of which the mean content of water in the tissues of deep-water fishes 

 is 2.5 times higher than that of shallow-water species (Walters, 1961). 

 The swim bladder loses its main purpose, that of maintaining neutral 

 buoyancy with changing pressure, and either disappears completely or is 

 reduced and filled with fat. The tissues of the bladderless bathypelagic 

 fishes are 2-4 times poorer in protein than those of the shallow-water 

 species (Denton, Marshall, 1958). The decrease in basic metabolism also 

 results from a reduction in the sensory organs. 



The reduction in musculature leads to simplification of the circula- 

 tory and excretory systems. In deep-water pelagic fishes, the surface of 

 the gill leaflets is greatly reduced, decreasing the respiration rate. 

 The low rate of respiration and secondarily simplified excretory system 

 indicate a low level of metabolism and a corresponding decrease in energy 

 expenditure. And, in fact, measurements of the metabolic rate of certain 

 deep-water fishes at the depth where they live have shown that it is two 

 orders of magnitude lower than that of shallow-water fishes of the same 

 size (Smith, Hessler, 1974). 



The weak development of musculature and the systems of organs which 

 serve it and the low rate of metabolism are possible only with extremely 

 limited mobility of the animals. Deep-water animals are almost all 

 predaceous, but cannot be predators which actively seek out and chase their 

 prey, but rather only passively attracting predators. This has been 

 confirmed by direct observations. For example, J.-M, Peres, observing the 

 behavior of C hauliodus sloanei from the bathyscaphe, writes, "These fish 

 float motionless in the water, the longitudinal axis of the body inclined 

 at an acute angle to the horizontal, so that the head is higher than the 

 tail. The long second ray of the dorsal fin, equipped with a luminous 

 organ, is extended forward above the head, so that the tip of the ray is 

 in front of the mouth." C yclothone and Gonostoma are "always immobile, 

 floating passively; they wait for their prey, rather than following it; I 

 saw no motion of the fins, with which they maintain their equilibrium" 

 (cited by Marshall, 1960, p. 110, 107). 



In many deep-water Decapoda, the carapax are much thinner than in 

 the animals living higher up; let us compare, for example, Hymenodora 

 frontal is , with its comparatively hard external skeletons, and the deeper- 

 water species H_. g lacial is , the external skeleton of which is quite fragile, 

 or the strong Systellaspis and O plophorus with the \/ery fragile Notostomus , 

 etc. In the deep-water cephalopod mollusks ( Vampyroteuthis , V itreledonel la , 

 Amphitretus , Japetella , Eledonel la , etc.) have lost most of their supporting 

 and muscular tissue. A thick layer of jelly-like subcuticular tissue has 

 developed, while the muscles have partially degenerated and are also 

 replaced by jelly-like tissue, so that the consistency of these cephalopods 

 is similar to that of medusae. Thus, even the large inhabitants of the 

 depths are passive planktonic animals, not active nektonic animals. 



Passive waiting, rather than active hunting, for food is characteristic 

 of deep-water invertebrates as for deep-water fishes. This is indicated, 

 in particular, by the (sometimes greatly) elongated extremities, allowing 



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