animal is a lion. Lions are the only cats that live in groups. If you want me to give you a shout out on the podcast, send an email to on wildlife dot podcast@gmail.com telling me your first name, where you're from and your favorite animal.

(06:07): Now back to the episode. Now let's talk about what and how horseshoe crabs eat The class meris that horseshoe crabs belong to means legs attached to mouth. So what is this supposed to mean? Well, their mouths are actually located at the center of the underside of their body, which is surrounded by their legs. Horseshoe crabs are omnivores and they're also mainly scavengers, so they'll eat most things that they can find. They like to eat worms, clams, small crustaceans, and algae. They also have six pairs of legs, so 12 legs in total, and the pairs have different purposes. Only five of these pairs are actually used for walking. The other pair is used to remove sediment when making nests, and we'll talk about that more in a little bit. The front most pair of legs are used to pass food into their mouths. This pair is called the cholera, and you'll also notice that they have small claws on each of their legs.

(07:10): These are to help them grab onto their food. They also use these claws to break up food before passing it to the mouth. Horseshoe crabs also have bristles on their legs that break down the food even more as they walk. They need these bristles because they don't have any mouth parts like teeth or jaws. Instead, they have a gizzard similar to birds. This is part of what allows them to digest solid and tougher foods. The gizzard is a muscular part of their throat that can crush food, that the legs didn't crush enough and they'll regurgitate anything that they can't digest, like bones or pieces of shells. Most horseshoe crab feeding and activity is done at night, so they'd be considered nocturnal. So if it's dark under the water when they're feeding, how do they find their prey? Well, this is where their legs come in again, they have chemoreceptors on their legs that allow them to taste and smell the environment around them.

(08:08): Most living things leave some kind of chemical footprint and horseshoe crabs use that to their advantage. Now let's talk about horseshoe crab reproduction. These animals are very particular about when and where they'll reproduce. They really prefer areas that don't have a lot of large waves like bays and coves. And this is why you can oftentimes find them in the Chesapeake Bay, but they get even pickier than that. Research has also shown that the sand at the bottom of the water has to be the right size and it has to be extremely porous. In addition to the sand, the water also needs to be very well oxygenated. All of these characteristics are factors that will help the embryos survive and develop correctly. They also prefer to spawn in very shallow waters, and they lay their eggs between the low and high tide lines, and they usually start to mate around May or June.

(09:07): The most popular time for horseshoe crab spawning is on evening tides during full or new moons. So what's the reproduction process like? Females will release pheromones, which are like chemical signals into the water so that males can find them. And at spawning events, there are a lot more males than females. There. Basically, males will swarm a female forming huddles at the edge of the water. Sometimes up to six males will swarm. A single female males have developed a specialized appendage called a Casper that lets them attach themselves to a female's shell. After attaching himself to a female, she will carry him to the shore. This ensures that they'll both be there to fertilize the eggs that the female lays. Remember that females are much larger than males, and this is one of the reasons why when at or near the surf line, the female will dig a small nest using those hind legs that we talked about earlier where she'll deposit four to five clutches of eggs containing up to 4,000 eggs per clutch.

(10:14): And their eggs look kind of like little green dipping dots. Once the eggs are laid, the males who were previously clinging onto the female will fertilize the nest. The eggs take on average 14 days to hatch into larva. However, external factors like wave action, temperature hatching activity, and abrasion can affect this. Some eggs hatch as early as four days and as late as 30 days. And eggs that are laid further up on the beach and exposed to warmer temperatures will develop slightly faster. And the larvae look exactly like an adult horseshoe crab, but with no tail. As soon as they hatch, the larvae begins swimming from the place that they were laid to where they will live in their juvenile years. Intertidal flats near beaches. After spending about two years in the intertidal flats, they'll then begin moving into deeper waters. As the horseshoe crabs grow, they'll undergo a series of molts where they'll shed their exoskeleton and grow a new one.

(11:17): Many animals that have exoskeletons will molt at some point in their lives. The length of time that it takes a horseshoe crab to molt depends on their size. With the first molt only taking around an hour and more advanced molts taking up to 24 hours, breeding is not a one and done occurrence. Females will return to the same beach to spawn again multiple times, and they can lay up to 80,000 eggs per season. These animals can take nine to 11 years to reach maturity, and they can live for around 20 years. Horseshoe crabs are really amazing animals, and we're going to talk about why they're so important to the ecosystems that they live in right after the break. Time for today's trivia question, which animal is able to fly faster than any other animal in the world? A dragonfly be a bald eagle, c, a brown pelican, or d a peregrine falcon? The answer is d. Peregrine falcons can fly at speeds of up to 186 miles per hour.

(12:53): Okay, we're back. So why are horseshoe crabs important? Well, they're known as keystone species in the ecosystems that they inhabit. Most importantly, because of their eggs, their eggs and larva are a major source of seasonal nutrition. For many animals like seabirds, fish, crabs, and snails, specifically in the Delaware Bay, their eggs are a major source of food for migrating shorebirds. As the Delaware estuary is the second largest stop for 11 different species of these birds, the eggs help the birds restore the energy that they expend from their flight all the way from South America. But don't worry, this feeding doesn't have a major impact on the breeding success of the horseshoe crabs, as most of the eggs that are eaten have already been washed away by a wave. Horseshoe crabs themselves are also a staple food source in Loggerhead sea turtle diets. So they're really important for those sea turtle populations and their shells are also a benefit for some smaller organisms.

(13:57): The shell of an adult horseshoe crab can provide a habitat for a bunch of different organisms that will attach themselves directly to the shell, such as sponges, mollusks, and snails. Many species of bivalves, such as mussels, are regularly found attached to horseshoe crab shells as well. Some species of snail even use their shells as a breeding ground, and there have been sightings of horseshoe crabs covered in mud snail and drill snail egg casings. And there's one species that has specifically adapted to use horseshoe crabs as a host the limulus leach. So all of these different organisms rely heavily on horseshoe crabs, but unfortunately they're struggling right now. Horseshoe crab numbers have significantly decreased before we could even figure out the full extent of all the good that they do for the environment. The north species of hermit crab is listed as vulnerable while the other three species are endangered and on the verge of extinction.

(14:59): This is all mainly due to human activity. One of the biggest problems that they're facing is over harvesting. Because the range is so vast throughout the coast of Asia, it's hard to regulate how many horseshoe crabs people should be harvesting from the ocean. In North and Central America, horseshoe crabs are harvested to be used as bait, and also for biomedical use. Like we talked about earlier, the leading cause of horseshoe crab deaths is over harvesting by a wide margin. The natural death rate is much lower. 10% of all adult horseshoe crab deaths are from accidental strandings on the beach. During spawning. As discussed earlier, horseshoe crabs are sought out for their blood too. The process of removing the blood is not outright fatal in itself. However, it requires the removal of around one third of the animal's total blood after the removal of their blood.

(15:56): Horseshoe crabs are released alive back into the ocean, and 15% of them don't survive this. In some areas of the world, they'll take all of their blood, not even giving them a chance to live. Like most other animals, horseshoe crabs are being negatively impacted by rapid human development and pollution as well. Beaches and estuaries, which are areas that horseshoe crabs rely on to reproduce, are being destroyed by developers. People want to build houses, resource, and hotels in these areas, which removes the natural habitat and leaves horseshoe crabs with less areas to reproduce. It also doesn't help that they take so long to reach maturity. Luckily, there are organizations that we can support to help horseshoe crabs and their environments. You should check out the Atlantic States Marine Fisheries Commission, the Connecticut Audubon Society, and the Ecological Research and Development Group. Thank you so much for coming on this adventure with me. As we explore the world of horseshoe crabs, you can find the sources that we use for this podcast and links to organizations that we reference at onwildlife.org. You can also email us with any questions at On Wildlife dot podcast@gmail.com, and you can follow us on Instagram under Wildlife or on TikTok at wildlife. And don't forget to tune in next Wednesday for another awesome episode, and that's On Wildlife.

Jess Avellino (17:34): You've been listening to On Wildlife with Alex Re. On Wildlife provides general educational information on various topics as a public service, which should not be construed as professional financial, real estate, tax, or legal advice. These are our personal opinions only. Please refer to full disclaimer policy on our website for full details.