New Zealand has a rich and varied shellfish fauna, although most of the species lack the brilliance of colouring and the polish of those from the tropical Pacific.
This country is but a remnant of a once continental land mass. Today over 1,200 miles of deep ocean separate it from Australia, the nearest existing continental mass. Conditions of more or less isolation from other lands have pertained during most of the Tertiary, that is, for a period of 40 million years or more. Fluctuations in climate have undoubtedly resulted not only in many extinctions but also in adaptations to changing conditions, without the stabilising influence, to any great extent, of the advent of new stock from outside areas. One looks in vain for the colourful cones and highly enamelled cowries of the tropical Pacific, yet both occurred here in the Oligocene and Miocene when the climate was considerably warmer than that of today. Nevertheless, despite these several adverse factors, New Zealand has developed a shellfish fauna almost unique in respect to the very high percentage of truly endemic species. Including land and freshwater forms, our shellfish fauna now numbers over 2,200 different species and subspecies belonging to 646 recognised genera.
Although relatively small in area, New Zealand is long and narrow, spreading over 13 degrees of latitude. This alone accounts for a wide variety of species, many with a restricted range determined by water temperatures. The fact that New Zealand straddles two great and distinctive water masses, the subtropical and the sub-Antarctic, makes for relatively sharp faunal distinctions between north and south. Other factors – ocean currents, both warm and cold, coupled with geographic and physical features – make it relatively easy to divide the shallow water faunas into five distinct marine faunal provinces. These, with their salient characteristics, are as follows:
Aupourian, Northland to East Cape and on the west coast, a division of some elasticity between Ahipara and Manukau Heads. It lies within the subtropical zone of surface waters and is even influenced by extropical warm currents, notably the East Australian Current which originates in the vicinity of New Caledonia, sweeps down the East Australian coast and thence in an upwardly deflected arc across the Tasman to New Zealand.
This current is responsible for occasional and even regular wanderings of certain larger marine animals to our shores. Instances are a number of species of fish, two species of sea-snakes, and occasional turtles. The East Australian Current also functions effectively in bringing to our shores, in their minute larval form, many species of invertebrates, particularly shellfish. The majority of these have become acclimatised and have thus introduced an element characteristic of warmer waters. Conversely, limited upwelling of cold water of sub-Antarctic origin on the Northland west coast brings in a small number of organisms characteristic of colder seas.
Cookian, southern part of the North Island and northern part of the South Island. This is a mixed zone of subtropical and sub-Antarctic waters and supports a mixed fauna from these two zones plus sufficient regionally distinct species to make the area recognisable as a marine province.
Forsterian, lower part of the South Island, Stewart Island, and the Snares Islands. This province is greatly influenced by sub-Antarctic waters and in particular by the active West Wind Drift, which sweeps the Southern Ocean. The province is made up of a high percentage of restricted species and many stragglers from the southern islands.
Moriorian, the Chatham Islands, which lie over 400 miles almost due east of Banks Peninsula. The fauna owing to long isolation, contains a high percentage of endemic species and is particularly noted for the absence of a number of species common on the mainland. In general, the fauna is most like that of the Cookian but there are northern and southern influences as well, the latter the more marked.
Antipodean, the southern islands of New Zealand, Antipodes, Bounty, Auckland, and Campbell Islands. These groups lie well within the zone of sub-Antarctic waters. The fauna is composed largely of the more temperature-tolerant mainland species and the endemic forms of these, with a strong admixture of restricted sub-Antarctic genera. It is an impoverished fauna suggesting many extinctions in the past, due to adverse conditions.
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.
Of special interest in the reconstruction of the past is the evidence afforded by our quite considerable fauna of native land snails particularly two genera, Paryphanta and Placostylus. Paryphantid snails have large, flatly coiled shells ranging from 2–3 ½ in. in diameter. The animal is carnivorous, feeding on earthworms and living in damp leaf mould on the forest floor, mostly at high altitudes. These snails belong to an ancient and primitive southern family that now ranges from South Africa to Melanesia. Their main distribution was probably achieved during the early Tertiary or even Cretaceous times. New Zealand has the largest number of species of this interesting group, and they could very well have originated in this part of the world. The fact that these snails produce large shelled eggs, resembling those of birds, and that these eggs will not survive dryness or immersion in water makes the distributional patterns of the Paryphanta species of special significance in the reconstruction of former geographical features and past land connections.
Similarly, the other genus of large New Zealand snails, Placostylus, is of equal significance in the reconstruction of the past. Placostylus snails are vegetarian, feeding on the dead leaves off coastal trees, mostly karaka. They grow up to 3 in. or more in length, and in shape are narrow and tall spired. They have been found in New Zealand only in Northland and outlying islands, never far from the sea and on the East Coast, not below Whangarei.
The special significance of Placostylus is that the genus is known elsewhere only from the islands of Melanesia, and northwards to the Solomons and eastwards as far as Fiji. This indicates some connection between New Zealand and the Melanesian area within comparatively recent times. This does not necessarily mean continuous land at any one time, but rather it suggests a series of give and take connections between island groups, and the gradual emergence of the present distributional pattern. The distribution of the kauri tree (Agathis) and of another plant (Xeronema), known only from New Caledonia and on islands of the Northland coast, lends support to this connection.
by Arthur William Baden Powell, Assistant Director, Auckland Institute and Museum.