The effects of reduced salinity on lobster ( Homarus americanus Milne-Edwards) metabolism: implications for estuarine populations

Abstract

During periods of substantial freshwater runoff, lobsters that inhabit estuaries, such as the Great Bay Estuary in NH, are exposed for several days to weeks to seawater that is diluted as low as 10 ppt. To assess the physiological stress imposed by these conditions, we measured the oxygen consumption, heart rate, ventilation rate and hemolymph osmolarity of lobsters while sequentially exposing them, for 24-h periods, to seawater of 20, 15, and 10 ppt. Measurements of hemolymph osmolarity confirmed previous results which demonstrated that at salinties below 20 ppt lobsters are limited osmoregulators; allowing their hemolymph osmolarity to drop as the environmental salinity is reduced, but always maintaining it higher than the ambient osmolarity. All animals exposed to 10 ppt, at 15 °C, were capable of surviving for at least 72 h. There was a nearly linear increase in oxygen consumption, heart and scaphognathite rates in animals exposed to dilute seawater, with almost a twofold increase in metabolic rate when animals were moved from 20 to 15 to 10 ppt. At the lowest salinity tested (10 ppt) the average oxygen consumption was higher for females than for males. We conclude that at low salinities the energetic demands of osmoregulation are greater for females than males, and for both sexes the physiological stress imposed may determine, in part, their distribution and/or movements in estuarine habitats.