MARINE BOTTOM COMMUNITIES

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Abstract
SUMMARY: The more important contributions to the literature of animal communities on the sea bottom are summarized. It is concluded that while there has been much disagreement about the constitution of the communities, the chief criticism has been directed against the view that they are units bound together by biological factors. There is little evidence for this idea. However, most workers agree that communities exist and that there is a correlation between their distribution and that of certain physical factors. Therefore any system of classification should be based on the external conditions. The factors affecting the distribution of communities are discussed. The significant factors are probably temperature, salinity, and the nature of the bottom deposit. A scheme of classification applying to the communities of the littoral system is suggested, and an attempt is made to define the range of physical conditions within which each exists. Only the Atlantic boreal region is sufficiently well investigated for the classification to be applied in detail. The communities are described as shallow when the animals composing them are eurythermal and euryhaline within wide limits, offshore when they are eurythermal and euryhaline within narrower limits, and deep when they are stenothermal and stenohaline. Each group is subdivided according to the type of bottom on which the communities occur. There is more or less intergrading between neighbouring communities, both vertically and horizontally. The more important species occurring in each community are listed and the probable place in the classification of the communities previously described is shown. The reasons for the existence of animal communities on the sea bottom are discussed. The morphology and mode of life of certain species seem to be important in restricting them to certain grades of deposit, and while analysis of feeding methods does not at present indicate that the latter are of paramount importance in this respect, it is possible that more detailed studies on individual species will prove them to be so. The quantitative investigations into the fauna of the sea bottom are discussed, and it is concluded that there is little probability of valuable results arising from evaluations at the present time. On the other hand, much useful information may be expected from further studies of the differences in production apparent on different deposits and between different areas, and of the changes in numbers that take place from time to time among the population of a single area.

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