Egg Stage
Adult horseshoe crabs bury their eggs on beaches. Females will dig a nest in the moist sand between the spring high-tide and low-tide lines. Nests may consist of 4,000 eggs or more and are located 15-25 cm from the surface of the beach. Each egg is 1.6mm in diameter or roughly the size of a pinhead. Once fertilized, the light green eggs begin to develop.

During the first 5 days after fertilization, the egg yolk becomes segmented and the embryo forms on the surface of the egg yolk. At day 6, the outer membrane of the egg called the “chorion” is shed. An inner membrane secreted by the developing embryo replaces the chorion and swells up to twice the size of the original egg. The new membrane is transparent and the developing horseshoe crab embryo is visible to the naked eye. The visible embryo appears to rotate and move within the swollen inner membrane. By day 10, two major body segments (i.e., prosoma and opisthosoma) are readily visible. In another few days, the embryo will hatch from the clear membrane as a miniature horseshoe crab. The newly hatched horseshoe crab lacks a visible telson and will remain within the moist sand for varying lengths of time. At this stage they can use their book gills to propel themselves through the water column. Lacking a completed formed digestive system, these post-hatch larval crabs will absorb their remaining embryonic yolk sack until their first post-hatch molt. Photos and movies of developing larvae

Most of the larval horseshoe crabs will emerge from the sand soon after hatching. Entering the water during a high tide, they will swim for a short period of time and then settle to the bottom and burrow into the sediments of the tidal flats off the beach where they were spawned. Here they will molt and remain in the sediments for up to a year. After this first post-hatch molt, the characteristic telson appears and they begin to feed on benthic organisms (i.e., polychaetes and marine worms).

Juvenile Stage
As juveniles, horseshoe crabs molt or shed their shell to grow. Molting occurs several times during the first two to three years and about once a year afterwards. Molting occurs approximately 16 to 17 times over a period of 9 to 11 years before sexual maturity is reached and once mature, it is believed they no longer molt. Females reach maturity one year later than males and consequently, go through additional molts. Juvenile crabs will move further away from the intertidal flats as they grow.

Adult Stage
Adult horseshoe crabs spend most of their time in deeper waters feeding mainly on marine worms and shellfish including razor clams and soft-shelled clams. They dig through the benthic sediments looking for food and use their chelicera to guide the food to their mouths. Because they lack jaws, horseshoe crabs crush and grind their food items using the spiny base of their legs and then push the small food particles into their mouths.

Horseshoe crabs can tolerate a wide range of temperatures and can survive in low oxygen environments. As long as their book gills are kept moist, horseshoe crabs can survive out of the water for extended periods of time, especially to spawn.

Mature horseshoe crabs annually migrate to inshore spawning areas every spring. The spawning period varies by latitude. In the Mid-Altantic region, horseshoe crabs spawn from May through July peaking in June. If a horseshoe crab can survive the rigors of spawning, disease or predators, it may live to 18 years of age.