THE PHYSICS AND BIOLOGY OF VERTICAL MIGRATIONS 



93 



Table 6. Resorbent surface-area : swimbladder volume ratios in various 



bathy pelagic fishes 



These ratios are higher than that found in the perch (using Saupe's (1939) figure for the area of the 

 oval). The two myctophids, the melamphaid and Maurolicus have ratios that are at least twice as 

 large as that of the perch. In Vinciguerria and Astronesthes this factor is 4 and 5 respectively. 

 ' Clearly, the concentration gradient in these fishes will be much higher than that ever existing in the 

 perch. At a depth of 500 m. the partial pressure of oxygen in the swimbladder will be some 40 

 atmospheres, a tension several hundred times greater than that in the blood flowing through the 

 capillary bed. This would indicate that the rate of oxygen removal would be limited by the rate at 

 which blood could flow through the resorbent area. 



Now, in myctophids the vein or veins leading from the oval to the cardinal vein are extremely 

 large. Furthermore, the by-pass vessel (or vessels) from the retial artery to the oval can take all the 

 blood that would flow through the retia during gas-secretion. Text-fig. 39 shows two large veins 

 from the oval of Myctophum punctatum, and they unite to form a single vessel running to the cardinal 

 vein. In the stomiatoids the resorbent surface is also fed with blood through a by-pass of the retial 

 artery while the venous return is through the venous capillaries of the rete (see pp. 78-79). 

 However, the retial flow will not be a limiting factor, which may be seen by using Poiseuilles' 

 law. For a series of n parallel tubes the resistance to the flow of blood will be proportional to : 



(Length of a tube) 



(Number of tubes) > (Diameter of tube) 4 



Using the figures for the diameter and length of the venous capillaries, and for the by-pass artery 

 and other vessels involved in the capillary bed, it can be shown that the retial capillaries have less 

 resistance than that developed in the larger vessels. It would thus seem that the resorbent vascular 

 system in both stomiatoids and myctophids will carry relatively large amounts of blood over a 

 given time. 



Temperature will also play a part, though a small one, in accelerating the diffusion of gas into the 

 capillary bed. In the subtropical and tropical ocean, a fish migrating from a depth of 500 m. to the 

 surface will pass from temperatures of about 8-1 8° C. to those ranging from 20 to 30 C. Thus over 

 wide areas of the warm ocean the rise in temperature is io° C. or more. However, in terms of absolute 

 temperature the percentage increase in diffusion rate in passing from io° to 25 C. is less than 

 10 per cent. 1 



1 Taking the rate at 20° C. as unity, the value of the diffusion constant rises 1 per cent per degree (Prosser et al. 1950). 



