Creature Feature: American Horseshoe Crab

Move over dinosaurs, there is a cooler fossil in town! Meet the American horseshoe crab (Limulus polyphemus); don’t be fooled, it is neither a horse, nor a crab, but a close relative of spiders and scorpions (class: arachnids) [1][3]. These marine invertebrates are considered living fossils as their body structure, which has made them so effective at surviving, has remained relatively unchanged since the Paleozoic era (540-248 million years ago). In evolutionary terms, this is akin to saying: “If it ain’t broke, don’t fix it.”

While this is an animal behavior blog, let’s pause for a second and orient ourselves to Earth’s long history (besides, I am a big fan of colorful figures). Life has been evolving on this planet for millennia, and most species that are alive today look nothing like their earliest predecessors that lived over 500 million years ago. Note the numbers (in Figure 1) labelling different eras of geological time, progressing chronologically from the earliest time period where life has been documented (the Paleozoic era) to our current era (the Holocene):

Figure 1: Geological time figure, adapted from the International Stratigraphic Chart, 2015. The oldest eras begin at the bottom and the newest are at the top. The numbers on the figure correspond to the numbers within the main body text. All sentences in bold highlight time periods relevant to the horseshoe crab. [Source]

1) The Paleozoic era is considered the “age of visible life” due to the rapid evolution of marine invertebrates, fish, plants, insects, and amphibians [2].

2) The Cambrian period was a time of evolutionary explosion, with a wide variety of animals appearing during this period. Arthropods, such as trilobites (the horseshoe crab’s closest ancestor), evolved in the Cambrian and the first horseshoe crab existed 500 million years ago (as indicated by fossil records) [1][2][3].

3) During the Devonian period, complex plant life (e.g. trees) appears and marine life became more diverse and complex (we start to see animals such as sharks and other fish species) [2].

4) The Permian period, at end of the Paleozoic era, is marked by a mass extinction event. 95% of all marine species went extinct, including trilobites…But not the horseshoe crab! [2]

5) The Mesozoic era begins and many dinosaur species came onto our planet’s scene; this period is also known as the “age of reptiles”. Many terrestrial species evolved, including some early mammalian classes (e.g. marsupials). The horseshoe crab began to diversify and evolve into several of different present-day species, meaning their features remained relatively unchanged for the next 200 million years [2][3].

6) Another mass extinction event occurs at the end of the Mesozoic era (the Cretaceous period), with the extinction of dinosaurs and 50% of marine invertebrates, but, once again, not the horseshoe crab! [2]

7) The Cenozoic era is considered the “age of mammals”, and fossils of marine mammals, rodents, large mammals, and primates have been found around the world [2].

8) We are currently in the Holocene epcoch, which includes Earth’s last 11,000 years (and when humans enter the scene). Currently, horseshoe crabs are thriving in some areas, but some populations are in decline due to over-fishing, pollution, and/or habitat loss.

(Side note: If you are having a hard time wrapping your head around the extensiveness that is geological time, check out what geological time would look like if it was squished into one calendar year!)

Alien Adaptations

Now that we have dabbled in some geology and evolutionary history, let’s shift our focus back to our little, ancient protagonist: the horseshoe crab! Poet Anne Stevenson accurately wrote that, “the sea is as near as we come to another world.” And the horseshoe crab certainly seems other-worldly. Looking somewhat like a helmet with legs and a butt-sword, scuttling across the bottom of sandy or muddy coastal shallows of eastern North America, the horseshoe crab (among a myriad of other sea critters) confirms her observation [3].

Horseshoe crabs have plenty of alien-like, other-worldly adaptations that have remained relatively unchanged for millennia. [Source]

Horseshoe crab anatomy includes a slew of unique adaptations that are so wonderfully distinct from any other organism on the planet. They have 3 distinct sections of their body: the prosoma, the opithosoma, and the telson [3]. The prosoma (which is the part of the body that inspired the name, as it resembles the shape of a horseshoe) houses all of the animal’s internal digestive organs, brain, and heart. Their digestive system is quite impressive: unlike their spider, scorpion, and tick relatives (subphylum: Chelicerata), horseshoe crabs can handle and digest solid food. The opithosoma section encompasses most of the muscles that control breathing and movement of the telson. Despite its appearance, this long appendage is not a poisonous barb or stinger, but is used by horseshoe crabs strictly to flip themselves over, should they be pushed onto their backs by wave action or a predator [5].

Horsehoe crabs have specialized limbs, including pedipalps & pincer-tipped legs, that are used for feeding. [Source]

Horseshoe crabs have 6 pairs of legs, each with specialized functions. Searching the intertidal zone [4] for benthic (a.k.a. bottom-dwelling) organisms, such as clams and worms, they dig into sand or mud, grasping and crushing prey and then using pincer-tipped legs, called chelicera, to place pieces of prey in their mouth [3]. Unlike a dog, that digs at an angle with two front limbs, horseshoe crabs can dig vertically (imagine going down an elevator rather than going down the stairs); thus, their mouth is actually located towards the center of their legs.

A horseshoe crab mating pile. Can you spot the large females and the smaller males? [Source]

The next pair of legs are called the pedipalps, and for males, these are specially used to grab onto the larger female during mating season. There is a male-biased sex ratio during the breeding season, meaning that males outnumber females and must compete for mating opportunities [3]. Males use their pedipalps to hold onto a female for dear life, in hopes that she will bury herself into the sand with the male and deposit up to 80,000 eggs for him to fertilize [3][4].

Then, there are the “pusher” legs that are used to scamper across the coastal sands. Legs aren’t the only locomotor adaptation they have; fluttering behind these pusher legs are 5 sets of book gills over which flowing water carries dissolved oxygen via thin membranes. The propulsion of water passing across each “page” creates movement, making the book gills doubly-adaptive, for both breathing and swimming! Covering the book gills is the operculum, which protects the respiratory system and is the location of the reproductive system.

Horseshoe crab dorsal eye locations.
Source: Wikimedia

In addition to having specialized appendages, and efficient, two-for-one functional body parts, this critter has an extensive vision system, that has very different functions from our own! Although we might initially notice the two compound lateral eyes that give them a human-like, furrowed-brow look, there are an additional 5 eyes on the dorsal (top) side of the prosoma! These compound eyes don’t have high resolution (i.e. can’t see objects very clearly) but can perceive a wide angle and have the ability to track fast-moving objects. The two median eyes, the endoparietal eye, and rudimentary lateral eyes are used for light sensitivity and processing ultraviolet light. But wait, there’s more! In addition to the 7 eyes on the shell, there are also a series of photoreceptors (i.e. light sensors) on the telson that helps the horseshoe crab maintain a circadian rhythm (i.e. day and night cycles), as well as two ventral eyes located near the mouth to assist in swimming and perhaps hunting efficiency. Why so many eyes, you ask? In addition to helping with hunting and food processing, horseshoe crabs rely on changes in daylight to trigger migrations from wintering grounds along the continental shelf (6m-20m) to the coastal shallows for breeding. These migrations occur usually on evenings of new and full moon tides in the early summer, meaning that light is so important to their reproduction that they evolved special eyes to detect event the smallest changes in light [3].

“One man’s “trash-fish” is another’s ancient treasure.

Although they are armored heavily with intimidating alien adaptations, and have certainly survived the long battle of time and evolution, horseshoe crabs are actually harmless to humans. In fact, it could be argued that humans are more dangerous to horseshoe crabs than the other way around. Considered historically as a “trash-fish”, horseshoe crabs have had little conservation protection. In the United States during the 20th century, their populations declined significantly since they were being harvested for fertilizer and livestock feed [3].

[Source: Walls, Berkson, & Smith, 2002]

The biomedical industry has also played a part in horseshoe crab harvesting, specifically for human benefits. Horseshoe crab blood has sensitive clotting capabilities to combat infection and after the discovery of this clotting agent (Limulus Amebocyte Lysate, or LAL), researchers pushed to have LAL be accepted in the medical field to efficiently detect and eliminate endotoxins [3]. The detection of endotoxins is important for ensuring that manufactured pharmaceutical products are sterile, and LAL has been used to detect bacteria in meat, fish, and dairy products, as well as to diagnose bacterial diseases such as meningitis. This unique immune adaptation led to massive harvesting of horseshoe crabs for their blood. Although horseshoe crabs that are caught for their LAL are required to be released within 72 hours of capture, differences in capture, travel, holding, and bleeding procedures can lead to a 10% increase in mortality after their return to the wild [3]. Research has also shown that while the blood volume can be regained within 1 week, it takes up to 4 months to regain the amebocytes (the mobile cells that produce the clotting enzyme that helps detect endotoxins) [3]. This delay may lead to released animals having a weakened ability to protect themselves from bacteria that they might encounter in their natural habitat.

While conservation and management of this “trash fish” has been nonexistent in the past, the 21st century brought increased awareness of the need for population data and habitat research of this unique creature. As American horseshoe crabs don’t reach sexual maturity until 9 or 10 years of age, a crucial long-term management step is to understand how the population is faring and how to protect critical habitat needed for successful breeding and juvenile survival [3][6]. By providing innovative ways to reduce the amount of bait needed in fisheries, and researching synthetic versions of LAL, humans can reduce the need of harvesting this living, marine fossil. For instance, a simple conservation plan is the “Just flip ‘em” initiative, which encourages beachgoers to assist a stranded (upside-down) horseshoe crab by scooping them up by their shell (and not their telson) and placing them right side up at the shoreline.

Help our fossil friends flip over! [Taken by Gregory Breese, Source]

To go back to that lovely-colored geological timeline (Figure 1), if we are to consider different eras as different generations on earth, humans are the “newborn babies” of Earth. Yet we have quickly been able to harness our Paleozoic “grandparent’s” adaptations to assist in our own advancement. Does that make humans the better-adapted species, as we are able to manipulate horseshoe crabs to: produce more nutritious crops, be bait for highly-coveted seafood, and use their blood (and lives) for life-saving medical procedures? Or does that make humans reckless? We have single-handedly threatened the survival of these ancient fossil critters which, up until this point, have survived several mass extinction events, massive geologic change, asteroid impacts, and even outlived dinosaurs…

If you want to learn more:

This website has great resources on the evolution, anatomy, and conservation research of the horseshoe crab!

Written by: Karli Chudeau, graduate student in the Animal Behavior Graduate Group and a part of the UC Davis Center for Animal Welfare. She is interested in conservation management and assessing animal welfare in wildlife rehabilitation settings. Her current research examines how we can use behavioral management interventions, such as environmental enrichment, to improve reintroduction success with pinnipeds. She is also an avid ocean nerd.


  1. Zhang, X. L., Shu, D. G., & Erwin, D. H. (2007). Cambrian naraoiids (Arthropoda): morphology, ontogeny, systematics, and evolutionary relationships. Journal of Paleontology81(sp68), 1-53.
  2. Walker, J.D., Geissman, J.W., Bowring, S.A., & Babcock, L.E., compilers (2018). Geologic Time Scale v. 5.0. Geological Society of America. DOI: 10.1130/2018.CTS005R3C
  3. Walls, E.A., Berkson, J., & Smith, S.A. (2002) The Horseshoe Crab, Limulus polyphemus: 200 Million Years of Existence, 100 Years of Study. Reviews in Fisheries Science, 10(1), 39-73, DOI: 10.1080/20026491051677.
  4. Lee, W.J. (2010). Intensive use of intertidal mudflats by foraging adult American horseshoe crabs Limulus polyphemus in the Great Bay estuary, New Hampshire. Current Biology, 56(5), 611-617.
  5. Penn, D., & Brockmann, H. J. (1995). Age-biased stranding and righting in male horseshoe crabs, Limulus polyphemus. Animal Behaviour49(6), 1531-1539.
  6. Chabot, C.C., & Watson III, W.H. (2010). Horseshoe crab behavior: Patterns and processes. Current Zoology, 56.
  7. Dubofsky-Porter, E.A., Chabot, C.C., & Watson III, W.H. (2017). Entrainment of juvenile horseshoe crab activity to artificial tides. Marine and Freshwater Behaviour and Physiology, 50(2), 125-140.

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