Ontogeny of cardiac and ventilatory function in the crayfish procambarus clarkii

Ontogeny of cardiac and ventilatory function was investigated in the direct developing crayfish Procambarus clarkii to determine basic developmental patterns and to evaluate diffusional and convective gas exchange. Animals were exposed to water ranging in Po₂ from 150 to less than 10 mmHg. Ontogeny of cardiac function follows a pattern unlike that observed in other developing organisms. Heart rate (f_H) decreases from the mid-point of embryonic development until hatching, and the decrease in / is accompanied by a concomitant loss in cardiac and ventilatory sensitivity to hypoxia. During larval development however, /H increases until a juvenile stage is reached. Heart rate then decreases again as the animal increases in mass. Cardiac and ventilatory responses to hypoxia are restored by the third larval instar. Ventilatory function is initiated within hours of hatching. Scaphognathite movement (f_Sc), which is initially uncoordinated, does not result in appreciable movement of water, but functional pumping is achieved within hours of hatching. Animals do not exhibit an adult-like response to hypoxic exposure until at least the third larval instar. The ontogeny of both cardiac and ventilatory function indicates that the direct developing crayfish is not physiologically mature until an early juvenile stage. The drop in embryonic JH and loss of hypoxic sensitivity late in development may indicate that oxygen requirements of embryos exceed the capacity of egg membranes capacity (surface area) to supply oxygen by diffusion.