The salinity of the water retained within the mantle cavity of mussels after the shell valves have closed in response to falling environmental salinities is influenced by the rate of external salinity change. At high rates of salinity change the retained water salinity is significantly higher than in animals exposed to slowly changing salinities. However, the mantle fluid salinity is not primarily determined by the timing of shell valve adduction, but by closure of the exhalant siphon. Once this siphon has closed mantle cavity irrigation ceases, and a mussel can remain in a gaping, partially isolated state, for periods dependent upon the rate of external salinity change, without suffering significant dilution of the mantle fluid. Eventually the inhalant siphon and shell valves close, but during the period of partial isolation the oxygen tension of the mantle fluid falls more slowly than in circumstances when the sequence of siphon and valve closure is rapid. It is suggested that this behaviour may give mussels a significantly increased period of oxygen availability at estuarine distribution sites. © 1979, Marine Biological Association of the United Kingdom. All rights reserved.
