The upper reaches of Milford Haven and the Daucleddau have an unusually rich fauna because of their depth and the abundance of hard substrata. The estuary is of commercial importance both in its potential for shell‐fisheries and its development as a major oil port, bringing risks of serious pollution. Recent extensive sublittoral studies have added numerous faunistic records and have also cast further light upon changes in the zonation of sedentary marine animals as they penetrate brackish regions. Re‐survey of a transit across the channel near Lawrenny has shown changes largely related to the spread of a vigorous immigrant competitor of the native oys