The majority of barnacles are sublittoral and limited to full strength sea water, but a number of thoracican species are amongst the most conspicuous and successful members of the littoral and brackish-water faunas. Salinity fluctuations are a feature of most littoral and brackish habitats, but the importance of salinity as a stress factor is variable. In many open coast situations, intertidal barnacles are likely to be subjected to lowered salinity levels only when emersion coincides with heavy rainfall or pronounced terrestrial run off. For these animals, behavioural adaptations to prevent irrigation of the mantle cavity by dangerously dilute sea water may be sufficient protection against an irregular, transient factor in their existence, overshadowed in importance by temperature fluctuations and the desiccating influence of aerial exposure.