Horseshoe Crabs

Sometimes called living fossils, these horseshoe shaped arthropods are brown in colour and totally harmless.

Horseshoe crabs (Limulus polyphemus) migrate to shallow water to mate and lay their green-coloured eggs. Horseshoe crabs are benthic and feed on worms and molluscs. Horseshoe crabs swim with their bodies upside down.

The Horseshoe crab is more closely related to spiders and scorpions than crabs. Horseshoe crabs are most commonly found in the Gulf of Mexico and along the northern Atlantic coast of North America. A main area of annual migration is the Delaware Bay.

Horseshoe Crab Characteristics

Horseshoe crabs can grow up to 20 inches (51 centimetres), on a diet of molluscs, annelid worms and other benthic invertebrates. A Horseshoe crabs mouth is located in the middle of the underside of the cephalothorax (the cephalothorax is an anatomical term used in arachnids and malacostracan crustaceans for the first (anterior) major body section.). A pair of pincers (chelicerae) for seizing food are found on each side of the mouth.

Horseshoe crabs possess five pairs of book gills located just behind their appendages that allow them to breathe underwater and also allow them to breathe on land for short periods of time, provided the lungs remain moist.

The outer shell of Horseshoe crabs consists of three parts. The carapace is the smooth frontmost part of the crab which contains 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’ (caudal spine) which is used to flip itself over if stuck upside down.

Horseshoe Crab Life Span and Reproduction

Horseshoe crabs can live for 20 – 25 years. Horseshoe crabs migrate into the shore in late spring, with the males arriving first. The females then arrive and make nests at a depth of 15 – 20 centimetres in the sand. Females deposit eggs into the nests which are subsequently fertilized by the male. Egg quantity is dependent on female body size and ranges from 15,000 – 64,000 eggs per female.

After the eggs split, the larvae form and then swim for about 5 to 7 days. After swimming they settle and begin the first molt. This occurs approximately 20 days after the formation of the egg capsule. As young horseshoe crabs grow, they move to deeper waters, where molting continues. Horseshoe crabs reach sexual maturity in approximately 11 years and may live another 10 – 14 years beyond that.