An Analysis of Scaphognathite Pumping Performance in the Crayfish Orconectes virilis: Compensatory Changes to Acute and Chronic Hypoxic Exposure

Abstract

Ventilation volume (V̇w), beat frequency of right scaphognathite (fRr), total stroke volume of the scaphognathite pair (Vsw), and the amplitude of the pulse (PP) and mean standing pressure (Pbr) developed within the postbranchial chamber by scapihognathite action have been measured in the crayfish Orconectes virilis. Data were recorded from crayfish acclimated to normoxia (PO₂ = 130 mm Hg), and during acute (< 24 h) and chronic (> 4 days) exposure to hypoxia (PO₂ = 40-60 mm Hg). At values of fRr below 100 beats/min V̇w varied as a function of both Vsw and fRr, both in normoxic and acutely hypoxic animals. Above 100 beats/min, Vsw remained constant in both groups. In acutely hypoxic crayfish, stroke volume was elevated above normoxic values at any given fRr. Long-term acclimation to hypoxia was accompanied by a significant decrease in V̇w through reduction in both fRr and Vsw. In addition, the stroke volume at higher scaphognathite frequencies was significantly lower than in either normoxic or acutely hypoxic crayfish. Calculations of power output for scaphognathite pumping indicate that the power required to produce a given V̇w was unchanged by hypoxic acclimation. Scaphognathite pumping accounted for 1%, 12%, and 4% of total oxygen consumption in chronic normoxia, acute hypoxia, and chronic normoxia, respectively. The scaphognathites of the crayfish thus emerge as variable-stroke, variable-frequency ventilatory pumps. Possible active and passive mechanisms involved in modulating scaphognathite stroke volume are discussed.