Horseshoe Crab Anatomy

• Horseshoe crabs are an ancient group, appearing in the fossil record over 300 million years ago.
• They are arthropods, related to spiders, crustaceans, and insects.
• Horseshoe crabs are invertebrates with segmented bodies, jointed appendages, and hard exoskeletons.
• The exoskeleton protects the internal organs.
• The name "horseshoe crab" comes from their shape.

Horseshoe Crab Anatomy

• The body has three sections: the prosoma, opisthosoma, and telson.

The Prosoma

• The prosoma (cephalothorax) is the largest, front-most section.
• It contains the majority of the nervous and cardiovascular systems.
• It is round and u-shaped, resembling a horseshoe.
• The prosoma has a hard exoskeleton made of chitin.
• Horseshoe crabs have 10 eyes of varying sizes and locations.
• Two compound eyes on the prosoma are for locating prey and mates.
• Eight other eyes are light receptors that detect movement and moonlight changes.
• Six pairs of legs are on the underside of the prosoma.
• Five pairs are walking legs (pedipalps).
• One pair is pusher legs used for digging and burrowing.
• Leg tips have gnathobases, spiny protrusions that guide food to the mouth.
• Leg movement and gnathobases help process food.

The Opisthosoma

• The opisthosoma (abdomen) is the mid-region, connected to the prosoma by a hinge.
• It is smaller and inverted trapezoid-shaped.
• The underside has gills for breathing and muscles for locomotion.
• Spines along the edges serve as defense mechanisms.
• The reproductive organs are in the opisthosoma.

The Telson

• The telson is the tail or rear section.
• It is used to flip the animal over and help with burrowing.
• The telson can have eyes that sense changes in light based on the lunar cycle for mating.

Internal Anatomy

• Horseshoe crabs breathe using gills on the underside of the opisthosoma.
• There are five pairs of gills with lamellae arranged like book pages.
• Oxygen is absorbed into the blood as water moves across the lamellae.
• A long, tube-shaped heart stretches from the prosoma to the opisthosoma.
• The heart circulates oxygen and nutrients.
• The nervous system controls respiration and circulation.
• There is a central nervous system (brain) in the prosoma.
• Eight pairs of nerves innervate other structures.
• Optic nerves control the eyes.

Gender Differences

• Adult females are larger than males.
• Males have specialized clasping legs to grab females during mating.
• Reproductive organs are associated with genital pores.
• Male genital pores are white and pointed.
• Female genital pores are broad and bump-shaped.

Horseshoe Crab Taxonomy

• Domain: Eukarya
• Kingdom: Animalia
• Phylum: Arthropoda
• Class: Euchelicerata
• Order: Xiphosurida
• Family: Limulidae
• Genus: Limulus
• Horseshoe crabs are invertebrates that evolved in shallow marine environments during the Paleozoic Era.

Horseshoe Crab Phylum

• Horseshoe crabs belong to the phylum Arthropoda, along with spiders (arachnids).
• The subphylum is Chelicerata, derived from Greek words meaning "claw" and "horn."
• Chelicerata refers to the hard exoskeleton.
• Chelicerata includes sea spiders, horseshoe crabs, and arachnids.

Horseshoe Crab Class

• The horseshoe crab class is Euchelicerata, meaning "well-covered exoskeleton."
• Euchelicerata includes horseshoe crabs and arachnids.

Horseshoe Crab Order and Family

• All horseshoe crab species belong to the order Xiphosura and family Limulidae.
• Xiphosura comes from Greek words meaning "sword" and "tail," referring to the telson.
• Order Xiphosura has four extant species with segmented bodies and jointed legs.
• Family Limulidae includes living and extinct horseshoe crab species and comes from the Latin word meaning "askance"

Horseshoe Crab Genus and Species

• Limulus polyphemus: Atlantic Coast, long, round front, moderately sized mid-region
• Carcinoscorpius rotundicauda: Southeast Asia, shorter front, broad mid-region
• Tachypleus tridentatus: Southeast Asia and Japan, short, indented front, medium mid-region
• Tachypleus gigas: Indo-Pacific, short, rounded front, broad mid-region