SEA FLOOR, ANIMALS OF
The sea floor may be conveniently divided into three regions: the Continental Shelf (to about 100 fm deep), the Continental Slope (from 100 to 2,000 fm), and the abyss or deep sea (from 2,000 to about 5,500 fm). While each region can thus be generally characterised, the boundaries between them are not abrupt, but consist of gradual changes in the physical conditions and in the animals present. In each region there are four main physical factors that control the distribution of the animals, namely, the temperature and salinity of the surrounding water, the nature of the sediments on the bottom, the depth, and the type of food available. These factors vary from place to place and so, consequently, does the population of animals. The variation of these factors is most marked in the shallow waters on the Continental Shelf and becomes progressively less as the water deepens. Hence animals in shallow water are more restricted to particular regions than those in the deep sea. Sea-floor animals have definite tolerances to physical conditions and definite food requirements, and their distribution is dependent to a great degree upon these factors.
Sea-floor animals may also be broadly classified according to the nature of their food and their method of feeding. First, there are those animals which are herbivorous, living on seaweeds or plant material. These animals are mainly confined to the shallow near-shore regions where the seaweeds live. Most of these animals are molluscs such as sea snails and periwinkles, chitons, limpets, and similar types, and include the common paua, Haliotis iris. Some deep-water echinoids (sea urchins) have been found feeding on plant debris washed out by rivers, but none is yet known from New Zealand waters.
Other benthic (bottom living) animals live attached to rocks and stones or in the sediments and get their food by filtering the tiny planktonic animals or plants out of the surrounding water. The sponges and sea mosses, the mussels, and the rock and mud oysters of Auckland and Foveaux Strait are typical of these filter feeders. Other filterers live in burrows in the mud or sand for protection and send tubes to the surface to suck in sea water to filter for food; the pipi and toheroa (Amphidesma ventricosum) are representatives of this group.
Another group includes the solitary corals, and related hydroids, the sea anemones, bryozoans (sea mosses), and crinoids or feather stars. Some of these are individual animals and some (the bryozoans) are colonies or multitudes of separate animals; the animal is firmly attached to rock or other firm foundation and feeding is accomplished by sweeping the surrounding waters with arms or tentacles. Still another group of animals comprises those that search the mud or sand where they live for small animals or remains. These forms include sea urchins, such as Echinocardium cordatum (the common heart urchin), sea cucumbers or holothurians, and brittle stars (Amphiura rosea is one widely distributed species), as well as many bivalve shellfish and worms. Some animals are carnivorous, moving over the sea bed preying upon others; the oyster borer, Lepsiella scobina, lives upon a diet of rock oyster; the olive shells (Baryspira), starfish, such as Coscinasterias calamaria, and octopuses all actively hunt for food.
Many other animals are scavengers, eating any dead animal remains that they find. Most of the crabs fall in this category, along with hermit crabs, some shrimps and prawns, and other crustaceans, such as amphipods and isopods.
Those animals growing up from the sea floor or moving over it are known collectively as the epifauna, while those living buried beneath the surface can be grouped together as the infauna.
The majority of the examples given are from those commonly known on the Continental Shelf. In contrast, the fauna of the deeper seas is not well known, and collections from the New Zealand Continental Slope and abyss are as yet very limited. Some of the animals occurring in these regions also occur on the Shelf, but many are restricted to the deeper sea. Generally, animal life is somewhat less abundant in the deep sea than in the shallow marginal Shelf. In the deeper sea below the levels at which light can penetrate and plants can grow, herbivorous animals are, naturally, almost completely lacking; hence the animals feed chiefly on microscopic plank-tonic life, on organisms living in the sediments, or are carnivores and scavengers. In some groups there is a tendency for the animals to grow long legs to enable them to move more rapidly over the soft sediments and so obtain more food.
Animals from the very deep sea have not been extensively collected in the New Zealand region. The conditions in this region are often very similar throughout the world, and many deep-sea animals are known to occur throughout the Pacific Ocean. The Danish research ship Galathea collected some of our abyssal animals in the very deep Kermadec Trench to the north-east of New Zealand, many of the animals being new to science. In general, the deep-sea animals are not of peculiar types, but fall into categories similar to those of the Continental Slope. They are, however, somewhat more sparsely distributed.
by Donald George McKnight, New Zealand Oceanographic Institute, Department of Scientific and Industrial Research, Wellington.
References to New Zealand sea-floor animals, although numerous, are very scattered and some difficult to obtain. Two books by A. W. B. Powell, Native Animals of New Zealand and Shells of New Zealand, are very handy for the beginner. From here, one may refer to many useful papers in the Transactions of the Royal Society of New Zealand and to work published by the various New Zealand institutions working in the marine sciences.