Dense populations of very young Modiolus modiolus do occasionally occur subtidally in estuaries, although the species is more poorly adapted to fluctuating salinity than other mussel species (Bayne 1976). Dense populations of adults are not found in low salinity conditions. Pierce (1970) established tolerance limits of 27 – 41 ‰ for Modiolus modiolus based on ventilation behaviour and byssus formation.

Coleman (1973) demonstrated that Modiolus modiolus exposed to air has an erratic heart rate, suggesting a lack of physiological adaptation to ariel exposure, and that it loses water rapidly due to an apparent inability to control its gape effectively. Dense populations therefore seem restricted to around 5 – 50 m in British waters, although bioherms have been recorded in over 80m in Nova Scotia (Wildish & Fader, in press). Lack of mobility, thin shell and restricted tolerance to changes in temperature and salinity has been suggested as reasons for the poor ability of Modiolus to colonise the intertidal (Davenport & Kjorsvik 1982).