Growth, longevity and breeding characteristics of populations of the musselXenostrobus securisin the Swan Estuary are remarkably variable, both between one year and another and between upstream and downstream localities. These characteristics and their variability may be accounted for by the species' physiological responses and limitations to variable salinity conditions. The Swan Estuary is subject to extreme temporal and spatial variations in salinity. Temperature has only a secondary significance on the biology of the mussel in this estuary.