Limulids

Limulids are a group of marine arthropods belonging to the class Merostomata, which also includes extinct relatives like eurypterids. The most well-known living member is the horseshoe crab (*Limulus polyphemus*), though the term "limulid" technically refers to the entire family Limulidae, which includes several other species found primarily in coastal waters of the Atlantic, Pacific, and Indian Oceans.

Limulids have a distinctive, flattened, horseshoe-shaped carapace that covers most of their exoskeleton, giving them their common name. Their bodies are divided into three main regions: the prosoma (head and thorax), opisthosoma (abdomen), and telson (tail spine). They possess five pairs of walking legs, with the first pair modified into pincers used for feeding. Unlike true crabs, they lack antennae and have compound eyes on stalks.

These ancient creatures have remained largely unchanged for over 450 million years, making them living fossils. They play a crucial ecological role by feeding on small invertebrates and serving as prey for larger marine animals. Their blue blood, containing the copper-based protein hemocyanin, is used in medical testing for bacterial contamination due to its sensitivity to endotoxin.

Limulids are oviparous, laying eggs in shallow water where they are guarded by the male until they hatch. Some species, like *Limulus polyphemus*, are commercially harvested for their eggs, which are considered a delicacy in certain cultures. Conservation efforts are underway due to habitat loss, pollution, and overharvesting, particularly for the Atlantic horseshoe crab. They are protected in some regions, and sustainable fishing practices are encouraged to preserve their populations.