Blue Crab Life History
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.