Developmental Changes in Ionic and Osmotic Regulation in the Dungeness Crab, Cancer magister

The ontogeny of osmoregulation and specific ion regulation was studied in the megalopa, 1st instar juvenile, 5th instar juvenile and adult of Cancer magister. Hemolymph Na+, Cl-, K+, Mg++, and Ca++ concentrations and osmolality were measured after 8-h exposure to 100%, 75%, and 50% seawater at 10°C and 20°C. The ability to hyperosmotically regulate is present in the megalopa, and ontogenic changes occur in both ionic and osmotic regulation. First instar juvenile crabs, which are exposed to the greatest extremes of salinity and temperature in the field, are less able to osmoregulate than are the other three stages examined. Changes in Na+, Cl-, and K+ concentrations parallel total osmolality in all four stages. Hemolymph Mg++ concentrations in megalopa and juveniles acclimated to 100% seawater are more than twice that of the concentration in the adult; after 8 h in 50% seawater, the megalopa and juvenile Mg++ concentrations decrease to the level of the strongly regulated adult Mg++ concentration. Ca++ is strongly regulated by megalopas and adult crabs exposed to reduced salinity compared to the two juvenile stages. Diminished predation pressure and high food availability are proximate factors that may outweigh short-term osmoregulatory stress encountered on the tideflats during development of the juvenile crab.