Horseshoe crabs are well known for their highly visible mating activities. Spawning in the Chesapeake and Coastal Bays usually begins in late May when large numbers of adults move onto beaches to mate and lay eggs. The peak in spawning activity usually coincides with a 3 or 4-day period centered on the new and full moon and evening spring tides. View a graph of the peak spawning periods.
Spawning adults prefer beach areas within bays and coves that are protected from rough water.
Preferred beaches are also close to tidal flats that offer an ample supply of food for juvenile horseshoe crabs. In addition, beaches need to have well oxygenated sediments that allow deposited eggs to develop.
Eggs are laid in clusters or nests along the beach in a broad band between high and low tide marks. Several nests are made during one beach trip and females will return on successive tides to lay more eggs. Females can produce 80,000 to 100,000 eggs per year.
During a spawning period, the smaller male crabs will move onto the beach and seek out female crabs at the tide line. They find female horseshoe crabs by the chemical attractants secreted by the females. There is also some evidence that the male horseshoe crab can find females visually.
On the beach, males outnumber females at a ratio of 3 to 1. Males attach to an unattended female using a specialized claw (see photo below) used to clasp onto the female’s abdomen. The coupling of horseshoe crabs during spawning is called aplexus. Numerous male horseshoe crabs will cluster around a single female along the tide line. This reduces the chances that the eggs of a spawning female will go unfertilized during a spawning event.
At peak spawning periods, area beaches will sometimes be covered in spawning horseshoe crabs at the tide line. The Delaware Bay beaches contain the highest densities and numbers of horseshoe crabs during a spawning season. Numbers of spawning crabs could reach the 100’s of thousands in the Delaware Bay on a given night. The Chesapeake Bay spawning population is thought to be much lower but thousands of crabs have been seen during peak spawning events. View a chart showing spawning relationships to the lunar cycle.
Fertilization of horseshoe crab eggs occurs externally. The female horseshoe crab digs a nest in the sand at the tide line. Females will sometimes be almost completely buried in the sand. At this time, she will lay up to 4,000 eggs. The attached male will then deposit milt into the sand reaching the eggs. If there are several satellite males surrounding the buried female, they will also contribute sperm to the nest. The female horseshoe crab will move a short distance away from the first nest and dig another nest to lay a new clutch of eggs. She may lay up to 5 clutches of eggs before returning to the water. Female crabs will return to nearby beaches to lay egg masses at least 3 times during the spring spawning season.
Heavy spawning activity
Several horseshoe crab mates
surround a female during spawning
or use the glossary.
Raising Horseshoe Crabs in the Classroom
- Stacy Epperson
Aquatic Resource Education Dept
Department of Natural Resources
580 Taylor Ave., E-2
Annapolis, MD 21401