Horseshoe crab
Scientific classification
Phylum: Arthropoda
Genus: Limulus
Species: polyphemus
Binomial name
Limulus polyphemus

The horseshoe crab, Limulus polyphemus, is an arthropod that is more closely related to spiders rather than crabs. They are most commonly found in the Gulf of Mexico and along the Northern Atlantic coast. They can grow up to 51 cm, on a diet of mollusks, wormss, and invertebrates, which they find under the sand where they spend most of there lives.

Horseshoe crabs have gills located just behind their appendages that allow them to breath underwater. The outer shell of these animals consists of three parts. The carapace is the smooth front most part of the crab; it has on it the eyes, the walking legs, the chelicera (pincers), the mouth, the brain, and the heart. The abdomen is the middle portion where the gills are attached as well as the genital operculum. The last section is the telson it is used to flip itself over if stuck upside down.

Limulus has been extensively used in research into the physiology of vision. It has a compound eye, and each ommatidium feeds into a single nerve fibre. Furthermore the nerves are large and relatively accessible. This made it possible for electrophysiologists to record the nervous response to light stimulation easily, and to observe visual phenomena like lateral inhibition working at the cellular level. More recently, behavioural experiments have investigated the functions of visual perception in Limulus. Habituation and classical conditioning to light stimuli have been demonstrated, as has the use of brightness and shape information by male Limuli when recognising potential mates.

Since 1964 a substance in their blood called Limulus amebocyte lysate (LAL) has also been used to test for bacterial endotoxins in pharmaceuticals and for several bacterial diseases. The animals can be returned to water after extraction of a portion of their blood (except in Massachusetts), so this is not necessarily a threat to the survival of horseshoe crabs. (Funk & Wagnalls New Encyclopedia)