Horseshoe Crabs – Anatomically Speaking

What’s in a Horseshoe Crab?

Top or Dorsal View

A. Prosoma:

The front section of the horseshoe crab’s body, which is also the largest section. It’s the part of the shell that’s shaped like a horseshoe, and contains the majority of its vital organs including its brain, heart, mouth, nervous system, and glands.

B. Opisthosoma:

This is the middle section of the horseshoe crab that contains the abdomen and the gills.

C. Telson:

This is the horseshoe crab’s tail, used to help the horseshoe crab turn over and steer like the rudder of a boat.

D. Compound Eye:

The horseshoe crab has two compound eyes, one on each side of its prosoma section. These eyes see very well, just like a spider sees, and help the horseshoe crab to find a mate.

E. Simple Eyes:

The horseshoe crab has a bump in the middle of its shell in the front that has two small black dots, one on either side. Many people think this is the horseshoe crab’s nose, but horseshoe crabs don’t have a nose, they have gills. Those two small dots are simple eyes, which can only see shadows and light. Horseshoe crabs have other light sensors on their bodies, with several near the tail.

F. Movable Spines:

The horseshoe crab has several moveable spines on either side of its opisthosoma, or middle section, that help to protect it.

G. Hinge:

The horseshoe crab has a hinge that connects the prosoma to the opisthosoma, and allows it to bend. There is a second hinge that connects the telson to the opisthosoma, which gives the horseshoe crab’s tail the ability to move.

H. Flange:

This is the rim around the prosoma.

Underside or Ventral View

A. Mouth:

The horseshoe crab’s mouth is in the center of the prosoma, and is surrounded by legs.

B. Gnathobases:

These are the horseshoe crab’s teeth, which surround its mouth and are at the base of the horseshoe crab’s legs. These teeth feel like the bristles from a toothbrush that are soft and easily bent. A horseshoe crab’s teeth aren’t sharp at all.

C. Exuviation Suture:

This is the hole the horseshoe crab makes in its own shell when it’s time to molt and grow. As pictured in this molted shell, the hole follows the edge of the “U” shape of the horseshoe crab’s shell on the underside of its body.

D. Chelicerae:

These are the horseshoe crab’s first pair of appendages or legs that capture food and send it towards the horseshoe crab’s mouth and bristles, or gnathobases.

E. Book Gills:

The horseshoe crab has five flaps that make up the book gills. A horseshoe crab uses its gills to breath; and by flapping its gills, the horseshoe crab propels itself through the water.

F. Anus:

Once its food is digested, waste products leave the horseshoe crab’s body through its anus.

G. Hairs:

The horseshoe crab has hairs in many places on its body. These hairs have a sense of touch, and the ones located near the bristles surrounding the horseshoe crab’s mouth have the ability to smell the approaching food.

H. Chilaria:

The horseshoe crab has two degenerate legs behind its pusher legs that help to catch and guide food towards the horseshoe crab’s gnathobases and its mouth.

I. Pusher Leg:

Horseshoe crabs have two pusher legs where the ends spread out to look like a daisy. These powerful legs help to push the horseshoe crab off in the sand.

J. Walking Legs:

The horseshoe crab has several pairs of walking legs, with weak pinchers on the ends of each leg. The horseshoe crab crawls with the use of these legs.

K. Pedipalps:

The second pair of appendages on a horseshoe crab are called pedipalps, which are different on male and female horseshoe crabs. The horseshoe crab pictured above is male. The male horseshoe crab’s pedipalps are hooks that are used to hold the female’s shell while spawning. The female horseshoe crab’s second set of appendages are pincher-like, similar to her walking legs.