THE PHYSICS AND BIOLOGY OF VERTICAL MIGRATIONS 95 



Gas resorption 



During its upward migration, the pressure on the same hatchet fish will be reduced by about 60 per 

 cent ; the figure for the lantern fish is about 90 per cent. It seems odd that the resorbent surface : swim- 

 bladder volume ratio of the hatchet fish Argyropelecus aculeatiis is greater than the values for the two 

 lantern fishes (see Table 6). But as already suggested, the figures for the two lantern fish in Table 6 

 are likely to be low. If the radius of their 'ovals' was actually 4 mm. instead of 3 mm., which may 

 well be under natural conditions, the ratios would be about 170 instead of 100. For it is a striking 

 fact that lantern fishes brought up in nets hardly ever have the viscera pushed into the mouth by 

 extra expansion of the swimbladder, whereas this is not uncommon in hatchet fishes. It is also 

 interesting that Astronesthes niger, which appears near the surface by night, has a much higher ratio 

 than Argyropelecus aculeatus, a partial migrator. 



PELAGIC AND BENTHIC FISHES, THE SWIMBLADDER, AND 

 ASPECTS OF THE ECONOMY OF DEEP-SEA LIFE 



There is a far-reaching correlation between the focal depths of the living-space and the presence or 

 absence of a swimbladder in fishes from the mid-waters of the ocean (pp. 82-83). While a highly 

 developed, gas-filled swimbladder is found in about half the species that swim in the upper reaches 

 (200-1000 m.) of this environment, this organ is absent or regressed in all fishes with populations 

 centred below the 1000-m. level. Is such a relationship with depth also found in fishes living on or 

 near the deep-sea floor ? The surprising fact, as already indicated, is that much of the answer is in 

 the negative. 



The main families to be considered are the Alepocephalidae, Bathypteroidae, Harpadontidae, 

 Ipnopidae, Synaphobranchidae*, Halosauridae,* Notacanthidae*, Moridae*, Macrouridae*, Zoar- 

 cidae, Brotulidae*, and Liparidae. In the families marked with an asterisk, a swimbladder is com- 

 monly present, but is absent in the others. Using Grey's (1956) list of species that are found below 

 a depth of 2000 m., at least half of a total of some 240 species should have swimbladders. 



Before giving special consideration to those species that range into the deeper, abyssal parts of the 

 ocean, appreciation is due to observers in bathyscaphes and to underwater photography. Until these 

 records were available, there was always the possibility that some of the supposedly benthic fishes 

 might not live near the bottom. Hjort (Murray and Hjort, 1912), believed that the Bathypteroidae 

 were mid-water fishes, but they have now been seen actually resting on the bottom, supported by the 

 two long pectoral rays and the single, elongated caudal ray (Houot and Willm, 1955). Representatives 

 of the Halosauridae, Moridae, Macrouridae and Brotulidae have also been observed near the bottom 

 (Fages et al. 1958; Peres, Picard and Ruivo, 1957). I have seen halosaurid, notacanthid and morid 

 fishes in photographs of the deep-sea floor taken by Dr A. S. Laughton of the National Institute 

 of Oceanography. 



Nybelin (1957) has listed the species that have been fished below a depth of 3000 m. There are 

 sixty-eight. (Alepocephalidae (7), Harpadontidae (2), Bathypteroidae (3), Ipnopidae (3), Synapho- 

 branchidae (3), Halosauridae (2), Notacanthidae (1), Macrouridae (16), Zoarcidae (1), Lycodidae (2), 

 Brotulidae (25), Liparidae (3).) Thus forty-seven species may at least be suspected of having swim- 

 bladders. 



However, there is the possibility that the deeper living species might have lost this organ. Yet 

 Giinther (1887) found a swimbladder in Typhlonus nasus, taken by the 'Challenger' at depths of 

 3934 and 4465 m., and in Acanthonus spinifer, trawled at 3242 m. Besides these two brotulids, a 



