Limulid

Limulid is a common name for members of the family Limulidae, a group of marine arthropods known as horseshoe crabs. Limulids are chelicerates, not true crabs, and have changed little in appearance over hundreds of millions of years. They possess a hard, horseshoe-shaped carapace that covers a broad prosoma and a narrower abdomen, a long tail spine called a telson, and a set of book gills for respiration. They typically range in size from about several centimeters to over a foot in length, depending on the species. The most familiar living limulids belong to the genus Limulus, including Limulus polyphemus, the Atlantic or American horseshoe crab.

Limulids inhabit shallow coastal waters, estuaries, and sandy or muddy bottoms. They are typically bottom-dwellers and feed on worms, mollusks, small crustaceans, and detritus. They are known for seasonal mass spawning on beaches, often timed with tidal cycles and moon phases. Reproduction is external: females lay eggs in nests in the sand, and males intercept and fertilize eggs as spawning occurs.

Limulids have a long fossil record, dating back to the Late Ordovician, and are often cited as living fossils due to their morphological stability. Their blood contains amebocytes that clot in the presence of bacterial endotoxins, a property exploited to produce Limulus amebocyte lysate (LAL) for detecting contamination in vaccines and medical devices. Because of this biomedical value and their sensitivity to environmental conditions, limulid populations are monitored in many regions, and harvesting is regulated to protect wild populations.

Conservation concerns include habitat alteration, pollution, and overharvesting in some areas, leading to region-specific protections and management measures to sustain populations.