Female mitten crabs migrate to brackish (salty) water to reproduce, brooding eggs that hatch as swimming larvae in winter through early summer. Larval crabs (zoea) develop primarily in high salinity waters until the last larval stage (the megalopa), when they settle in brackish near-shore waters. Juvenile crabs migrate from estuaries to freshwater rivers, lakes and marshes in the late summer and fall. The juvenile crabs can migrate inland up to 1500 km. The crabs stay in freshwater environments from two to five years, before migrating back to the estuaries in late winter or early spring to reproduce.
Mitten crabs go through seven larval stages (a prezoea stage, five zoeal stages, and a megalopa stage), before developing into juvenile crabs. Larval mitten crabs can hatch and complete development in a wide range of temperatures and salinities, and environmental tolerances vary with larval stage. The salinity optimum shifts from brackish to seawater during zoeal development and back to brackish by the megalopa stage, corresponding to larval dispersal patterns.
Larval crab, zoea.
Larval crab, megalopa.
Photo by Sarah H. Fernald,
Hudson River National Estuarine Research Reserve