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Horseshoe crabs have several distinct stages to their life cycle. Stages of a horseshoe crab’s life cycle are:
Larval horseshoe crabs develop in 21 different stages before hatching from their embryos as juvenile crabs. These stages occur in a span of 14 days. However, the time period for development is variable depending on temperature and salinity at the nest site.
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).
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.
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|Updated July 29, 2005|