GRAVITY, PRESSURE, AND SOUND 



139 



Most fishes maintain an internal density 

 about equal to that of the surrounding 

 water. For those with air bladders, this is 

 done by appropriate exchange of gases 

 between the swim bladder and the blood. 

 When such a fish descends to a deeper posi- 

 tion in the water, the increased hydrostatic 

 pressure compresses tlie gases in the blad- 

 der to a point at which the swim bladder 

 no longer helps support the fish. Under 

 these conditions, fishes adapt themselves by 

 putting more gases into the bladder. As 

 fishes rise in the water and pressure is re- 

 leased, the bladder is overbuoyant, and 

 some of the gases are absorbed. The mech- 

 anisms whereby these changes are brought 

 about have not yet been demonstrated (von 

 Ledebur, 1937; Brown, 1939). 



The present summary indicates clearly 

 that the ecology of the deep sea is not yet 

 understood. There is a need for precise 

 observations and experimental studies, par- 

 ticularly of the effects produced by con- 

 tinued exposure to different pressures with- 

 in the ecological range. It is clear that we 

 cannot understand the ecological complex in 

 the depths of the ocean on the basis fur- 

 nished by our more extensive knowledge 

 of relationships at the surface even when 

 reenforced by principles derived from 

 physical, chemical, and physiological re- 

 search on the physiology of high pressures. 

 We do know that hydrostatic pressures 

 within the ecological range may aflect 

 such basic matters as the velocity of chem- 

 ical reactions, the viscosity of certain fluids, 

 the imbibition of water, and the physio- 

 logical activity of some bacteria and bac- 

 teriophages. Pressures greater than the eco- 

 logical range bring about irreversible 

 changes in proteins; they inactivate most 

 enzyme systems and strongly affect the bac- 

 terial toxins and the viruses. 



The simpler forms of life— bacteria and 

 Protozoa, for example— are more sensitive 

 to pressure than are nonliving systems, and 

 the sensitivity increases in general with in- 

 creasing complexity of organization. Most 

 aquatic invertebrates are less sensitive to 

 pressure than are fishes, and fishes lacking 

 an air bladder are much less sensitive than 

 are birds and mammals. The latter relation- 

 ship may be stated more generally as fol- 

 lows: Animals are much more resistant to 

 marked changes in environmental pressure 

 in the absence of free air or gas within 

 the body. 



Many instances have been recorded in 

 wliich small increases in pressure are stim- 

 ulating; and although apparent exceptions 

 occur, this, too, may prove to be a general 

 condition. Greater pressure is uniformly de- 

 pressing and becomes lethal if sufficiently 

 increased. The changes produced are revers- 

 ible in the lower ranges, and high pressures 

 are less hkely to be harmful if compression 

 is relatively slow anxl particularly if de- 

 compression is gradual. 



Eurybathic animals exist that have a 

 wide vertical range; Anthozoa (Coelen- 

 terata) furnishes examples. Many plankton 

 and nekton organisms move vertically 

 through great pressure changes in the daily 

 routine of their existence; malacostracan 

 crustaceans, for example, make diurnal mi- 

 grations of 200 and possibly of 600 meters 

 (Waterman, Nunnemacher, Chace, and 

 Clarke, 1939). Other animals are restricted 

 in vertical range; that is, they are steno- 

 bathic. Ail-breathing animals or fishes with 

 air bladders are surface, stenobathic forms, 

 and fishes of the Macrurus type are steno- 

 bathic animals of the ocean depths. 



SOUND, SUBSTRATAL VIBRATIONS, 

 AND MECHANICAL SHOCK 



Sounds are produced and conveyed by 

 mechanical vibrations. Although they may 

 be caried through fluid or solid media, 

 sounds of ecological importance are best 

 known as vibrations transmitted through 

 air. They vary primarily in pitch and in- 

 tensity. Those of low pitch, which result 

 from vibrations of low frequency, grade 

 into vibrations that are detected by touch 

 rather than by an auditory organ. At certain 

 relatively low frequencies, both methods of 

 detection may be used. Mechanical vibra- 

 tions with too low frequencies to produce 

 physiological sound may be carried as sub- 

 stratal vibrations, and these are reacted to 

 by a variety of animals. When such vibra- 

 tions are sudden and intense, they produce 

 mechanical shock, a stimulus to which a 

 wide range of animals react. These three 

 physical phenomena— sound, substratal vi- 

 brations, and mechanical shock— are closely 

 and inextricably interconnected. 



All three types of vibrations are produced 

 by nonliving forces in nature. Waves lap 

 gently on the beach or crash heavily in 

 storms. Winds whistle through rock crev- 

 ices. There is the sharp crash and roll of 



