Not So Crabby
Spawning horseshoe crabs in New Jersey in May 2020 on the Delaware bay shore.
I can’t exactly put my fin on why I love horseshoe crabs so much. They are beautiful, old and quiet but all at once imperative to the ecosystem and people alike. These creatures are gentle and wise, but often misunderstood and undervalued. Older than dinosaurs, they have seen many billions of tide cycles. If they could speak to us, they would tell us the stories of their ancestors as they evolved in the ocean and migrate each year back to our beaches. Tethered to the moon phases and the tides, the horseshoe crabs symbolize rhythm and tradition.
Horseshoe crabs flipping themselves over using their telsons that are under the water.
Horseshoe crabs have been on the planet for over 400 million years. They are living fossils that have changed little over the millenia. Each species (4 worldwide) vary slightly, however all of them sport the unmistakable horseshoe shaped head, a midsection and a tail (Prosoma, Opithosoma, and telson). They are peculiar looking, beautiful in their own right. Horseshoe crabs are brown and smooth with a shine as they break the surface of the water. Even with many ‘eyes’ that serve different levels of detecting light, their most recognizable eyes are two compound eyes on either side of their prosoma, carapace, or more commonly understood as their head. Their many types of eyes are extremely sensitive to light allowing them to navigate at night. The tail is extremely important for their survival. To humans, it is often mistaken for a poisonous stinger or barb that is sharp and threatening. Ironically it is the most vulnerable when lifted by its tail, as the tail is attached with such small, fragile tissue. Their tail serves only to right the crab if it is turned upside down. The underside of the crabs can also frighten people. With 4 pairs of legs, pincers, a bristly mouth and flexing book gills, they can look scary. Closer observation reveals their pincers are harmless and weak, their legs controlling their mouth that is totally harmless to people. They eat crustaceans and worms that they find along the bottom and will crush them between their legs before passing the food to their mouth. These interesting creatures often swim upside down on an angle using their legs and book gills to propel them gracefully through the water column. The book gills flap in a ripple, extracting oxygen from the water.
Atlantic Horseshoe crabs (Limulus polyphemus)have a common name that is a misnomer because in reality they are not crabs but more closely related to spiders, a part of arthropoda phylum. Like other arthropods, they wear their skeleton on the outside (exoskeleton) of their bodies shedding it periodically to grow larger. Someone might even be lucky enough to find a perfect shed! The shed is clear, much lighter than the crab itself, odorless, and looks like an exact replica of the living crab.
Horseshoe crabs sit deep in the ocean at the precipice of the continental shelf in the Atlantic Ocean until May when they pull themselves along the bottom of the ocean floor many miles to get a taste of air where the sea meets the shore. Their favorite time to come to break the surface at the shore is during full and new moon high tides. At this time all mature crabs hustle to the beach just at the edge where they can still remain in the wash. The large regal females crawl and push sand while multiple smaller males hang on jostling for a chance to fertilize her eggs. As she lays her eggs in the depression she has made in the sand, the males release sperm simultaneously. Some crabs seem older than others with a larger carapace covered in hitchhikers such as barnacles, slipper shells, algaes and more. Some crabs are smooth and if one lays on the sandy beach, you can look into their eyes as they hustle to continue their species and make their way back into deeper waters. I recently sat quietly with the crabs both in Delaware Bay and Barnegat Bay as they moved through the wash dragging each other around. The sound of shorebirds and waves totally drowned any sound that could possibly be coming from the crabs. From what I observed, they were silent. They didn’t seem to care whether or not people, mermaids, or flocks of birds were there. The crabs moved about minding their own business. As I stared into their large compound eyes, I wondered about their consciousness, their ability to see me and acknowledge my presence. The females were so large, effortlessly dragging the males around as they worked together to procreate. Worry set in as I began to remember their importance and their growing scarcity
Horseshoe crabs spawning in the wash of the Delaware bay shore as a cacophony of shorebirds search for food and consume the crab eggs.
While their eggs will carry the next generation of horseshoe crabs, they also serve an incredibly important role in the life of migrating shorebirds, particularly the red knot shorebird. The most numerous horseshoe crabs can be found all at once in the Delaware Bay during the end of May. For hundreds of years, the mass horseshoe crab migration phenomenon has been timed with the northern migration of red knots as they make their way from the southern tip of South America to the Arctic (one of the longest migrations of all birds). The eggs of the crabs are high in fat and protein feeding the depraved birds along their journey. In recent years, crabs have been showing up less numerous and late, adding to the threats these migrating shorebirds face as they struggle to find food. So what’s negatively affecting the horseshoe crabs?
They serve humans in multiple facets that until recent years have been imperative. Conch and eel fisherman in the United States harvest these crabs for bait. The medical industry harvests their blood containing LAL which aids the biomedical industry in detecting harmful bacteria in human medicines. All while climate change affects their ability to survive and destroys their habitat. So as horseshoe crab numbers decline, more pressure is put on the shore birds that depend on their eggs for food.
These crabs do so much to help people. They are gentle creatures that follow their own patterns year after year leaving nothing but pincer tracks and eggs in their wake. So how can we honor these wise and ancient creatures with respect and thoughtfulness? We can return the favor and flip them over so they are not vulnerable to predators. Most importantly take care of their telson by handling them from their carapace. It is important to give them space on our estuary beaches to spawn. We can avoid bulk heading our estuary shorelines where they come to spawn as they require gently sloping sandy beaches. This affects the birds as they arrive on our bay beaches during migration searching for food. We can continue to monitor the crabs through volunteer surveys that count and tag crabs for future data collection. Ask the biomedical industry to prioritize finding a synthetic alternative to LAL. Finally we can educate our peers about horseshoe crab importance to people, shorebirds, and their harmlessness to us. Next time you are walking in the shallow sand flats of Barnegat Bay, look for their gentle track lines in the sand as they lead to the beach and back. These creatures are a prime example of the vein of significance that connects our estuaries to our oceans. Both habitats serve the horseshoe crab at different points in its life cycle. The undoubted connection the horseshoe crabs have to other animals in the kingdom is remarkable. There is hope that we can continue to show respect for these creatures so they may live for many more millennia to come.
If you would like to learn more about these crabs and interact with them, you can visit Jenkinson’s Aquarium in Point Pleasant Beach, NJ upstairs in their touch tank exhibit or at your local aquarium facility. You can also learn more about the Atlantic Horseshoe crab by following the links in this article. Thank you for caring for our estuaries!
-Brizo