Counting "Living Fossils"
by Dr. Sarah D. Oktay
Managing Director UMass Boston Nantucket Field Station
You may have noticed people over by “the Creeks,” at Eel Point, or on the beach at the UMass Boston Nantucket Field Station wading in the water carrying PVC stakes and clipboards and wondered, “what the heck are they doing?” 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. Separate teams of volunteers from the Maria Mitchell Association, the Nantucket Conservation Foundation, the Nantucket Land Council, the UMass Boston Nantucket Field Station, the Marine Mammal Stranding Team, and many other volunteers are methodically searching at the high tide twice a day (as I write this my next survey is scheduled for 1:00 am) for these ancient graceful swimmers along the beach.
Horseshoe crabs are sometimes referred to as “living fossils” which are defined as species who have not changed much from their ancient evolutionary roots and yet have very few close living relatives and few “branches” protruding from their phylogenetic tree. You might be surprised to find out how many plants and animals as diverse as the ginkgo tree, the koala, and the nautilus are classified as living fossils.
The horseshoe crab was first named by Carl Linnaeus (1707 – 1778) a Swedish botanist, physician, and zoologist, who laid the foundations for the modern scheme of binomial nomenclature (two part names that are often Latinized). Linnaeus is known as the father of modern taxonomy, and is also considered one of the fathers of modern ecology. At the time of his death, he was widely renowned throughout Europe as one of the most acclaimed scientists of the time and his system for naming, ranking, and classifying organisms is still in wide use today. Linnaeus is a fascinating person, to learn more, check out www.ucmp.berkeley.edu/history/linnaeus.html and any of the many books written by and about him. He called this creature the Limulus polyphemus. The genus, “Limulus,” from the Latin, meaning “somewhat oblique, odd, or askew” and referring to the sideways placement of the compound eyes and the species, “polyphemus,” from the Greek, meaning one-eyed giant and referring to the simple eyes on the front of the shell
There are only four living species of the horseshoe crab family; our local buddies, 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. 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, a staggering 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, crustaceans, and the horseshoe crabs which are in the own class called the Merostomata or “legs attached to the mouth.” Horseshoe crabs are the closest living relatives of the now extinct trilobites.
Horseshoe crabs have three main parts to the body: the head region, known as the “prosoma,” the abdominal region or “opisthosoma” which is attached to the “head” by a hinge, and the spine-like tail or “telson.” It is the tail that earns this order its name Xiphosura, which derives from the Greek for “sword tail.” The sexes are similar in appearance, but females are much larger than males. The carapace is shaped like a horseshoe, and is greenish grey to dark brown in color. On the underside of the prosoma there are six paired appendages, the first of which (the chelicera) are used to pass food into the mouth. The second pair, the pedipalps, are used as walking legs; in males they are tipped with 'claspers' which are used during mating to hold onto the female's carapace. The remaining four pairs of appendages are the 'pusher legs', also used in locomotion. The opisthosoma bears a further six pairs of appendages; the first pair houses the genital pores, while the remaining five pairs are modified into flattened plates, known as book gills, that are used in breathing. There is a compound eye on each side of the prosoma, five eyes on the top of the carapace, and two eyes on the underside, close to the mouth, making a total of nine eyes. In addition, the tail bears a series of light-sensing organs along its length.
A further unique and intriguing feature of this ancient species is that it has copper-based blood, which turns blue (as opposed to red) when it encounters oxygen, in other words, they are “blue bloods.” According to http://horseshoecrab.org, in the 1960s, Dr. Frederik Bang, a Johns Hopkins researcher working at the Marine Biological Laboratory in Woods Hole, Massachusetts, found that when common marine bacteria were injected into the bloodstream of the horseshoe crab, massive clotting occurred. Later, with the collaboration of Dr. Jack Levin, the MBL team showed that the clotting was due to the presence of a gram-negative bacterial toxin called endotoxin. These investigators were able to localize the clotting phenomenon to the blood cells, called amebocytes, of the horseshoe crab, and, more importantly, to demonstrate the clotting reaction in a test tube. The cell-free reagent that resulted was named Limulus amebocyte lysate, or LAL. This reaction was then used to look for endotoxins in a variety of pharmaceutical and medical devices to ensure that they were not contaminated.
Why do horseshoe crabs have the ability to form clots around this bacteria? Unlike mammals, the horseshoe crab lacks an immune system so it cannot develop antibodies to fight infection. However, the horseshoe crab does contain a number of compounds that will bind to and inactivate bacteria, fungi, and viruses. The components of LAL are part of this primitive "immune" system. For example, they not only bind and inactivate bacterial endotoxin, but the clot formed as a result of activation by endotoxin provides wound control by preventing bleeding and forming a physical barrier against additional bacterial entry and infection.
Horseshoe crabs are collected using dredges or clam rakes, packed into trucks (sometimes refrigerated) driven to labs to have 1/3 of their blood removed, then returned to the ocean. Some studies estimate 10 to15 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. More information on how the test was discovered, what groups do to maintain and evaluate horseshoe crab safety and greater details on the science behind the LAL test can be found at http://www.horseshoecrab.org/med/med.html.
Horseshoe crab spawning season varies according to latitude, but it generally peaks in May and 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 appendage to "clasp" themselves onto the back of the female. We call these appendages “boxers gloves.” Sometimes, several males will attempt to attach to a female and will form clusters with satellite males jostling for position.
By the beginning of the spawning season, each female will have developed about 80,000 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. Go to this site to see a movie of a horseshoe crab larvae: www.ocean.udel.edu/horseshoecrab/Multimedia/movieScreen.html , which I'll admit is a little creepy.
Juvenile horseshoe crabs generally spend their first and second summer on the intertidal flats feeding before the daytime low tide and burrowing in the sand for the rest of the day. As they grow, young crabs move into deeper water. Like other members of the phylum Arthropoda that have a suit of armor or exoskeleton, the horseshoe crab must molt and shed its shell in order to grow larger. Before molting, a new shell begins to form. When this new shell is ready, the horseshoe crab absorbs water through its gills, making itself bigger. The old, hard shell cannot expand and splits in the front where the top and bottom join. The horseshoe crab crawls out the front, leaving the old shell behind. It takes about 24 hours for the new soft shell to harden. With each molt, the horseshoe crab increases in size by an estimated 25-30%. By the end of its first year, the crab will have molted several times, but will still be very small, clocking in at about one-half inch in diameter. Horseshoe crabs initially molt an average of three or four times a year. Sub-adults (horseshoe crabs that are five to seven years old) appear to molt annually, usually in July or August. Males are sexually mature at their sixteenth molt, which is usually their eighth or ninth year. During their final molt, they develop specialized clasping claws for holding the female during reproduction. Females need at least 17 molts, or one more than the males, so they mature in their tenth year or even later and are, on the average, 30% larger than the males. A small percentage of horseshoe crabs continue to molt after reaching sexual maturity. Scientists are not sure how long a horseshoe crab can live, but conservative estimates are at least 20 years.
Horseshoe crabs carry a variety of hitch-hikers during their journey, kind of like the world's oldest Country Squire station wagon. The typical 5-10 year old horseshoe crab has slipper shells and barnacles and algae growing on the top of the shell. I explain to my students that many of the mollusks and snails, the horseshoe crab is a convenient movable platform that helps keep them safe and moves them closer to food. Tiny crabs may hide along the inside “lip” of the shell and the Limulus leech (Bdelloura) is a flatworm that is found around the book gills and leg joints of crabs, especially on older females that have not shed for a long time.
Back to our local research: In 1990, the first organized survey of spawning horseshoe crabs in the Delaware Bay began. Now, every May and June during the full and new moon evening high tides, volunteers donate their time to count crabs on key beaches throughout Delaware and New Jersey. Each of the four peak spawning tides are bracketed with a count two days before and two days after, bringing the total number of survey nights to twelve. This project has expanded up the Atlantic coast, and is spearheaded by the University of Rhode Island researcher Mary Jane James-Pirri (gsosun1.gso.uri.edu/~mjjp/) and overseen by local survey coordinators from various state, government, educational, and conservation organizations (our local coordinator is Dr. Bob Kennedy of the Maria Mitchell Association).
Why are we concerned about their population? Besides the fact that it is frankly kind of rude to decimate an animal who has peaceably and successfully been existing for hundreds of millennium, the horseshoe crab eggs and larvae are also the favorite food for a huge variety of animals. Everything from migrating shorebirds to sea turtles, fish, crabs, and whelks dine on a horseshoe crab egg and larvae diet. When the horseshoe crabs are bigger, only the persistent seagulls have the strength and fortitude to flip them over and eat them.
The horseshoe crab's main strategy to avoid predators is to be most active at night, feeding and spawning under the cover of darkness. In fact, during spawning season, you will find 100 times more crabs on shore at night than during the day. During high tide when large aquatic predators are swimming nearby, the juvenile horseshoe crabs bury themselves in the sand for protection. At low tide, young horseshoe crabs emerge from the sediment, but now they must be cautious of predators on the shore. If the crabs are turned upside down, they will use their telson to flip over, this movement always reminds me of a slow moving windshield wiper. The need for them to keep their telson intact is the primary reason why you should never pick up a horseshoe crab by its tail. Instead lift it by the shell and flip it back over, avoiding the telson. Shorebirds cannot penetrate the horseshoe crab's coat of armor as long as the crab is upright.
The huge drop in population along the east coast has been attributed to a variety of factors, including habitat alteration (bulkheads, groins, and associated development can remove naturally occurring beaches), overuse for medical purposes, harvesting for chitin, and overfishing for use as bait in the conch and eel fisheries and for fertilizer and animal feed. According to www.horseshoecrab.org, two million individuals were killed in 1996 due to fishing mortality. Bait bags and alternative bait gear is now used to wean fishermen off of the previously abundant horseshoe crabs. Anecdotally, Nantucket Harbor and the surrounding water supported several thousand horseshoe crabs over the past 50 years, Those numbers have declined noticeably in recent years. Obtaining baseline data 20 years ago would have been ideal, getting it now is better than nothing.
To learn more go to horseshoecrab.org, and www.ocean.udel.edu/horseshoecrab. Activities for kids can be found at www.marine.usf.edu/pjocean/packets/f01/f01u5p3.pdf