Limulus

Best known through its single living representative today, the Atlantic horseshoe crab (Limulus polyphemus). Found along the eastern coasts of North America, from Maine to the Gulf of Mexico, this ancient creature has walked the shores for more than 200 million years. While other related genera, such as Tachypleus and Carcinoscorpius are found in Asia, Limulus is the only horseshoe crab genus native to the Americas. This makes it unique not only in its evolutionary history but also in its ecological role along western Atlantic coastlines.

Like other horseshoe crabs, Limulus is easily recognized by its rounded shell, long spiny tail (telson), and multiple pairs of legs. Their breeding behavior is also remarkable: during spawning season, thousands of Limulus gather in massive numbers on sandy beaches, with smaller males clinging to the backs of larger females in what looks like a living “conga line.” Their eyes are another marvel—besides their compound eyes on the sides of their shells, they also have simple eyes on top that remain sensitive even at night, making them excellent navigators in shallow, moonlit waters.

What makes Limulus particularly special is its role in both ecosystems and human health. Every spring, females lay millions of eggs in sandy nests, creating a feast for migratory birds such as the red knot. These eggs are so crucial that the survival of entire bird populations depends on their abundance. Beyond its ecological role, Limulus is of oversized importance in modern science. Its blue blood, rich in copper, contains cells that detect bacterial toxins instantly. This discovery led to the creation of the Limulus Amebocyte Lysate (LAL) test, now used worldwide to ensure that vaccines, IV drugs, and surgical tools are free of harmful contamination.