Carnivores & Our Coast: Going back to the basics for ground-breaking research on horseshoe crabs

NORTH CAROLINA (QUEEN CITY NEWS) – You’ve probably seen little horseshoe crabs on so many of our Carolina beaches, “people call them rumbas of the ocean; really, they’ll eat anything in the sediment that they can feasibly get into their mouth,” explains Alexis Longmire.

They use their front legs to grab onto dinner, one they find with one of their eleven eyes! They have “two main, what we call compound eyes, the rest of them are what we call photophores.”

Photophores line their whole body but only detect changes in light, “so it’s just like if you were looking up towards the sun or light and you close your eyes and you like run your hand across your eyelids, you can still see the shadow,” explains Longmire.

As you can tell, Alexis Longmire is an expert in all things horseshoe crabs, “that’s how they breathe, and they’re called book gills because in the water they flap like pages in a book.” She’s a Ph.D. student in marine ecology at the UNC Institute of Marine Sciences.

Crab by name, but not by genetics: horseshoe crabs are actually closer cousins with scorpions and spiders, not crabs nor crustaceans. Longmire explains, “We call horseshoe crabs living fossils,” they used to crawl the earth with the dinosaurs, “they’ve been relatively unchanged for 450 million years,” she adds.

But today, we are challenging them like never before, “we know this creature can survive a lot of conditions,” reflects Longmire. Warming oceans, sea level rise, and land development are renovating their homes, “but we’re introducing conditions that this planet hasn’t seen, at least not since humans and horseshoe crabs evolved,” she adds.

We know their environment is changing, but we don’t know how it’s impacting these little ones, “there’s really been no studies or papers published about juvenile horseshoe crab ecology.”

So Longmire is breaking barriers by going back to the basics, “it’s definitely exciting but also can be very nerve-wracking because you’re the one that has to figure it out.” We don’t know how horseshoe crabs move, eat, grow, hide or survive as babies in their nurseries.

Longmire adds, “we have to figure out what we can attach, we’re figuring out how big and how small they can be, we’re figuring out how many times they molt,” and we need these answers to conserve them better.

So Longmire and her all-lady crew load up the boat hit the gas, and steer full speed ahead into Shackleford Banks in Carteret County to find these answers.

The team gets to an intertidal zone; Longmire explains, “it’s exposed in low tide, covered during high tide.” It’s where the baby horseshoe crabs live after they hatch on the beach.

Longmire has eight research sites that can range from the size of one to four football fields, “I’ll do a near-shore zone, closer to the beach. We’ll do a near reef zone right up against the reef, and we’ll do a behind the reef zone,” she assigns her team some spots to search for the horseshoe crabs.

Her team marks down not-so-friendly neighbors like blue crabs and sting rays, “we would take note of these bird tracks as well; birds can also be a potential predator to the juvenile horseshoe crabs,” she instructs.
“Atlantic horseshoe crabs do bury themselves, and this is a way for them to hide from predators,” Longmire points out that their love for burrowing into the sand can make them harder to find and track. “So to make sure we get a really good survey of how many horseshoe crabs are here, we have to pull these clam rakes through the sediment so we can see if we can unearth them.”

Sometimes science isn’t so fancy, “sometimes we just use this tracking or scouting method, which I guess is a scientific way of just saying ‘look for them,'” laughs Longmire. Good ol’ fashioned eyes are needed since the tiniest ones can slip through the cracks.

Horseshoe crab babies can be as small as a dime, “knowing how big they are is a proxy to how old they are,” Longmire adds adults can be as large as tire hub caps. Bigger horseshoe crabs tell us how cozy and safe their home is, “maybe there’s more resources in protected nurseries that allows them to get bigger faster,” observes Longmire.

Another sign of healthy growth is molted that are often left behind on the beach, “it’s kind of like a snake, where they have to shed their skin to get bigger.” Longmire describes, “when they’re ready to molt, they basically open up right here, it pushes water out, and they just crawl straight out the front bigger.” They can molt upwards of twenty times, sometimes just days apart, which makes it extremely difficult to tag and track them.

“Nobody has successfully tagged juvenile horseshoe crabs; tagging juvenile invertebrates that molt a lot is just difficult in general,” explains Longmire.

So back in the lab, these pioneers are testing everything from nail polish to ping pong balls and injectable dyes to keep them tagged. Longmire explains, “I inject a color dye into their legs, and when I find them in the field and flip them over, you can see the colors… there’s a red and a yellow tag.”

No one has ever used these dyes to track horseshoe crabs before, and as Longmire adds to her color scheme, “it can give us a better idea of mortality and movement if I can successfully tag them.”

Knowing how babies become successful adults is crucial to the fisheries economy, “we call it a trophic cascade; there’s this cascading effect through the food web, where if you lose one thing, the rest of the ecosystem suffers.” Longmire explains that they live among our favorite sea creatures, like sharks, sting rays, and clams.

But they also house a secret sauce; Longmire adds, “horseshoe crabs have a compound in their blood called Limulus amebocyte lysate or LAL for short.” It’s used as a biomedical resource, an estimated billion-dollar industry; Longmire adds, “LAL is used to test injectable equipment and even vaccines for bacterial contamination.” This compound from horseshoe crabs was even used in the COVID-19 pandemic.
So as the changing tides of sea level rise renovate their childhood homes, “it can alter predator access to nurseries, it could prevent the movement they need to find food and other resources,” explains Longmire.

We need to understand how to help them create stability going forward; Longmire predicts, “if nurseries degrade, you don’t have juveniles becoming adults, less adults means less spawning, and then less juveniles. And it becomes a really horrible cycle, and population declines, especially for fisheries we rely on very heavily in North Carolina.”

The estuaries they live in are some of the most vulnerable on our coast, the heart of where land, fresh water, and ocean all meet, “which means anything that impacts those like coastal development on land, damming of fresh water, or pollutants in the ocean, all converge into estuaries,” she adds.

So while Longmire is at the helm of figuring out the basics, “later we’ll be able to build off of this with these climate change syndromes to really see how much we need to conserve and how vulnerable these intertidal nurseries are,” it’s the first step to nurturing a long future for these little guys.