Reproduction
The Ancient Spawning Ritual in Delaware Bay
Horseshoe crabs spend most of their time in salt water on
the bottom of bays and shallow coastal areas, resting or plowing through the
sediments in
search of their favorite foods — clams
and worms. In places where the water stays warm year-round, horseshoe
crabs remain active all year. Where water temperatures are low in winter,
they burrow down in the mud to wait for spring.
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Males surround a female during spawning. |
As the days lengthen, adult horseshoe crabs begin to move from deeper waters
in the bay or continental shelf toward the beaches to spawn. Mating activity
peaks during the full and new moons of late May and early June when a million
or more horseshoe crabs may appear on the beaches of the Delaware Bay. They
can be observed spawning during the day and night, but by far the highest
numbers are seen at night when they are protected by the darkness.
Adult males arrive on the beaches in late spring, a few weeks before the
females, and begin patrolling the near-shore waters for mates. When
the females arrive, they release into the water a pheromone, a natural attractant
that acts as a sexual stimulant. Horseshoe crabs also use their compound
eyes to spot potential mates.
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A male is carried along by a female. |
Upon finding a mate, the males hook their pedipalps (the specially
modified second set of clawed appendages) onto the opisthosoma of a female as she heads toward the beach. Sometimes additional males
will attach themselves to the male, forming a chain.
The female drags the male to the water's edge. Once on shore, she uses her
pusher legs to form a shallow nest between four and six inches deep
between high- and low-tide lines. Here she deposits 5-7 clumps of 2000-4000 eggs each, or up to 20,000 eggs in aspawning episode. The attached male and any additional or satellite males that are surrounding the spawning female, move with her as she lays each clump of eggs. She will repeat this process several times over the spawning cycle laying 90,000 eggs or more in a season. It is estimated that less than ten of these eggs will survive
to adulthood.
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A female looks half-buried
as she digs her nest. |
The pair repeat this process several times before returning to the water.
Scientists believe that on average female horseshoe crab mature at 10-11 years and males 8-9 years, which is why the fishery can be easily impacted by over fishing - time to sexual maturity is a long time so the fishery is difficult to manage.
Click here to find a guide to
horseshoe crab spawning hot spots on the United States' east
coast. |
