Blue Crab Life History

 Blue crabs live in estuaries where freshwater rivers meet the saltwater ocean.  Crabs occupy both saltwater habitats and freshwater habitats at different parts in their life cycle.  Females carry their eggs to offshore locations to release their larvae into the open ocean.  These larvae (zoea) develop for approximately 30 days in nearshore coastal waters before coming back in shore.  The last zoea stage molts into a postlarvae (megalopae) which is the life stage that rides the tides into the estuary and settles into shallow seagrass or marsh grass habitats.  The megalopae molts into a first stage juvenile crab.  Juvenile crabs slowly move upriver into the freshwater rivers over the next 18 months growing larger and moving further inland.  When they reach sexual maturity, females release a pheromone to attract a male for mating.  Mating usually begins in the early spring and may continue until the fall.  Once females are mated, they will begin their downriver migration back to the open ocean to complete their life cycle.

Crabs know how far to migrate upriver by the salinity of the water.  During drought years when less freshwater is flowing into the estuary, the salinity from the open ocean extends further upriver.  This increases the distances that crabs must migrate to complete their life cycle.  This increased distance of travel has several potential negative consequences to crabs.  First, juvenile crabs grow faster at intermediate salinities (20-25 psu or practical salinity units).  During drought years it takes longer for juveniles to reach this range of optimal growth.  Second, crabs encounter a parasite (Hematodinium sp) only in high salinity water.  So droughts increase exposure and mortality due to disease.  Third, females expend a lot of energy migrating upriver to find males and then again downriver to release larvae.  This extra energy could be used to make more eggs in years when drought conditions are not present.  Finally, females use salinity cues to know when they have returned to the open ocean for the release of their larvae.  If females encounter full seawater (35 psu) before they leave the mouth of the river, the larva may not make it off-shore to complete their life cycle.