Differential physiological and behavioural responses of the intertidal mussels, Choromytilus meridionalis (Kr.) and Perna perna L., to exposure to hypoxia and air: a basis for spatial separation

Abstract

Two abundant South African mussels show intrashore habitat segregation and marked differences in their responses to physiological stress. Choromytilus meridionalis (Kr.) occurs mainly low on the shore and on rocks associated with sand, while Perna perna L. occurs higher on the shore on rocks which arc not usually influenced by sand. The sand tolerant C. meridionaus withstands prolonged exposure to hypoxia (< 1.00 ppm O 2: LT 50 > 30 days) and shows significantly better survivorship in air (LT 50≈ 7 days) than the sand intolerant species P. perna. with LT 50 values of ≈6.5 days and ≈ 1.5 days for hypoxia and air exposure, respectively. C. meridionalis appears to activate anaerobic pathways more readily than P. perna when exposed to air and hypoxia. In cither medium it shows a much stronger tendency to close the shell valves and a more rapid induction of reduced heart rate (bradycardia). Also, some individuals of P. perna showed an absence of oxygen debt when returned to seawatcr after exposure to air. With declining oxygen tension C. meridionalis regulates oxygen uptake down to a lower level (2 ppm O 2) than P. perna (3 ppm O 2). Mortality of P. perna in hypoxic seawatcr is suggested as being linked to a decline in cellular energy level, while in air it is more likely to he caused by an acid-base disturbance. The above behavioural and physiological responses can explain the exclusion of P. perna from sand-influenced rocks on the lower shore, where oxygen availability is likely to he reduced. However, the greater tolerance of air exposure by C. meridionalis docs not correlate with its exclusion from the higher regions of the shore where P. perna lives.