Abstract
Estuaries are highly variable environments in which organisms must withstand rapid changes in several physical parameters, including salinity and temperature, or suffer the consequences. As microhabitat temperatures increase in the summer, and atmospheric and oceanic termperatures continue to warm over the next century, many intertidal organisms will be pushed closer to the thermal limits. In addition, local convective thunderstorms and stream discharge during the wet season can be amplified during El Nino years resulting in estuaries with reduced salinities. The flatback mud crab, Eurypanopeus Depressus, lives on intertidal oyster reefs that experience abrupt changes in both salinity and temperature (air and water) on a seasonal, daily, and hourly basis. Eurypanopeus Depressus has previously been used as a model species for understanding oyster reef communities and may serve as an indicator species for climate changes studies. Therefore, it is important to determine the thermal maximum of E. Depressus and hoe acute changes in salinity and temperature affect its physiology. Thermal tolerance was measured using the righting response (behavioral thermal limit) and mortality (lethal thermal limit) as endpoints. The behavioral thermal limit, the temperature at which 50% of the population was unable to right themselves, increased as salinity increased. The lethal thermal limit, the temperature at which mortality was found in 50% of the population, increased between 5 and 10 ppt but decreased slightly between 10 and 15 ppt. Eurypanopeus Depressus was also exposed to an acute temperature increase (24° to 27, 30 or 33°C) at 5, 10, and 15ppt as well as an acute salinity decrease (15 ppt to 10, 5, or 0 ppt) at 34 and 36°C for 48 hours. No distinct trend in changes in hemolymph osmolality emerged with regards to multiple stressors. Oxygen consumption rate (VO2) of crabs increased as temperature increased. At all salinities, VO2 was significantly lower at 24°C than at 33°C. In addition, after an acute decrease in salinity at 36°C, VO2 was significantly lower at 0 ppt and was significantly higher at 5 ppt than at all other salinities. VO2 at 34°C resulted in no significant differences in VO2 due to salinity. Observations at 0 ppt and 34 and 36°C revealed no crabs survived past 8 and 24 hours, respectively. This work shows how E. Depressus responds to multiple stressors and that multiple stressors can elicit responses that are different from those to a single stressor. This study also demonstrates that the capacity for E. Depressus to withstand high temperatures is compromised at reduced salinities (>5 ppt) over a period of 48 hours.