24 THE BIOLOGY OF MARINE ANIMALS 



siderable swelling takes place and normal activity returns only after some 

 hours (8, 35). 



Animals are usually dead when brought to the surface from great 

 depths. In this process various factors besides pressure changes are also 

 operative, such as mechanical disturbance and temperature alterations, 

 and these may be lethal in themselves, quite apart from changes in pres- 

 sure per se. 



The effect of these pressure changes on the functioning of tissues is 

 still imperfectly understood. It has been shown that the compressibility of 

 muscle (rabbit) is approximately 88 % of that of pure water, and at 500 

 atmospheres the volume of the muscle is decreased by almost 2%. Some 

 interesting data are available about the physiological effect of pressure on 

 various tissues and extracts, but all derived from shallow-water or ter- 

 restrial animals. Pressure inhibits gelation of protoplasm, solation results, 

 and the cell loses its power of contraction. Disappearance of movement 

 and contraction following rise of pressure has been observed in Amoeba, 

 dividing egg cells (Arbacia) and chromatophores (Fundulus). Effective 

 pressures for producing these effects lie between about 350-550 atmo- 

 spheres. Pressure also reduces the beat of the embryonic fish heart 

 {Fundulus), affects ciliary movement (Mytilus), abolishes muscle contrac- 

 tion (striated muscle, frog) and blocks nerve conduction (frog), the effec- 

 tive absolute pressures varying with the tissues under investigation 

 (8, 32, 33). 



Moderate hydrostatic pressures (300-500 atmospheres) are known to 

 affect many biochemical processes. On luminous bacteria the effect of 

 pressure is to alter light intensity at temperatures departing from the 

 optimum. When highly purified preparations of luciferin and luciferase 

 (luminescent substrate and activating enzyme) are tested, it is found that 

 pressure reversibly increases light intensity over a wide temperature range 

 once the reaction is under way. Effects of this kind are explained on the 

 grounds that some oxidative enzyme reactions proceed with a large volume 

 increase, which is opposed by pressure and results in change of reaction 

 rate and displacement of equilibrium. Each reaction displays its own 

 peculiarities and must be interpreted separately. For example, the positive 

 pressure effect shown by the luminescent reaction of ostracod extracts is 

 due to a shift in equilibrium of a non-luminous oxidation of substrate, 

 which proceeds with volume increase and is opposed by pressure. As a 

 result, the concentration of luciferin substrate available for the lumi- 

 nescent oxidation reaction is increased (6, 24). 



It is generally believed that the most significant effect of pressure on 

 biological systems lies in the volume changes which it brings about. At 

 high pressures protein molecules are compressed, denatured and altered 

 in structure and chemical activity. In the ocean depths two factors, low 

 temperatures and high pressures, both of which affect the rate of bio- 

 logical processes, are acting concomitantly, and the animals of the abyss 

 must be genotypically modified to withstand the conditions obtaining there. 



