Effects of salinity fluctuations on the respiration rate of the southern oyster drill Thais haemastoma and the blue crab Callinectes sapidus

Abstract

Respiration rates of Thais haemastoma and Callinectes sapidus were determined as a function of salinity with a flow-through respirometer at 20°C. Respiration rates were measured at 10, 20 and 30‰ S for acclimated animals. The effects of 10-5-10‰, 20-10-20‰, 30-10-30‰ and 10-30-10‰ S semidiurnal cycles (12 h) of fluctuating salinity on the rate of respiration of the oyster drill were studied. During each cycle, salinity was changed from the acclimation salinity over a 4 h interval, held at that salinity for 2 h, returned to the acclimation salinity over 4 h and held at that salinity for 2 h. The effects of diurnal (24.8 h) salinity cycles on respiration in the oyster drill and blue crab were also studied. Salinity was changed from the acclimation salinity over a 10.4 h interval, held at that salinity for 2 h, then returned to the acclimation salinity over 10.4 h and held at that salinity for 2 h. The respiration rate of 30‰ S acclimated oyster drills (679 μl O₂ g dry weight⁻¹ h⁻¹) was significantly higher than for individuals acclimated to 10‰ S (534 μl O₂ g dry weight⁻¹ h⁻¹). Blue crab respiration was 170 μl O₂ g dry weight⁻¹ h⁻¹ at 30‰ S, and was significantly higher at 10 and 20‰ S than at 30‰ S. With the exception of the 20-10-20‰ S semidiurnal cycle, the respiration rate of oyster drills declined as salinity fluctuated in either direction from the acclimation salinity and increased as ambient salinity returned to the acclimation salinity. Semidiurnal cycles (12 h) of fluctuating salinity produced greater changes in the respiration rate of snails than analogous diurnal cycles (24.8 h). A 10-30-10‰ S pattern of fluctuation caused a greater percentage reduction in the steady state respiration rate of oyster drills than the 30-10-30‰ S pattern. The respiration rate of blue crabs varied inversely with fluctuating salinity. Relatively minor changes occurred in blue crab respiration rate with fluctuating salinity. Blue crab respiration rate characteristically dropped during the initial phase of declining salinity at a rate directly proportional to the rate of salinity decrease, perhaps representing a metabolic adjustment period by the blue crabs. The respiratory response of T. haemastoma to salinity is consistent with its incomplete volume regulation, while the response of C. sapidus is compatible with its ability to regulate extracellular fluid osmotic and ionic composition.