Apart from these broad faunal divisions, our marine shallow-water fauna can be readily grouped ecologically into distinctive zones relevant to tide level and in association with different kinds and combinations of substrate materials, such as sand, mud, soft or hard rock, gravel, shingle, and rock pools, further modified by water salinity and the degree of exposure to wave action.

Three papers serve to introduce these topics. One, Marine Littoral Plant and Animal Communities in New Zealand (Oliver, 1926) attempts to classify the main associations of marine plants and animals in New Zealand. Many of the more striking ones are named; for example, the Corallina-Hormosira Association, which is a common mid-tidal community developed on clean to silty rock platforms in fairly sheltered situations. It is characterised by a low, stiff, limy seaweed, Corallina officinalis and the grape-like seaweed, Hormosira banksii. The most abundant mollusc of this association is the cat's-eye periwinkle, Lunella smaragda.

In deeper water below the strong influence of tides, different animal communities occur and for these the reader is referred to Animal Communities of the Sea Bottom in Auckland and Manukau Harbours (Powell, 1937) and a later paper, Some Animal Communities of the Sea Bottom from Queen Charlotte Sound, New Zealand (Dell, 1951).

In the Auckland Harbour and vicinity four main communities are recognised: (1) Echinocardium formation; (2) Maoricolpus formation; (3) Tawera and Glycymeris formation; and (4) Arachnoides formation. The dominant animals in (1) are the heart urchin, Echinocardium australe, the bivalve Dosinia lambata, and the brittle star Amphiura rosea which develops only in soft mud where the salinity of the water does not fall below 34 per mille. This means that it is absent under estuarine conditions. The dominant animal for (2) is the mollusc Maoricolpus roseus which in Auckland waters is thickly strewn over the current-swept shelly bottom within the harbour. Outside the harbour limits, on clean-swept, shelly bottom in the channels, (3) occurs, the dominant animals being the molluscs Tawera spissa, Glycymeris laticostata, Perna canaliculus, and the crab Petrolisthes elongatus. This is the richest fauna of the four main communities both for number of species and for number of individuals. (4) develops on a substratum that is 95 per cent fine sand with practically no silt, and is found in sheltered areas of relatively high salinity, usually just within a harbour entrance. The dominants are the cake-urchin Arachnoides zelandiae and the mollusc Zethalia zelandica. These four communities have since been found in other parts of New Zealand, and several are clearly recognisable in the Pleistocene and Tertiary beds of the Wanganui district.