Horseshoe crabs are typically active at night, with activity peaking around the time of the full moon. They dig for food, such as worms, algae and molluscs in the sediment (3).
During the spring and summer, adult horseshoe crabs migrate in huge numbers towards sandy beaches and congregate in the shallow water (4). Breeding is associated with the lunar and tidal cycles, with most adults arriving at the full or new moon and within a couple of hours of high tide. The direction of the waves guides the females towards the beach (3). Male horseshoe crabs patrol along the bottom of the beach in the shallow water, waiting to intercept beach-bound females (4). Pairs make their way to the high tide mark and the male fertilises the eggs as they are laid into a 15 centimetre deep nest in the sand (3). From 2,000 to 20,000 eggs may be produced in a single clutch (4). Very often there may be more than one male accompanying each female; in some cases there have been as many as 14 males to one female. As the tide begins to retreat, the horseshoe crabs make their way back to the sea (3).
The sticky eggs hatch after around five weeks (3), but this is dependent on temperature (4). The larvae, which are known as ‘trilobite’ larvae, may remain buried in the sand in aggregations for a number of weeks (4) before emerging at high tide. After they enter the water, they undergo a ‘swimming frenzy’ of constant, vigorous activity. Six to eight days after emerging, they moult into the first juvenile stage, which is very similar in appearance to the adult stage. At this point they cease swimming and start to live on the bottom (3). Horseshoe crabs are slow-growing. Males reach sexual maturity at 9 to 11 years of age and females between 10 to 12 years. Although it is difficult to assess age in this species, the average life-span is thought to be 20 to 40 years (4).
The horseshoe crab is an essential part of the ecosystem in which it occurs. Their eggs provide a valuable source of food for many species including wading birds, sea turtles, alligators and fish. Furthermore, the action of the horseshoe crab as it ploughs the sea bed in search of food aerates the substrate, resulting in a higher level of species richness (2).