Physiological and biochemical aspects of the valve snap and valve closure responses in the giant scallopPlacopecten magellanicus

Summary

• 1. The giant scallopPlacopecten magellanicus responds to stimulation with starfish extract by snapping the valves in a series of rapid adductions. After approximately 25–35 adductions, the scallop does not respond to further stimulation, and usually closes the valves.

• 2. Snapping of the valves results in an accelerated heart rate, increased stroke volume and enhanced cardiac output. The supply of oxygen to the adductor muscle will be increased but must be insufficient, as blood$P_{{\text{O}}_{\text{2}} }$ decreases to 15 mm Hg and energy-producing anaerobic pathways are invoked (de Zwaan et al., 1980). There is an increase in the total carbon dioxide content of the blood and a fall in blood pH. When the scallop reopens, there is a transient increase in oxygen uptake.

• 3. The restoration of physiological functions to normal values takes several hours.

• 4. During valve closure, the$P_{{\text{O}}_{\text{2}} }$ of the mantle water decreases from 145 mm Hg to 80 mm Hg, blood$P_{{\text{O}}_{\text{2}} }$ from 85 mm Hg to 40 mm Hg. This is sufficient to maintain a degree of aerobic metabolism, and oxygen uptake continues. There is no change in heart rate, but blood pH decreases gradually.