ABSTRACT
Measurements of branchial pressure and ventilation volumes were used to calculate the work and power of gill ventilation in Carcinus maenas, during spontaneous as well as forced unilateral ventilation. With increasing ventilation rate (fR), the stroke work increases as a result of an elevated gradient of branchial pressure, while power output increases as a result of enhancement of both the pressure gradient and the flow rate. To a first approximation, the stroke work is proportional to fR1·5, and the power output is proportional to fR2·5. The available evidence suggests that flow is mainly laminar through the branchial chamber but turbulent through the pumping chamber. Evidence is presented which suggests that the crab is able to vary the resistance to the flow of branchial water.
The increased branchial pressure at elevated ventilation rates constitutes an increased load on the ventilatory muscles. Measurements of isometric tension confirm that the muscles compensate for this increased load by generating greater force. Electromyograms support the notion that the change in force results from appropriate changes in the output from the central pattern generator.
