The physiological ecology of Mytilus californianus Conrad : 2. Adaptations to low oxygen tension and air exposure.

Mytilus californianus regulated its rate of oxygen consumption (VO2) during decline in oxygen tension, but did not acclimate VO2 when held at 58 mm Hg PO2. In spite of a capacity to consume oxygen while exposed to air (the average VO2 in air being equivalent to 0.74xthe standard rate of oxygen consumption in water), these mussels acquired an "oxygen debt" during aerial exposure which was discharged on subsequent reimmersion. During exposure to air the oxygen tension of the fluid in the mantle cavity was rapdly reduced to approximately 40 mm Hg, in animals both in the laboratory and on the shore. Heart rate was also reduced during air exposure, though not to the point of cardiac arrest. The concentration of ammonia in the fluid of the mantle cavity increased during aerial exposure, but the rate of excretion of ammonia was much lower than during immersion. Observations of mussels on the shore at low tide indicated that dehydration of the tissues was not a serious threat, possibly due to the large volume of fluid retained in the mantle cavity. During aerial exposure some end-products of anaerobic metabolism (alanine and malate) accumulated in the posterior adductor muscle. Malate accumulation was rapid during the first hour of exposure; alanine accumulated more gradually. It is concluded that during aerial exposure M. californianus resorts to anaerobiosis in spite of a capacity to extract some oxygen from the atmosphere. This results in a metabolic deficit during each period of low tide which, coupled with the reduced time available for feeding, imposes a physiological stress on mussels distributed on the shore.