G. caespitosa is an osmoconformer surviving within the range of 100-60% seawater. However, at lower salinities, the worms are able to maintain high body-fluid osmotic concentrations, for a limited period, by behavioural osmoregulation. Thus, G. caespitosa is largely confined to the open coast of its geographic distribution, while also being able to survive estuarine conditions where periodic flooding is of limited duration and extent. Populations of G. caespitosa can survive greater excursions into estuarine conditions, provided suitable substrates are available. Here, however, they can be subjected to high mortality from time to time, and reconstitution of these populations must take place by subsequent settling of larvae from more permanent populations.