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Blue crabs live in estuaries along the United States' Atlantic and Gulf coasts. They are mobile predators whose salinity requirements change at different stages in their lives.
Adult male crabs live in the low-salinity waters upstream, while adult female crabs live in the higher-salinity waters near the mouth of the estuary. During the crabs' mating season (May to October), the high-salinity preference of the female overlaps with the lower-salinity preference of the male (Zinski, 2003). After mating, female crabs migrate offshore, sometimes up to 200 km, to high-salinity waters to incubate their eggs. The females release their larvae, called zoeae, during spring high tides. The zoeae, resembling tiny shrimp, develop in the coastal waters. Zoeae require water with a salinity over 30 ppt (parts per thousand) for optimal development, which is only found in the ocean. Winds and coastal currents keep the larvae near the ocean shore, until they return to the estuary as young crabs, called megalops.
When the megalops return to the estuary, they swim up and down in the water in response to light and tides. This is called vertical migration (Zinski, 2003). The young crabs use nighttime flood tides to move upriver into the shallow parts of the estuary. Eventually, the young crabs take up life on the bottom of the estuary, seeking out shallow-water habitats like seagrass beds and submerged aquatic vegetation (SAV), where they feed and gain protection from predators.
MouseOver the crabs for details of the blue crab's life cycle.