This year the horseshoe crab population has shown signs of improved numbers and the red knots that depend on their eggs to give them the energy to complete their journey to their breading grounds in the Artic are getting enought eggs to bring them up to optimum weight.

While this is a very good sign that things are turning around, the red knot population is still critically low and more work has to be done to ensure that the red knot and horseshoe crab population makes a full recovery to previous over harvesting days.

There are some things you do need to do in order to keep your horseshoe crab alive and healthy.

First you need to set up your marine aquarium correctly and with the right equipment. If you have a marine set up already then adding a horseshoe crab is pretty straight forward. They just need to be acclimatized to the new tank slowly like you would do with any new member to your marine environment.

If you buy a horseshoe crab from a local fish store, be sure not to take all their advice at blindly. I have visited a lot of fish stores and questioned them on keeping horseshoe crabs and the advice they gave was in most cases wrong or incomplete. So beware.

horseshoe crabs coming to shore to spawn

This is the time of year when the weather gets nice and love is in the air in Delaware. At least for the horseshoe crabs.

From now until about the middle of June the Delaware bay area beaches will be covered with horseshoe crabs as they come to spawn as they have done for thousands of years.

Delaware bay area beaches have more horseshoe crabs by far then any other place on the east coast. The horseshoe crabs love Delaware’s quiet protected beaches to lay their eggs by the millions.

This is also the time of year that the horseshoe crab sensus will be taken of the horseshoe crabs to try to get an estimate of the current population of horseshoe crabs and whether conservation efforts over the last decade is helping these ancient creatures to maintain a healthy population.

Finding horseshoe crab shells in shallow water

What they are looking at are not dead horseshoe crabs but their molts that they need to shed at least once a year. In areas where Limulus is common, the shells, exoskeletons or exuviae (molted shells) of horseshoe crabs frequently wash up on beaches, either as whole shells, or as disarticulated pieces.

The shell of these animals consists of three parts.

The carapace is the smooth frontmost part of the crab which contains the eyes (five pairs), one pair of small pincers/chelicerae used to move food towards the mouth, five pairs of walking legs (the first four with claws, the last with a leaflike structure used for pushing), the mouth in between the legs, 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 (i.e., tail or caudal spine) which is used to steer in the water and also to flip itself over if stuck upside down.

The horseshoe crab can grow up to 60 centimetres (24 in) in length (including tail); the female is typically 25 to 30 percent larger than the male.

Limulus means “odd” and Polyphemus refers to the giant in Greek mythology. It is based on the misleading idea that the animal had a single eye.

Former scientific names include Limulus cyclops, Xiphosura americana and Polyphemus occidentalis.

The horseshoe crab is a ‘living fossil’: forms almost identical to this species were present during the Triassic period 230 million years ago, and similar species were present in the Devonian, a staggering 400 million years ago. Despite their common name, they are not crabs but are related to arachnids (spiders, scorpions, ticks and mites), and are the closest living relatives of the now extinct trilobites.

Horseshoe crabs have three main parts to the body: the head region, known as the ‘prosoma’, the abdominal region or ‘opisthosoma’ and the spine-like tail or ‘telson’. It is the tail that earns this order its name Xiphosura, which derives from the Greek for ‘sword tail’.

The sexes are similar in appearance, but females are much larger than males and the male has a boxer’s glove shaped front claw that is used to hang onto the femail horseshoe crab during spawning.

The carapace is shaped like a horseshoe, and is greenish grey to dark brown in colour. A wide range of marine species become attached to the carapace, including algae, flat worms, molluscs, barnacles and bryozoans, and horseshoe crabs have been described as ‘living museums’ due to the number of organisms that they can support.

On the underside of the prosoma there are six paired appendages, the first of which (the chelicera) are used to pass food into the mouth. The second pair, the pedipalps are used as walking legs; in males they are tipped with ‘claspers’ which are used during mating to hold onto the female’s carapace.

The remaining four pairs of appendages are the ‘pusher legs’, also used in locomotion. The opisthosoma bears a further six pairs of appendages; the first pair houses the genital pores, while the remaining five pairs are modified into flattened plates, known as book gills, that are used in ‘breathing’.

There is a compound eye on each side of the prosoma, five eyes on the top of the carapace, and two eyes on the underside, close to the mouth, making a total of nine eyes. In addition, the tail bears a series of light-sensing organs along its length. A further unique and intriguing feature of this ancient species is that it has blue copper-based blood

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 Delaware Bay, although stray individuals are occasionally found in Europe.

The other three species in the family Limulidae are also called horseshoe crabs. The Japanese horseshoe crab (Tachypleus tridentatus) is found in the Seto Inland Sea, and is considered an endangered species because of loss of habitat. Two other species occur along the east coast of India: Tachypleus gigas and Carcinoscorpius rotundicauda.  All four are quite similar in form and behavior.

The extinct diminutive horseshoe crab, Lunataspis aurora, 4 centimetres (1.6 in) from head to tail-tip, has been identified in 445-million-year-old Ordovician strata in Manitoba.