INTRODUCTORY 23 



The biological effects of pressure are certain to be complex. The volume 

 of water changes only slightly under compression: this amounts to about 

 0-46% at 1,000 m, 3-30% at 8,000 m, and 5-01% at 10,000 m (0 C C). At 

 any given depth the pressures inside and outside the animal will be the 

 same, and even if the animal changes its level, mechanical changes due to 

 alterations in volume will be very small and will be equally distributed so 

 long as air is absent. It is conjectural whether pressure limits the vertical 

 range of motile species, since other factors may intervene before they 

 exceed the pressure change which they can tolerate. Some species carry 

 out vertical migrations of up to 400 metres (equivalent to 40 atmospheres), 

 and can be designated eurybathic, or tolerant of pressure change. There 

 are other benthic species which are known to have extraordinary vertical 

 ranges, e.g. Henricia sanguinolenta, an asteroid, has been reported from 

 inshore waters to depths of some 2,500 metres, equivalent to a pressure 

 range of 0-250 atmospheres. Conditions in vertebrates are somewhat 

 different, since many pelagic teleosts have an air-filled hydrostatic organ, 

 and cetaceans dive with inflated lungs. Teleosts are often brought to the 

 surface with inflated sounds as the result of the sudden pressure change, 

 but this gives no indication of their powers of accommodation to pressure 

 alterations which take place more gradually. The reader is referred 

 to p. 401 for a discussion of the physiology of the teleostean air- 

 bladder, and to p. 172 for a discussion of problems of respiration in 

 cetaceans. 



The ability of marine organisms to survive great changes in pressure 

 has been investigated experimentally to a limited extent. Particularly 

 interesting data have been obtained for bacteria, which may be expected 

 to throw some light on the effect of pressure changes on biochemical pro- 

 cesses. Marine bacteria are abundant in nearly all bottom deposits, and 

 Zobell (45) has obtained samples from the Philippine trench at depths 

 exceeding 10,000 metres. When cultured, many of the bacteria from these 

 abyssal depths survived at a temperature of 30C and at atmospheric 

 pressure. Significantly larger bacterial counts, however, were obtained in 

 cultures incubated at 2-5 C and under pressures of 1,000 atmospheres, 

 than at 30 C C and at atmospheric pressure (46). 



Some experiments have been carried out on the ability of marine 

 animals to withstand great changes in pressure. Beebe attached a lobster 

 (Panulirus argusl) to his bathysphere during one of his descents, and 

 recorded that the animal survived a dive to 671 metres (pressure of about 

 67 atmospheres). The effect of hydrostatic pressures up to 1,000 atmo- 

 spheres has been tested by Regnard (35) on a wide variety of organisms. 

 Experiments of this sort have shown that it is always possible to kill 

 shallow-water animals by exposure to a sufficiently high pressure, some- 

 where between 400 and 1,000 atmospheres. Molluscs (Cardium), annelids 

 (Nereis), crustaceans (Eupagurus) and tunicates are inactivated by an 

 exposure of one hour to 400-600 atmospheres. Some other forms are more 

 resistant and survive an hour's exposure to 1,000 atmospheres, but con- 



