Two pairs of moveable plates cover the operculum which can be sealed by a purplish-brown flap when the barnacle is exposed above water or it is not feeding.

Its most northerly extent is in southern areas of England and Wales,[3] and at this limit of its range it can be greatly affected by severe winters.

[5] It is found on rocks and man-made structures from mid-shore to the neritic zone and also forms part of the fouling community on the hulls of ships.

[8] In the habitats occupied by P. perforatus it is often associated with sponges and encrusting red seaweeds on shady overhanging rocks and cave entrances and also bryozoans and ascidians in deeper shade.

Another isopod crustacean, Naesa bidentata, normally lives in rock crevices and under seaweed and stones but with the spread of P. perforatus, it has adopted the empty shells of the barnacle as its home.