ii 4 DISCOVERY REPORTS 



SWIMBLADDER STRUCTURE AND CLASSIFICATION (pp. 50-58) 



The structural plan of the swimbladder has thrown new light on the classification of these groups. 

 The stomiatoids are revealed as a compact (paraphysoclistous) 1 group with a single, bipolar rete 

 mirabile at the posterior end of the swimbladder which has a resorbent capillary bed, obtaining its 

 arterial blood through a by-pass branch of the retial artery. Differences in swimbladder structure 

 may also be useful in distinguishing genera. 



The deep-sea salmonoid fishes have a very different (euphysoclistous) 1 form of swimbladder with 

 micro-retia mirabilia supplying the gas-gland. The myctophid swimbladder is also euphysoclistous 

 (with an oval) and three unipolar retia mirabilia enter the anterior end of the sac to carry blood to the 

 three-lobed gas-gland. The families Anoplogastridae, Melamphaidae and Stephanoberycidae (sub- 

 order Anoplogastroidea, Berycomorphi) yet again have a euphysoclistous swimbladder (with an oval) 

 and one or two unipolar retia mirabilia, which run forward from the posterior end of the sac. These 

 and other findings are used to discuss the development of a closed swimbladder in the deep sea, 

 particularly in the predominantly physostomatous 1 Isospondyli, and to consider evolutionary re- 

 lationships. 



The swimbladder wall (pp. 59-65) 



The fine structure of the swimbladder wall in bathypelagic teleosts is much like that of other physo- 

 clistous groups. Excluding the peritoneal investment, an outer, thin but tough layer of collagen fibres 

 (tunica externa) is separated from the inner epithelial layer by a more voluminous reticulum of fibres 

 developed within a semi-fluid, gelatinous matrix. There is also a layer of smooth muscle fibres near 

 the inner epithelium. Following descriptions of the fine structure of the swimbladder wall in certain 

 species, there is some discussion of its mechanical and gas-proofing qualities. In particular, attention 

 is drawn to the role of the submucosa in a swimbladder undergoing compression during a migration 

 into deeper waters. The semi-fluid submucosa would seem to allow the tissues to relax in a uniform 

 manner and the sac to maintain its ellipsoidal shape. 



Fat-invested swimbladders (pp. 65-68) 



In certain bathypelagic fishes (the stomiatoids, Cyclothone spp., Gonostoma elongatum, Polyipnus 

 later natus, Borostomias antarcticus, Stomias colubrinus, S. affinis, the myctophids, Latnpanyctus 

 leucopsarus, Diaphiis theta and the anoplogastroid, Anoplogaster longidens), the swimbladder regresses 

 after metamorphosis and becomes invested with fat, which is deposited between the peritoneum and 

 the tunica externa. It is pointed out that this replacement of gas by fat can have but little effect on 

 the ' credit side ' of the ' buoyancy balance sheet '. 



The swimbladder as a hydrostatic organ and the structure of 

 the gas-producing and resorbent parts (pp. 68-81) 



Like species living nearer the surface or over the continental slope, the volume of the swimbladder in 

 bathypelagic fishes is about 5 per cent of the body volume. It thus functions as a hydrostatic organ, 

 making its possessor weightless in water. But compared to shallow-water species the gas-producing 

 complex (rete mirabile and gas-gland) is highly developed : 



1. The product of the number and length of the capillaries forming the retia is high compared to 

 the dimensions of the swimbladder. These two features, together with the form of arrangement of the 

 capillaries, are also considered in relation to the design of the retia as counter-current systems allowing 



1 See p. 50. 



