5. Communities of Abyssal Macrofauna . (Z. A. Filatova) 



The benthic abyssal communities have not been thoroughly studied. 

 There is a shortage of data on their species composition, geographic 

 area and conditions of habitation. Very little is currently known about 

 the physical and chemical conditions on the bottom and near the bottom, 

 the force and direction of movement of the water above the surface of 

 the bottom. These data, where available, are scattered and largely 

 random. In order to explain the peculiarities of distribution and 

 quantitative predominance of various species of macrofauna on the ocean 

 floor, it is not sufficient to know the depth of the location, the 

 nature and distribution of bottom sediments and the macroscopic bottom 

 relief. We also need information on many other important factors 

 determining the conditions of life in the bottom layer of water and on 

 the bottom itself, as well as how they change with time and space. 

 Nevertheless, using the data which have been accumulated, we can attempt 

 to imagine the composition and structure of abyssal benthic macrofauna 

 biocenoses of the ocean, on the example of the best studied eutrophic 

 zone, that of the northern Pacific. As a first step, let us use the 

 available, though incomplete, data on the specific composition and 

 quantitative nature of the biocenoses, and their distribution over the 

 sea floor. 



Deep-water macrobenthic fauna is arbitrarily subdivided into three 

 dimensional groups. The meiofauna (meiobenthos) measures 0.04-1 mm 

 (Thiel, 1966, 1975) or 0.5-5 m (Sokolova, 1970), and is the most 

 massive group of animals. The macrofauna (measuring 5-10 mm) and 

 megafauna (>1 cm) are the remaining dimensional groups. The nature of 

 the distribution of these groups of animals on the sea floor is related 

 to their dimensions: The smaller the organisms, the more evenly they 

 are distributed on the bottom. Such groups as the small Xenophyophoria, 

 Tanaidacea, Nematoda or the agglutinating Rhizopoda frequently form a 

 sort of continuous "carpet" in a number of regions of the eutrophic zone 

 of the ocean. Therefore, the meiofauna and smaller macrofauna 

 (primarily the megabenthos juveniles) are most completely accounted for 

 by the bottom digger. The larger the organisms, the less evenly they 

 are distributed on the bottom and the more frequently they form various 

 aggregations, "spots" or accumulations. This is a result both of the 

 small-scale peculiarities of distribution of conditions of habitation, 

 and of the nature of breeding and behavior of the larvae and fry. 



It is characteristic for deep-water benthic fauna that the species 

 usually have limited geographic distribution, whereas many genera are 

 almost cosmopolitan (Hessler, 1974). This is quite important for the 

 study of the abyssal biocenoses. Studies of the abyssal biocenoses will 

 yield the greatest results where they are most completely developed, 

 i.e., in the eutrophic abyssal zones, e.g., in the North Pacific. 



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