Dinosaurs Amoung Us
by Dr. Sarah D. Oktay
Managing Director UMass Boston Nantucket Field Station
A few nights ago, I was walking with a friend chest deep in the salty surf at high tide at 2:44 am. Did I lose a bet? Am I an extremely creative sleepwalker? Nope, just seeing how many horseshoe crabs are coming inshore to do what they have done for eons, which is to have crab relations, lay eggs, and continue the species. There's nothing more surreal than wading up to your chest in the pitch black new moon darkness along the beachfront with headlamps & lanterns looking for silent ancient living fossil creatures.
I wrote about horseshoe crabs two years ago, but I’ve had a lot of questions and interest in them lately so I thought I would readdress the topic with some new information that we've gained over the last three years worth of research on island. Teams of volunteers from the Maria Mitchell Association, the Nantucket Conservation Foundation, and here at the UMass Boston Nantucket Field Station are going out during the day and night at the times of the highest tides on the new and the full moon (and two days before and after) in order to measure the horseshoe crab population. We are part of a multi-state effort to count the number of spawning horseshoe crabs along our shorelines to determine how the population is doing. The horseshoe crab population appears to be increasing slightly over last year's very low numbers. Many years ago, Nantucket beaches, coves, and marshes were covered with horseshoe crabs. They were extremely abundant on the Cape and Island and throughout New England. Their numbers have declined over the past few decades due to a combination of factors, including overfishing, excess mortality occurring from harvesting them for their blood, and habitat loss.
Horseshoe crabs have been around a long, long time. In fact, no other creature has remained so close to its early form and shape. The horseshoe crab has been relatively unchanged since the Triassic period 230 million years ago, and similar species were present in the Devonian, 400 million years ago. Despite their common name, they are not crabs but are in the phylum Arthropoda (animals having an articulated body and limbs) which includes insects, arachnids, and crustaceans. Horseshoe crabs are in the own class called Merostomata or “legs attached to the mouth.”
There are only four living species of the horseshoe crab family; our local creatures (and the most abundant of the species) Limulus polyphemus, and three species found in the Indo-Pacific: Tachypleus gigas, Tachypleus tridentatus and Carcinoscorpius rotundicauda. Limulus polyphemus is found along the western Atlantic and Gulf coasts from southern Maine to the Yucatan Peninsula, with the Delaware Bay as the center of the population. According to the Maryland Department of Natural Resources website (http://www.dnr.state.md.us/education/horseshoecrab/species.html): “Each major estuary along the coast is believed to have a discrete horseshoe crab population that can be distinguished by adult size, carapace color, and eye pigmentation.“ In fact, Cape Cod Bay horseshoe crabs can be distinguished from Nantucket Sound ones (http://www.capecodtravel.com/attractions/nature/crabs_0700.shtml).
The horseshoe crab was first named by Carl Linnaeus (1707–1778) a Swedish botanist, physician, and zoologist. Linnaeus is known as the father of modern taxonomy, and is also considered one of the fathers of modern ecology. He named these creatures, Limulus polyphemus. The genus, Limulus, from the Latin, means “somewhat oblique, odd, or askew” and refers to the sideways placement of the compound eyes. The term polyphemus, from the Greek, means “one-eyed giant” and refers to the simple eyes on the front of the shell. Horseshoe crabs have several pairs of eyes. Two large compound eyes on the prosoma are sensitive to polarized light and can magnify sunlight 10 times. A pair of simple eyes on the forward side of the prosoma can sense ultraviolet light from the moon. In addition, multiple eye spots are located under the prosoma, with more on the underside of the tail. Horseshoe crabs occasionally swim upside down and may once have used these eyes more than they do today (see their confusion upon finding out boots are not girlfriends above).
Horseshoe crabs use book gills (flaps resembling the pages of a book) to get oxygen from the water. If these primitive gills stay moist, horseshoe crabs can remain out of water up to four days. Crabs stranded on the beach during spawning bury themselves in the sand or fold themselves in half to conserve water until the tide rises again. Horseshoe crabs have no jaws or teeth. Instead, they have an impressive array of spiny mouth bristles at the base of five pairs of legs to maneuver food items such as razor clams, soft-shelled clams and marine worms into their centrally located mouth. To chew its food, the crab must simulate walking movements. Horseshoe crab eggs and larvae are a seasonal food item of invertebrates and fish. Striped bass and white perch eat horseshoe crab eggs. In addition, American eel, killifish, silver perch, weakfish, kingfish, silversides, summer flounder and winter flounder, most crab species and several gastropods including whelks eat eggs and larvae. Many sea turtles eat them; abundant stocks of adult horseshoe crabs may be an important component of ensuring the long-term survival of loggerhead sea turtles in the Delaware and Chesapeake Bays.
Horseshoe crabs were the inspiration for the vocal effects provided by voice master Peter Cullen in the 1987 (and subsequent) “Predator” horror /science fiction movies. In an interview, Cullen says, “as I watched the Predator take off his helmet, I remembered the sounds of an upside down horseshoe crab bubbling in the sun. The sounds of the clicking bursting bubbles came to me. The horrible underside of the dying crab and the face of the Predator just intertwined.” Rumor has it that H. R. Giger, the designer of the “Alien” xenomorph drew the “facehugger” based on the horseshoe crab anatomy, but that is a coincidence (http://www.hrgiger.com/). The actual design was based on a combination of two hands and, well, if you must know, human “naughty bits.”
And this correlation brings us back to why they are so abundant along our beaches in May and June. Adults spend the winter in deep bay waters and off-shore areas. Horseshoe crab spawning season varies according to latitude, but it generally peaks in May & June, with peak spawning occurring on evening high tides during the full and new moons (the higher-than-normal “spring” tides). The adults seek beaches that are at least partially protected from surf, within bays and coves. When the Limuli head for shore, the males patrol along the foot of the beach, awaiting the females. The female horseshoes give off chemical attractants called pheromones, which the males can detect. Although there may be other means of identification, these attractants, the directional movement and the number of males involved (often several times the number of females) reduce the chance of a female reaching the beach without a boyfriend or two. Males, who are about 30% smaller than females, use a specially developed “boxer’s glove” shaped appendage to clasp onto the back of the female. Female front appendages look a bit like feather dusters. Sometimes, several males will attempt to attach to a female and will form clusters with satellite males jostling for position. During our surveys, we find that as we walk around in the water with waders, our feet will become irresistible, female-like objects to any circling males which quickly demonstrate that sight is not their most highly developed sense.
By the beginning of the spawning season, each female will have developed about 80,000-100,000 eggs, which are located in dense masses near the front of her shell. She will return to the beach on successive tides, laying 4-5 clutches of eggs with each tide. Each cluster contains about 4,000 eggs and a female will lay about 20 egg clusters each year. Newly laid horseshoe crab eggs are opaque, pastel-green in color, and about 1.5 mm (1/16 inch) in diameter. It takes two weeks for the horseshoe crab to progress from egg to larvae to hatchling. If an egg is exposed to air for long it will dry out, but it will form an important source of food to migrating shorebirds.
Some folks aren't aware that horseshoe crabs molt. When you're walking in an area like the Creeks and you find all these little shells of horseshoe crabs, you're finding the leftovers after the crab wiggles out of its too-small shell in order to become larger. Horseshoe crabs will molt at least 6 times in their first year of life and about 18 times before they reach sexual maturity. Females may molt more than males to reach their larger size. Once crabs are sexually mature, which takes at least 9 years, they won’t shed their shells again. Older adults are adorned with slipper shells barnacles, and other hitchhikers along for the free ride. Adult crabs may live another 8 to 10 years, making the total lifespan of a horseshoe crab as long 20 years.
Arguably the most bizarre feature of horseshoe crabs is their copper-based blood, which turns blue (as opposed to red) when it encounters oxygen, in other words, they are “blue bloods.” Researchers at the Marine Biological Lab at Woods Hole found that bacteria injected into horseshoe crabs triggered a gram-negative bacterial toxin called endotoxin to form clots around the bacteria. These investigators were able to localize the clotting phenomenon to the blood cells, called amebocytes, of the horseshoe crab. This reaction was then used to look for endotoxins (which can be dangerous pathogens) in a variety of pharmaceutical and medical devices to ensure that they were not contaminated (http://horseshoecrab.org/med/med.html).
Horseshoe crab blood quickly became a commodity. Horseshoe crabs are collected using dredges or clam rakes or by hand, packed into trucks (sometimes refrigerated) and then driven to labs to have 1/3 of their blood removed, then returned to the ocean. Some studies estimate 10 to 15 percent of animals do not survive the bleeding procedure, which accounts for the mortality of 20,000 to 37,500 horseshoe crabs per year. Blood volume returns to normal in about a week, though blood cell count can take two to three months to fully rebound. A single horseshoe crab can be worth $2,500 over its lifetime for periodic blood extractions. According to this amazing video (http://www.poetv.com/video.php?vid=92968), one quart of horseshoe crab blood is worth $15,000. That is an animal worth protecting! Because of the lucrative market in their blood and the abuse of their collection as bait for conch and lobster fisheries, it’s illegal in some states to harvest or even possess a live one without a proven scientific purpose.
Their eggs are an important source of food for shorebirds, especially birds like the red knot. According to the U.S. Fish and Wildlife Service (http://www.fws.gov/northeast/pdf/horseshoe.fs.pdf), “more than half of the total flyway population of red knots, ruddy turnstones and semipalmated sandpipers depend on Delaware Bay’s horseshoe crab eggs as a rich food supply.” A reduction in horseshoe crab population cascades through the populations of the birds and other creatures who depend upon their eggs for food. People have harvested horseshoe crabs for centuries. Prior to the European colonization of North America, native tribes used the telson as spear tips and used the shell as containers. These small and localized harvests had little impact on horseshoe crab populations. In the late 1800s and early 1900s, up to 4 million horseshoe crabs were harvested annually and used as fertilizer or animal food. Currently, crabs are harvested for bait in conch and American eel fisheries on the Atlantic Coast. Horseshoe crabs suffered a substantial increase in harvest in the 1990s that spurred the need for management on a coast-wide scale. In 1998, the Atlantic States Marine Fisheries Commission, representing 15 states from Maine to Florida, developed a horseshoe crab management plan. The ASMFC plan and its subsequent addenda established mandatory state-by state harvest quotas and created the 1,500-square-mile Carl N. Shuster Jr. Horseshoe Crab Sanctuary off the mouth of Delaware Bay. A combination of management efforts, research into alternatives for bait for conch fishermen and harvest quotas have started to very slowly turn the tide for these creatures. Their recovery will take some time because it takes nine years on average for horseshoe crabs to reach sexual maturity.
Not much is known about where horseshoe crabs go (other than into deeper waters) when they are not entertaining us on shore. For the past two years, horseshoe crabs have been tagged by Katherine Terkanian (whose feet can be seen in the attached photo) as she completes graduate research as part of her position as one of the two Horseshoe Crab Conservation Project Managers with Mass Audubon at Wellfleet Bay Wildlife Sanctuary. In addition, Dr. Jennifer Karberg, Research Supervisor with the Nantucket Conservation Foundation, has been tagging some of the horseshoe crabs observed in our surveys in collaboration with Massachusetts Department of Fish and Wildlife. If you see a tag, please call the number on it or email Dr. Karberg. Even thought they look pretty scary, horseshoe crabs are relatively docile and they use their impressive sword like tail, or telson, to help themselves flip back over if overturned in the surf. Please DO NOT pick them up by the tail as you can damage them and they need their tails to survive. For teachers out there, this web site http://horseshoecrab.org/ offers a variety of facts and information to use in your classroom.
