VERTICAL DISTRIBUTION AND THE SWIMBLADDER 83 



occurrence may vary in space and time. Species living in very transparent waters live further down 

 than their relatives in less clear waters (comparison of Beebe's (1937) and Grey's (1955) data with 

 those obtained in other parts of the Atlantic (Murray and Hjort, 1912) reveals this). Furthermore, the 

 higher the latitude the less the depth of maximum concentration (Murray and Hjort, 1912). There 

 may also be seasonal changes, species tending to live nearer the surface during the winter months 

 (Jespersen and Taning (1926), Taning (1918)). 



Order Isospondyli. Suborder Stomiatoidea. Apart from those listed in section B, most stomiatoids 

 seem to belong here. Suborder Salmonoidea : Opisthoproctus, Winteria, Bathylagidae. Apart from 

 Bathylagus argyrogaster, which was taken mainly above 500 m., the commoner species taken during 

 Discovery Investigations appear to be concentrated between 500 and 1000 m. (Norman, 1930). 



Order Itiiomi. Most alepisauroids (Marshall, 1955) and Myctophidae. 



Order Giganturoidea, Regan, 1925. 



Order Allotriognathi, Stylophorus chordatus (Bruun et al. 1956). 



Order Berycomorphi: Melamphaidae. The commoner species taken in Discovery nets (Norman, 1929, 

 1930) appear to come from depths between 500 and 1000 m. 



B. Fishes with centres of concentration between depths of about 1000 and 4000 m. 



Order Isospondyli. Suborder Stomiatoidea: Gonostomatidae, Cyclothone microdon, C. livida, C. 

 acclinidens, C. obscura, Gonostoma bathyphilum. 



Order Lyomeri. The centre of distribution of Eurypharynx pelecanoides seems to lie between 1400 

 and 2800 m. (Grey, 1956). 



Order Apodes. Certain of the deep-sea eels such as Cyema atrum, Serrivomer parabeani and Avocettina 

 infans appear to be commoner at these deeper levels (Grey, 1956). 



Order Berycomorphi: Melamphaes nigrescens (Grey, 1956) and possibly M. cristiceps (Norman, 1929, 

 1930) occur below 1000 m. 



Order Pedicidati: Suborder Ceratioidea. Metamorphosing, adolescent and adult deep-sea angler 

 fishes are mainly to be found at or below a depth of 1500 m. (Bertelsen, 1951). 



By referring to the descriptive part of this report, it will be clear that the populations of bathy- 

 pelagic fishes with fully developed gas-filled swimbladders are centred in the upper reaches (200- 

 1000 m.) of the deep-sea. The main groups are the Gonostomatidae (most species), Sternoptychidae 

 (hatchet fishes), Astronesthes spp. Myctophidae, and Melamphaidae (most species). While numerous 

 species from these levels lack a swimbladder [the Melanostomiatidae, Stomiatidae, Chauliodontidae, 

 Idiacanthidae, Malacosteidae, Bathylagidae, Alepisauroidea and Giganturoidea are the main groups 

 (but see also the summary on p. 81-82)], this condition is universal in the fishes centred between depths 

 of 1000 and 4000 m. 



Hydrostatic pressure increases by one atmosphere for each 10 m. of depth and the average depth 

 of the ocean is about 4000 m. It might thus be supposed that the limitation imposed on the develop- 

 ment of a gas-filled hydrostatic organ is simply related to the pressure factor. Clearly, the aspects to 

 be considered concern the compressibility of gases and the amounts of energy and gas required to 

 keep the swimbladder inflated at the appropriate volume against high hydrostatic pressures. 



