In a survey of the British species of Gammarus inhabiting estuarine waters (Spooner, 1947), it was found necessary to divide G. zaddachi Sexton into two well-marked morphological forms, with different salinity optima, which in fact behave as distinct species. They were, however, on account of their similarity in important structural features, retained as a single species and named G. z. zaddachi and G. z. salinus respectively. At the time little could be said about a third subspecies, essentially marine in its distribution (but ranging into brackish water), which Segerstråle (1947) independently described as G. z. oceanicus.