(a) Hauraki Gulf
A barnacle zone is ubiquitous in situations from moderate shelter to extreme exposure in the upper midlittoral, except on movable boulder substrates, where colonisation appears to depend on the degree of stability of the boulders and the intensity with which they are insolated. Subordinate species are listed in the individual accounts of the writers already mentioned (cf. Table II). Suffice it to say here that a number of communities have been recognised within this zone,
according to the prevailing dominants in a localised area. This present survey amply illustrates the replacement of C. columna by E. modestus in more brackish, silted up areas, and by C. brunnea with a change to oceanic conditions. All stages of gradation may be observed between these extremes. Where C. brunnea and C. columna occur together, the former occupies a belt above the latter (Stations 9, 15, 18–22, 28–30, 32–34).
Within the wide vertical range of dominance of C. columna its place may be taken by any one of the co-dominants. According to each writer's preference these may be regarded as belonging either to the barnacle community or as dominants of a number of separate ecological groups.