METAMORPHOSIS OF THE BLUE-CRAB CALLINECTES-SAPIDUS - EFFECTS OF HUMICACIDS AND AMMONIUM

Postlarvae (megalopae) of the blue crab Callinectes sapidus are transp orted from offshore/coastal areas into estuaries where they metamorpho se (molt) to the first crab stage. Metamorphosis is accelerated by exp osure to chemical cues in estuarine water. The present study tested th e hypothesis that estuarine dissolved humic acids and ammonium serve a s cues for accelerating metamorphosis. Estuarine humic acids are attra ctive as estuarine specific cues because they enter estuaries through freshwater inflow but precipitate as the salinity increases. Thus, est uarine humic acids are pre sent in estuaries but occur in very low con centrations in coastal/offshore areas. Chemical cues that accelerate m etamorphosis 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 aci ds. Thus, dissolved humic acids can serve as a cue for being in an est uary. High concentrations of ammonium chloride (25 mu M and greater) d elayed 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.