Description
Limulus polyphemus are ancient marine arthropods that are closely related to arachnids like spiders and scorpions, rather than true crabs.
Physical Description & Anatomy
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Body Structure: The body is divided into three main sections hinged together:
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Cephalothorax (Prosoma): The large, U-shaped or horseshoe-shaped front shell that houses the brain, heart, digestive organs, and most of the limbs.
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Abdomen (Opisthosoma): The smaller, segmented middle section containing the book gills used for breathing.
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Telson: A long, sharp, spike-like tail used as a rudder for steering and for flipping themselves right-side up if they get turned over. It is harmless and not venomous.
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Eyes & Vision: They have multiple eyes (around 10 in total), including a pair of compound eyes on the shell and smaller photoreceptors or light-sensing organs along the tail and underside.
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Legs & Feeding: They have 10 legs. The first pair (chelicerae) grasp prey, while the remaining legs feature spiny bases that crush and grind food (such as worms and clams) before pushing it into the mouth located underneath.
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Size & Color: Adults have a hard, brownish-green exoskeleton and can grow up to 2 feet (60 cm) or more in length.
Unique Features
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Living Fossils: Their lineage dates back hundreds of millions of years (older than dinosaurs).
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Blue Blood: Their blood is bright blue because it relies on copper-based hemocyanin instead of iron-based hemoglobin. Their blood contains special cells (amebocytes) that rapidly clot around bacterial toxins, making it vital for testing safety in pharmaceutical drugs and medical devices
