4 DISCOVERY REPORTS 



running forward in parallel to supply the gas-gland, and superficially these small blood-vessels look 



very like the fibres in a muscle. 



However, a year before the publication of Giinther's Report, Coggi (1886) had described certain 

 structures in the swimbladders of G. denudatum and Scopelus benoiti ( = Hygophum benoiti). He re- 

 marked that the ' red bodies ' (retia mirabilia) of these two species were very like those of physoclistous 

 teleosts and that the cells in the epithelial body (gas-gland) of Gonostoma were very large. In the 

 other species Coggi figured three retia mirabilia, each supplying a corresponding lobe of the gas- 

 gland. We shall see later that this is a constant feature of the myctophid swimbladder. But his finding 

 of an opening into the ' pneumatic duct ' is an error. Presumably he found the opening into the gas- 

 resorbing part of the swimbladder, which in myctophids is an ' oval ', and is placed at the front of the 

 sac (see pp. 30-42). 



Apart from this early account, the only other detailed work on the structure of the swimbladder of 

 bathypelagic fishes has been that of Rauther (1922) on the lantern fish, Diaphus rafinesqnei and 

 Nusbaum-Hilarowicz (1920) on the hatchet fishes Sternoptyx and Argyropelecus, and a species of 

 Cyclothone. Ray (1950) was the first to find and figure a fat-invested swimbladder, this being in the 

 lantern fish, Lampanyctus leucopsarus. In his report on deep-scattering layers in the Monterey Bay 

 area, Barham (1957) referred to work by Jollie (1954, Ph.D. Thesis) on the anatomy of this organ. 



There are numerous records of the presence or absence of the swimbladder in both pelagic and 

 benthic deep-sea fishes in Alcock's (1899) Investigator Report. Concerning benthic fishes only, Holt 

 and Calderwood (1895) described the swimbladder of the deep-sea gadoid, Mora and those of certain 

 macrourids. Presence or absence observations may also be found in Garman (1891) and in a few 

 other papers which will be referred to in the descriptive part of this report. Lastly, Beebe and 

 Vander Pyl (1944) gave the dimensions of the swimbladder in two lantern fishes, Myctophum affirie 

 and Lampanyctus macropterus, while Kanwisher and Ebeling (1957) measured the volume and gas- 

 content of this organ in a number of bathypelagic fishes. 



Considering only the observations made during the latter half of the nineteenth century, there is 

 evidence that the swimbladder is found in numerous deep-sea fishes. But despite this early work, 

 various authorities imposed a pattern on nature without further investigation. They decided that a 

 swimbladder could not function in the deep-sea environment owing to the high hydrostatic pressures, 

 and that it was therefore absent in deep-sea fishes. This organ is certainly absent in pelagic fishes 

 whose living-space is centred below the 1000-m. level, but it is present in many fishes that swim 

 above this depth and in numerous species living below, near the deep-sea floor. 



The survey in this report has involved the examination of about ninety species of deep-sea fishes, 

 certain aspects of which have already been published (Marshall, 1950, 1951, 1954, 1955; Jones and 

 Marshall, 1953; Denton and Marshall, 1958). The first part is concerned with the description of the 

 swimbladders of bathypelagic fishes. It will be found that this organ is commonly present in a 

 number of major groups (e.g. the Gonostomatidae, Sternoptychidae, Myctophidae and Melam- 

 phaidae). In these and other fishes the gas-producing structures are highly developed. 



In teleosts, the structural features of the swimbladder are not often of use in classification, but a 

 gratifying feature of this survey has been the discovery that this organ has a distinctive design in the 

 Stomiatoidea, deep-sea Salmonoidea, Myctophidae and Anoplogastridae. This aspect is considered 

 in the second part of the report and leads to discussion of the evolution of bathypelagic fishes. 



Then follow sections concerned with the larval organ, the fine structure of the swimbladder wall, 

 and fat-invested swimbladders. In a number of species the swimbladder contains gas during the 

 larval phase which is passed in the surface-waters, but the organ regresses and is invested with fat 

 after metamorphosis to the adult form. 



