Species that are exposed at low water to air for long periods have a breathing hole, micropyle, in the closed tergoscutal flaps. If a micropyle is observed, the species is one of the following but, if no micropyle is seen, they should not be discounted as it may be held shut: (from left to right in above images) Chthamalus stellatus, Chthamalus montagui, Austrominius modestus, and Semibalanus balanoides.

Full INTRODUCTION TEXT at 01 Barnacle Introduction
Albums with full accounts of all British acorn barnacle species at marinvert.senckenberg.science/image-browse/7215763361…