Metamorphosis of the blue crab Callinectes sapidus: effects of humic acids and ammonium

Postlarvae (megalopae) of the blue crab Callinectes sapidus are transported from offshore/coastal areas into estuaries where they metamorphose (molt) to the first crab stage. Metamorphosis is accelerated by exposure to chemical cues in estuarine water. The present study tested the hypothesis that estuarine dissolved humic acids and ammonium serve as cues for accelerating metamorphosis. Estuarine humic acids are attractive as estuarine specific cues because they enter estuaries through freshwater inflow but precipitate as the salinity increases. Thus, estuarine humic acids are pre sent in estuaries but occur in very low concentrations in coastal/offshore areas. Chemical cues that accelerate metamorphosis were present in river water entering an estuary. The time to metamorphosis decreased upon exposure to increasing concentrations of humic acids extracted from river water and to commercial humic acids. Thus, dissolved humic acids can serve as a cue for being in an estuary. High concentrations of ammonium chloride (25 mu M and greater) delayed metamorphosis in estuarine water and reversed the accelerating effects of humic acids. Since high levels of ammonium usually occur in areas of low oxygen levels, this response may prevent megalopae from metamorphosing in these areas, which are less conducive to survival.