Each year, by the light of full moons between May and June, beaches along the U.S. eastern seaboard bear witness to masses of domed, ancient arthropods emerging from the waves. These groups of Limnus Polyphemus coordinate their arduous journey from sea to land. Aggregating under cover of darkness, they plod ashore, filling the beach and shuffling about in search of partners, depositing the next generation of pearly round eggs safely under the sand. Over the course of a breeding season, a female can lay over 80,000 eggs. Polyphemus, more commonly known as the Atlantic horseshoe crab, is one of four species of these living fossils originating 350 million years ago. All four species, L. Polyphemus, C. rotundicauda, T. tridentatus and T. gigas are morphologically similar and use this coordinated egg laying technique in an attempt to produce more eggs than predators can possibly consume at once, thus ensuring a percentage laid will evade initial predation and survive to hatching. Migratory birds rely heavily on this rich food source and regularly return to take advantage of the bounty.
The Atlantic horseshoe crab is easily identified by its squat, domed prosoma, flattened spikey opisthosoma, and long, rigid telson (tails). They walk on 5 pairs of pedipalps (walking legs) with a mouth at the center and have two front feeding appendages. Book lungs allow the crab to both take up oxygen and swim. Males are smaller in size, and sport modifications on their first pair of appendages to attach a female’s terminal spines during mating. Their drab coloration make them inconspicuous along the ocean bottom, blending in with the shallow mud and sand. These arthropods are more closely related to spiders and ticks than true crabs, though they do share the growth method of ecdysis, or shedding of the skin. As they outgrow their exoskeleton, a hard, protective covering, a new, soft shell forms underneath with the growing crab’s body. Upon molting, this softer shell quickly hardens to become the new, larger protective cover. One-week-old larvae will settle and molt several times before heading for deeper waters. Leftover molts are often found washed ashore and sometimes found in inlets amidst flotsam and seagrass bundles.
The Atlantic horseshoe crab is currently listed as threatened under CITES, with their future survival outlook hinging upon medical use and conservation efforts. Their hemocyanin-rich blood contains copper, making it appear blue, and mobile amoebocyte cells are harvested to make limulus amoebocyte lysate (LAL), used to detect certain bacteria. Bloodletting for LAL has fueled the harvesting of these creatures while simultaneously making the species hold economical value. Near shore habitat degradation, nesting beach development, overharvesting, transportation stress, and questionable release practices all threaten this species today. In response to falling numbers, several places along the eastern seaboard have already placed restrictions on harvesting with promising results. Continued tagging and monitoring, blood harvesting accountability and habitat preservation are all key to upholding this bizarre, ancient creature.
Learn more about the horseshoe crab’s critical role in the survival of migratory red knot birds in the Delaware bay:
Watch a time lapse tri-spine horseshoe crab juvenile molt:
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