INTRODUCTION 5 



The remaining sections are devoted to functional-morphological and biological aspects. Now, 

 development of the first aspect obviously depends on adequate knowledge of both form and function. 

 Having undertaken a survey to the point where generalizations on morphology could safely be made, 

 it was a pleasure to turn to the work of Dr P. F. Scholander and his colleagues on functional aspects 

 of the swimbladder in deep-sea fishes. Full reference is made to this work at a later stage. In parti- 

 cular, Scholander's (1958) concept of counter-current exchange in the retia mirabilia and his review 

 of this principle in biology has been very helpful. 1 I am also indebted to Dr Ragnar Fange's (1953) 

 fine work on structure and functions of the euphysoclist swimbladder. In the section on the swim- 

 bladder as a hydrostatic organ, findings on the structure of the retia mirabilia and gas-glands are 

 considered in the light of this physiological work, a task that was made easier by my former colla- 

 boration with Dr F. R. Harden Jones (Jones and Marshall, 1953). 



In the final biological section, the structural development (and absence) of the swimbladder in 

 bathypelagic fishes is considered in relation to vertical distribution and vertical migrations. This has 

 involved some consideration of the physical and biological environment and leads to the final part, 

 which begins with a survey of the swimbladder in benthic deep-sea fishes. Considering only the 

 fishes taken below a depth of 2000 m., at least half the number of species have capacious swimbladders. 

 In the pelagic environment, as already mentioned, this organ is absent in fishes concentrated below 

 the 1000-m. level. This surprising difference led me to consider, to use a concept more familiar to 

 botanists, the ' life forms ' of fishes from the upper and lower reaches of the bathypelagic environment. 

 Again, I was helped by former collaboration, this time with Dr E. J. Denton (Denton and Marshall, 

 1958). The buoyancy balance-sheet of a fish with a capacious, gas-filled swimbladder is such that it 

 can carry a firm skeleton and well-knit muscles. But these two tissue-systems and others are much 

 reduced in pelagic fishes living below a depth of 1000 m., although this is not surprising in view of the 

 poor food-supply around them. These fishes may well have lost the swimbladder simply because 

 there is not sufficient potential energy in their environment to maintain the ' extra ' tissues that can 

 be carried by this hydrostatic organ. In pursuing this, and other ideas, I have tried to follow the 

 thought behind the quotation that heads this report. 



MATERIAL AND METHODS 



Most of the fishes used in this report have been taken during the Discovery investigations. The 

 positions at which they were caught are listed under the name of each species in the descriptive 

 section (pp. 6-50) and unless otherwise indicated the station numbers refer to the Discovery col- 

 lections. 



In nearly all instances, the swimbladder was found by dissecting the fish under a binocular micro- 

 scope. After noting the position of the organ, and where possible, tracing the blood supply, the swim- 

 bladder was removed and cut open so as to display the retial system and gas-glands. As these are 

 well-developed in deep-sea fishes, their structure was readily appreciated. Where sufficient material 

 was available more than one dissection was made. 



In certain species (Cyclothone braueri, C. Hvida, Argyropelecus olfersii, A. aculeatus, Polyipnus 

 laternatus, Opisthoproctas soleatus, Vinciguerria attenuata, V. nimbaria, Myctophum punctatum and 

 Diaphus dofleini) the fine structure was studied by means of serial transverse sections. An account of 

 this work appears in the sections dealing with the swimbladder wall (pp. 60-65), fat-invested swim- 

 bladders (p. 65-68) and the swimbladder as a hydrostatic organ (p. 68-81). 



1 To Scholander's list of organs in which this physiological principle is operative (the retia mirabilia of the teleost swim- 

 bladder, the gills of fishes, the placenta, the vertebrate kidney and the retial vascular structures of mammals) may be added 

 the choroidal gland of the eyes of bony fishes (see Barnett, 1951). 



