SUMMARY u 5 



of gaseous exchange between the arterial and venous capillaries. There is evidence that the deeper 

 living species tend to have longer retia. 



2. The development of the gas-gland, as expressed by its surface area, is much more pronounced 

 in bathypelagic than in epipelagic species. There appear to be three main types of gas-glands; (a) with 

 many giant cells, (b) with medium sized cells, and (c) with small cells. Intracellular capillaries are 

 found in the first type. Certain cytological features of the gas-gland cells of Vinciguerria seem in 

 keeping with the accumulating evidence that the cells actively transport gases from the blood plasma 

 to the lumen of the swimbladder. 



The structural features of the resorbent part of the swimbladder, the part by means of which the 

 gas-content can be reduced, are reviewed. Certain aspects of form and function are considered. 



The swimbladder and vertical distribution (pp. 82-85) 



A well-developed gas-filled swimbladder is only to be found in bathypelagic fishes with centres of 

 concentration above the 1000-m. level. While numerous other species in the same environment lack 

 such a swimbladder, this condition is universal in bathypelagic species that occur belozv a depth of 

 1000 m. The loss of the swimbladder in the deeper living species might seem to be related to the 

 pressure factor, to the compressibility of gases and the amounts of energy and gas required to keep 

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



The swimbladder and vertical migrations (pp. 85-95) 



Many of the bathypelagic fishes living in the upper oceanic reaches (200 to 1000 m.) undertake 

 diurnal vertical migrations. After reviewing the evidence for these movements, the physical and 

 biological background of these migrations is considered. While a number of species cross the near- 

 surface thermocline during their upward migrations, other species are rarely if ever taken in the 

 surface-layers and may be called partial migrators. The physical problems of gas secretion and 

 resorption are considered for both types of migrator. While these fishes are small, active species with 

 a relatively large gill-surface, the provision of enough gas to fill the swimbladder during and after 

 a downward migration seems an immense physiological task unless the gas-gland can store oxygen 

 in a combined form. Concerning the loss of gas during an upward migration, the high ratio between 

 the resorbent surface and the volume of the swimbladder and the very steep concentration gradient 

 between the tensions of the swimbladder gases and those in the blood, may well mean that the rate 

 of gas resorption is high enough to keep pace with the reduction in hydrostatic pressure. 



the swimbladder and the economy of life in the deep sea (pp. 95-113) 



At least half of the benthic species that range below a depth of 2000 m. have well-developed, gas-filled 

 swimbladders, with very long retia mirabilia. Considering also the bathypelagic species, there is 

 a direct correlation between the length of the retia and the depth of the living space, a further striking 

 indication of the function of the retia as systems for the counter-current exchange of gases. Apart 

 from implying that the swimbladders of these fishes are functional at great depths (down to 5000 m.) 

 these facts suggest that the loss of the swimbladder in bathypelagic fishes with centres of concen- 

 tration below the 1000-m. level is not due to the pressure factor. The most potent influence is seen to 

 be the food-poor environment of these fishes, one without the necessary nutriment to support the 

 ' extra ' tissues that can be carried at neutral buoyancy by a hydrostatic organ. Compared with the 

 fishes with swimbladders living in the upper reaches (200-1000 m.) of the bathypelagic environment 

 (and with benthic species), the tissues of these deeper living species are much regressed. The life- 

 forms of diverse fishes from these different levels are no less striking than those found in plants. 



15-2 



