5 o DISCOVERY REPORTS 



Order PEDICULATI 



Suborder CERATIOIDEA 



The combined researches of Garman (1899), Waterman (1948), R. Clarke (1950) and Bertelsen (1951) 

 show that the deep-sea angler fishes have no swimbladder. Bertelsen's work reveals that this organ 

 is also absent in the larvae. 



STRUCTURE AND SYSTEMATICS 



The foregoing survey has revealed that the species of major groups of bathypelagic fishes (Stomia- 

 toidea, Salmonoidea, Myctophidae and Anoplogastroidea) have swimbladders conforming to an 

 individual structural pattern or bauplan. Within each group there are, so to say, variations on an 

 original structural theme. The bearing of these findings on classification will now be considered. 



Order Isospondyli 



Deep-sea isospondylous fishes with a swimbladder are all physoclists : they lack a pneumatic duct, 

 which in physostomatous Isospondyli connects the swimbladder to the foregut. The acquisition of 

 a closed swimbladder in these stomiatoids and deep-sea salmonoids must have been imposed on them 

 by their living-space in oceanic mid-waters. The physostomatous isospondyls live in the relatively 

 shallow seas over the continental shelves, or in freshwater. While at least some of these forms can 

 secrete gas, this is a slow process, but they are readily able to inflate their swimbladders by gulping 

 in air at the surface and forcing it down the pneumatic duct. On the other hand, a visit to the surface 

 by a hypothetical physostome living several hundred metres below the surface is clearly ' out of the 

 question '. Such a fish would not only be faced with a long climb to the surface, but having replenished 

 the swimbladder gases, would then have to resolve the problem of the gradient in hydrostatic pressure 

 as it returned to its level in the ocean. After diving to a depth of say, 500 m., the volume of the 

 swimbladder would be compressed to about one-fiftieth of its capacity at the surface. If the fish is to 

 use its swimbladder as a hydrostatic organ, it must then secrete gas, and to make up the volume (to 

 5 per cent of the body volume, see p. 68) at a pressure of 50 atmospheres would obviously require 

 highly developed retia mirabilia and gas-glands. The retention of a pneumatic duct and air-gulping 

 habits by a mid-water, deep-sea fish is therefore a biological reductio ad absurdum. 



Suborder Stomiatoidea 



The stomiatoid swimbladder has a character complex which may be defined as follows : It is para- 

 physoclistous 1 with a single, bipolar rete mirabile at the posterior end. As might be expected, the 

 blood supply to the rete comes from vessels originating behind the swimbladder. The capillary 

 network of the resorbent surface arises from a side branch of the retial artery and part of the venous 

 circulation to the gas-gland. The venous blood thus eventually passes through the rete. The gas- 

 producing complex is highly developed, the gland consisting of one to four lobes. A schematic 

 diagram is shown in Text-fig. 30 A. 



Comparison of this definition with that of any of the following groups (pp. 53-56) will fully reveal 

 the very characteristic design of the stomiatoid swimbladder. It will also provide further support 

 for the classification of the stomiatoids as a distinct suborder of the Isospondyli. 



Regan (1923) was the first to appreciate the essential unity of the stomiatoids. His definition of the 



1 This term was introduced by Rauther (1922) and refers to simple closed swimbladders in which the glandular and 

 resorbent parts are not sharply localized. It is contrasted with the euphysoclistous type, the resorbent part of which is clearly 

 separate from the glandular part, being either in the form of an oval, or an anterior or posterior section of the sac. In the 

 latter forms, the two parts are often divided by a diaphragm. Fange (1953) has also pointed out that in euphysoclists the 

 resorbent part is thin-walled, while the glandular part is thick-walled. In paraphysoclists there is no such distinction. 



