(Wolff, 1956; Birshtein, 1963; Belyaev, 1966). However, V. B. Tseytlin 

 (1975) has shown that the primary factor determining the increase in 

 dimensions of zoophagous forms with depth is the decrease in the quantity 

 of food resources available. The drop in temperature also plays some 

 role. Based on food consumption and its variation with temperature, 

 V. B. Tseytlin has developed a method for estimating changes in the 

 dimensions of similar species of carnivorous animals with increasing 

 depth and decreasing food concentration and temperature. The results 

 produced correspond well to those observed in nature. 



In nonpredatory groups, changes in dimensions are more difficult to 

 follow, but it has been noted that in the abyssopelagic zone, smaller 

 forms predominate than in the higher layers. This is probably related to 

 the fact that the fine filter feeders, which satisfy their demands with 

 smaller quantities of foods, have an advantage there. The decrease in 

 the mean dimensions of filter feeders does not result from a decrease in 

 the size of species in the genus--their dimensions may even increase, as 

 in the case of the predators--but rather due to predominance of genera 

 with smaller species. 



The principle of economy of energy expenditures leads to radical 

 restructuring of the entire organism (Denton, Marshall, 1958; N. B. Marshall 

 1960; Walters, 1951; Vinogradov, 1968; etc.). The basic adaptations move 

 in the direction of creation of neutral buoyancy, so that the animal can 

 remain in the suspended state without expending muscular effort, thereby 

 decreasing the rate of active metabolism. 



The comparatively good food supply for fishes found in the surface 

 zone allows them to lead an active mode of life. Preservation of buoyancy, 

 which is near neutral, with extreme changes in depth of habitation (and, 

 consequently, hydrostatic pressure) and temperature of the environment is 

 possible only by active, rapid changes in the volume of the swim bladder. 

 This requires well developed musculature, a strong skeleton for the 

 attachment of this musculature and a system for rapid liberation of oxygen 

 from the blood into the swim bladder and for its rapid absorption by the 

 blood. All of this requires significant energy expenditures. Active 

 hunting is possible only with high mobility, while developed sense 

 organs (particularly vision), a complex brain structure, strong musculature, 

 and, consequently, intensive metabolism. All of this draws a basic distinc- 

 tion between the nekton inhabiting the rich epipelagic and mesopelagic 

 layers from the true deep-water residents. 



In fish which do not rise into the surface zone and live at depths 

 of over 500-1000 m, as well as most of the deep-water pelagic invertebrates, 

 neutral buoyancy is achieved by means not requiring additional expenditures 

 of energy. A decrease in specific weight is achieved primarily by 

 reduction of the content of heavy tissues. The bony skeleton is replaced 

 by a cartilaginous skeleton, and muscle fibers are weakened. The sub- 

 cutaneous layers and spaces between muscle fiber bundles are filled with 

 gelatinous mesenchyma, and a portion of the subcutaneous cavities--with 

 fat. Most of the soft tissues are gelatinous in consistency, as a result 



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