HARPER, S.L.*; REIBER, C.L.: Cardiac and respiratory adjustments of the tadpole shrimp in response to chronic and acute hypoxia

We have investigated the physiological basis of crustacean hypoxia tolerance using tadpole shrimp as a model organism. In particular, we examined the cardiorespiratory mechanisms employed by tadpole shrimp to survive euryoxic conditions. We reared tadpole shrimp under normoxia (19-21 kPa O₂), moderate hypoxia (10-13 kPa O₂) or severe hypoxia (1-3 kPa O₂) to determine if adult tadpole shrimp physiology is predetermined by the rearing environment or if the compensatory mechanisms of its adult response have a large range over which it can maintain regulation. Growth rates appeared to be higher during early development but plateau sooner in tadpole shrimp populations reared under severe hypoxia. Respiratory surface area was significantly decreased in animals reared under severe chronic hypoxia. However, despite the decrease in surface area, animals reared under severe hypoxia were able to take up oxygen at the same rate as animals reared under normoxia. It was found that oxygen consumption of the tadpole shrimp is dependent on Po₂, however, sex of the animal and rearing Po₂ had no effect on the metabolic rate or response to an acute hypoxia. Ventilatory amplitude was significantly different between the normoxic and severe hypoxic groups when exposed to a severe acute hypoxia (1kPa O₂). Hemoglobin concentrations were significantly elevated in those animals reared under severe hypoxia, whereas the concentrations were significantly reduced in those that were reared under moderate hypoxia when compared to normoxic animals. Tadpole shrimp appear to have a wide range of Po₂‘s over which they can be successful rather than committing to plastic traits during ontogeny that depend on the developmental environment.