Horseshoe crabs (Xiphosurids) are perhaps one of the most well-known examples of “living fossil” because their bodies have changed very little over a vast length of time. They are not true crabs (which are crustaceans), but instead are chelicerate arthropods, making more closely related to spiders and scorpions. They are known from marine environments as far back as the early Ordovician (about 450 million years ago). Today, there are four living species, including three in South and Southeast Asia and one found along the American Atlantic coast, from Maine to Mexico. They are usually found in shallow water on the ocean floor, where they prey on small animals including worms, mollusks, crustaceans, and fish.
Modern specimen of the horseshoe crab Limulus polyphemus from the Atlantic Ocean. Specimen is from the teaching collections of the Paleontological Research Institution, Ithaca, New York. Approximate length of specimen is 28.5 cm.
Fossil specimen of the horseshoe crab Mesolimulus walchi from the Jurassic Solnhofen Limestone of Germany (PRI 50591). Specimen is on display at the Museum of the Earth, Ithaca New York. Length of specimen is approximately 14 cm.
Fossil specimen of the horseshoe crab Paleolimulus signatus from the Carboniferous of Kansas. Specimen is from the paleontological collections of The Ohio State University, Columbus, Ohio. Length of specimen is approximately 6.5 cm.
The horseshoe crab’s entire body is protected by a hard shell or carapace. It has five pairs of clawed legs for walking, swimming, and moving food into the mouth. Behind the legs are several pairs of book gills, which have a folded appearance similar to the pages of a book. These are used for breathing, and also occasionally for swimming. (These features are annotated in the 3D model of the modern specimen shown above.)
Horseshoe crabs also have 10 eyes! In addition to the two faceted compound eyes on top of the shell, eight other eyes are arranged around the shell and are mainly used for sensing light. Horseshoe crab eyes have the largest rod and cone cells of any known animal (about 100 times the size of those in humans), and their eyes are extremely sensitive to light, but they have relatively poor vision.
Horseshoe crabs mature slowly, taking 10-12 years to reach reproductive age, and often die during growth to adulthood. Many marine creatures prey on juveniles, and less than 3 out of 100,000 survive their first year. Once they reach adulthood, horseshoe crabs have few predators other than humans, although they are occasionally eaten by turtles and seabirds. Adults begin their annual spring mass-migration from the shelter of deeper waters to their spawning beaches. There, each female burrows into the sand and lay masses of eggs, which are then fertilized by the males around her. Females lay as many as 25 egg clusters each year, or around 100,000 eggs. The eggs of the American horseshoe crab provide a critical food source for several species of migrating seabirds. Horseshoe crab’s can survive extended periods out of water (such as when they come ashore to reproduce), but this causes significant physiological stress.
There are four living (or, extant) species of horseshoe crabs. Limulus polyphemus lives in the western Atlantic Ocean, while Tachypleus tridentatus, Tachypleus gigas, and Carcinoscorpius rotundica are Asian species.
Modern specimen of the horseshoe crab Limulus polyphemus from Northport, New York. Specimen is from the teaching collection of the Paleontological Research Institution, Ithaca, New York. Length of specimen is approximately 21 cm.
A modern specimen of the tri-spine horseshoe crab Tachypleus tridentatus from Hong Kong. Specimen is from the research collections of the Paleontological Research Institution, Ithaca, New York. Approximate length of specimen is 13.5 cm.
Modern specimen of the mangrove horseshoe crab Carcinoscorpius rotundicauda from Hong Kong. Specimen is from the research collections of the Paleontological Research Institution, Ithaca, New York. Length of specimen is approximately 13.5 cm.
Horseshoe crabs carry oxygen through their blood using hemocyanin—a copper-rich molecule that gives the blood a light blue color. Their blood also contains cells called amebocytes, which play a similar role to the white blood cells of vertebrates in defending against infection. These amebocytes are extremely sensitive to chemicals—called endotoxins—produced by infectious bacteria.
Today, nearly every blood product, intravenous drug and vaccine, and implantable medical device (e.g., heart valves and orthopedic devices) in the world is screened using LAL or a similar test.
"The Importance of LAL" by Charles River Labs (YouTube).
In the U.S., the crabs are typically collected in the wild and taken to a laboratory, where up to 30% of their blood is drawn, and then they are released back into the sea. Most of the animals survive the process; mortality is correlated with both the amount of blood extracted from an individual animal, and the stress experienced during handling and transportation. Estimates of mortality rates following blood harvesting vary from 10–30%. Bleeding may also affect the reproduction of female horseshoe crabs. Approximately 500,000 crabs are harvested for biomedical purposes each year in the U.S.
Some alternative endotoxin tests require much less or no horseshoe crab blood, such as the recombinant factor C (rFC) test. Widespread adoption of these alternative tests, however, is challenging due to complex validation procedures and necessary redesigns of the manufacturing process. Because of these difficulties, some pharmaceutical companies are instead focusing on making existing tests more sustainable—requiring less blood, reducing mortality associated with bleeding, and culturing amebocytes in the laboratory.
In addition to medicine, horseshoe crabs have been harvested by humans for other purposes. In U.S. during the late 19th and early 20th century, they were heavily harvested for fertilizer and livestock feed, but this widespread practice ended in the 1920s. Their use as bait in commercial fishing became popular in the 1990s.
Horseshoe crab harvested for the production of fertilizer. Image from the Delaware Public Archives.
Today, over 500,000 horseshoe crabs are harvested in the United States each year for use in the eel and whelk fisheries. In Southeast Asia, horseshoe crabs are also fished for human consumption. Habitat destruction due to development and degradation of coastal habitat, as well as climate and sea level change, has the potential to threaten horseshoe crab populations worldwide.
The International Union for Conservation of Nature (IUCN) currently lists the conservation status of the American horseshoe crab (Limulus polyphemus) as “vulnerable” and the three Asian species as “data deficient”, which means there is not enough data to assess them. Anecdotal information shows that all of the Asian populations may be declining, and the IUCN has predicted declines of at least 30% over the next 40 years.
Throughout the twentieth century, when it was the most heavily harvested, the American species showed significant declines. Starting in the 1990s, however, regulation by states along the Atlantic coast—including fishing moratoriums in New Jersey and South Carolina—seems to have stabilized the overall abundance of the species. However, local declines are still occurring in some areas.
Conservation regulations covering the three Asian species are much less rigorous than those in the U.S., and this has raised the concern that depletion of Asian populations—combined with growing demand for horseshoe crabs, especially from biomedicine—will eventually put additional pressure on the American species.