structure and systematics 63 



Mechanical properties of the swimbladder wall 

 Apart from Alexander's (1959a, b, c) studies of the swimbladder wall of cyprinid and other fishes, 

 little is known of the physical properties of this tissue-complex. In cyprinids the posterior chamber 

 of the swimbladder is remarkably inextensible, a property that is related to the high collagen content 

 of the walls: the anterior chamber is more extensible (Alexander, 19596). The swimbladders of pike 

 and trout are much more extensible and much weaker than cyprinoid swimbladders (Alexander, 

 1959^). Judged by their fine structure, this is also true of the swimbladders of bathypelagic fishes in 

 comparison with those of cyprinids. 



Some preliminary notion of the physical properties of the swimbladder wall can be deduced from 

 the observations of Scholander, Claff, Teng and Walters (1951) and Jones (1952). These workers 

 considered how far a closed swimbladder might restrict rapid vertical movement in a fish above a 

 certain level — the level at which it was in hydrostatic equilibrium. Scholander and his colleagues 

 suggested that a fish would not move upwards beyond a level involving more than a 25 per cent 

 change in the volume of its swimbladder, which is in good agreement with Jones's figure of 22 per cent. 



An increase of 25 per cent in the volume of a swimbladder will not lead to more than a moderate 

 extension of the walls. As this figure may well apply to deep-water pelagic fishes, we may briefly 

 consider what would happen to the ellipsoidal swimbladder of a myctophid. (A fish with a standard 

 length of 70 mm. will have a swimbladder with a volume of about 0-25 ml.) An increase of 25 per 

 cent in the volume will be accompanied by rather less than 10 per cent increase in the lengths of the 

 major and minor axes, assuming the walls are uniformly elastic and the sac is free to expand. 



There is a further consideration : when fully expanded, the oval is by far the thinnest part of the 

 swimbladder of a myctophid, and during the hauling of a fish to the surface, the oval may be blown 

 out in the form of a blister. (This condition was found in several specimens of Myctophum pimctatum 

 and Diaphus rafinesquei (see Text-fig. 15^/). Taking an oval with the walls distorted in the form of a 

 hemisphere, the surface area will be increased by a factor of two. If this amount of expansion can 

 be sustained by the flimsiest part of the swimbladder, it is unlikely that the limitation of rapid upward 

 movements is set by the range of extensibility of the walls. (In Argyropelecus also it is clear that the 

 swimbladder can sustain more than a 10 per cent increase in the lengths of the major and minor axes.) 

 The restriction is more likely to be related to the fishes' incapacity to cope with more than a certain 

 decrease in its density, there being a limit to what can be done by compensatory movements of the 

 fins and body. 



Whatever may be the limits of extensibility, it is rare to find a bathypelagic fish with a burst swim- 

 bladder or ruptured body-wall. While it may be easy to miss a small hole in the swimbladder of 

 a preserved fish, distension of the organ (so that the viscera are forced out of the mouth or anus) is 

 also unusual, except perhaps in hatchet fishes (see also p. 95). 



In a letter Dr F. R. Harden Jones has told me that perch (Perca fluviatilis) with burst swimbladders 

 do not appear to have ruptured body-walls, although in some instances the pressure may be sufficient 

 to evert the rectum. Out of a sample of 106 cod taken by trawl at 20 fathoms, he found 45 per cent 

 with broken swimbladders. At greater depths (30 fathoms and above), all were broken. But there is 

 no evidence that the swimbladder walls of vertically migrating deep-sea fishes are any tougher in 

 relation to the size of the sac than those of perch or cod. Saupe (1939) gives 250-300// as the thickness 

 of the walls in the perch, but makes no mention of the size of the fish. This may be compared with 

 the measurements (20-300//) given earlier in this section, for various species of stomiatoids and 

 myctophids. Furthermore, there is no indication that the collagenous elements in the swimbladder 

 wall of bathypelagic fishes are more closely packed than those of shallow water species. 



