The amazing Horseshoe Crab is more closely related to terrestrial spiders and scorpions than it is to the crabs it was named for. These animals are very strange in appearance and can be unique captives for someone with a larger aquarium.

Horseshoe Crabs are bottom dwellers, and in the wild usually burrow in mud or sand flats in tidal areas or estuaries. In the aquarium, your Horseshoe Crab will generally do best in a sandy substrate. They are often to be considered helpful in cleaning an aquarium or sifting sand. In the substrate, the Horseshoe Crab will move about, scavengiong detritus or other materials that have fallen to the bottom. Algae, smaller bivalves, worms, and bits of fish are the mainstays of a Horseshoe Crab's diet. To grow, the Horseshoe Crab sheds a layer of its carapace, or molts. This process is accomplished at longer and longer intervals as the Horseshoe Crab ages. Older crabs may retain their hard shells long enough to collect barnacles or other organisms on them. Directly after molting, the remaining carapace is soft and does not provide much protection for the Horseshoe Crab, which should not be handled or disturbed at this time. In general, Horseshoe Crabs are not aggressive; they are scavengers, not hunters. Nevertheless, some smaller crustaceans may be consumed. For this reason many people do not consider Horseshoe Crabs to be reef safe. However, they are quite easy to care for, and are tolerant of a wide variety of temperatures. Horseshoe Crabs are hardy and can be a good choice for beginners, given that a large enough tank is provided.

Horseshoe Crabs have blue blood, because the chemical that carries oxygen through their bloodstreams turns blue upon contact with air. Their name stems from the appearance of their carapaces, or the hard shells that protect their bodies. The entire Horseshoe Crab, including the carapace, is a shade of brown. The carapaces are shaped like horseshoes, the curved part serving as the cephalothorax, and the flat end terminating at the end of the body. Extending beyond the carapace is the telson, the sharply pointed tail like structure that attaches to the Horseshoe Crab's body with a ball and socket joint. This joint allows a wide range of movement for the telson, which can be used to right the crab when it is overturned, or to assist the crab in burrowing into the sand. Two antennae, called chelicerae, terminating in pincers assist the Horseshoe Crab in feeding. Five pairs of legs, all with claws at the ends, are located along the sides of the crab. The fifth pair ends in large claws and allows the Horseshoe Crab to move forward with a lurching motion. Each leg has spines located along the midsection, which are used to break up food before it is placed into the Horseshoe Crab's mouth. The mouth is located at the bases of the legs. Gill books, similar to the book lungs of terrestrial arachnids, are found on the underside of the Horseshoe Crab. These are protected by an operculum, which houses the genital organs as well. Two large, compound eyes are located on the dorsum of the Horseshoe Crab. A number of other, more primitive eyes, and photoreceptors, are located on the telson, the ventral surface and the midline of the Horseshoe Crab. If these are counted along with the two compound eyes, the Horseshoe Crab could be considered to have ten eyes! A mature Horseshoe Crab can measure between 20 and 24 inches (51 and 61 centimeters). Often, females are larger than males.

Although many people believe that Horseshoe Crabs are creatures that have not changed since prehistoric times, their genus is not even old enough to date back to the fossil record. However, its similar ancestors have been fossilized and may date back 520 million years. The species of Horseshoe Crab addressed in this record inhabits the Atlantic coast of North America and ranges from as far north as Maine to as far south as Mexico's Yucatan Peninsula. It has no natural predators other than man. Through the centuries, various peoples have used the Horseshoe Crab's tail as a spear head. They were also once harvested in large numbers and ground up to use as fertilizer. In more recent times, it has been discovered that their blood contains a compound useful in medical laboratories; and in a harmless procedure, blood is drawn from them and they are re-released.

Horseshoe Crabs may grow too large for many aquariums. They can tolerate a variety of temperatures, from about 57 to 78 degrees Fahrenheit. The pH should be 8.3 to 8.4, and the specific gravity should remain between 1.023 and 1.025. If there is not enough detritus in your aquarium to keep your Horseshoe Crab fed, it may be offered small pieces of clam, squid, or fish.

At between 9 and 12 years of age, the Horseshoe Crab reaches sexual maturity. The male can be identified by hooks on the first pair of his claws, which allow him to hold on to the female during amplexus. Mating generally occurs at high tide, when Horseshoe Crabs migrate to protected beaches. In full and new moon phases of spring and summer, the female Horseshoe Crab digs a nest on such a beach, depositing 2 to 30 thousand eggs there. These are fertilized by the male, and covered with sand. The eggs form a gelatinous mass, and at the full moon, larvae about one centimeter long will hatch. These must crawl to the surface of the sand, where they will be washed away from the water. The young Horseshoe Crabs are underdeveloped; for example, of five adult pairs of book gills, the young have two. The larvae will develop over time, when size and new organs are attained with each molt. In captivity, Horseshoe Crab eggs can be reared in an aquarium with well-aerated water and an air temperature kept around 70 degrees Fahrenheit. The eggs normally require several weeks to hatch, though they may require several months. The larvae may be fed on brine shrimp nauplii and frozen brine shrimp placed in the tank with a pipette after they have been defrosted. If the water becomes fouled, you may be overfeeding. Young Horseshoe Crabs seem to be able to tolerate bad water conditions, however, some people choose to begin filtration in the incubation tank. If you do wish to use a filter, wait until the larvae have acquired tails and settled at the bottom of the tank. You may wish to place a piece of nylon mesh over the filter intake, in order to keep the young crabs out.