PELAGIC AND BENTHIC FISHES, SWIMBLADDER, ECONOMY OF DEEP-SEA LIFE 109 

 1954), 1 and in a few species such as Aphyonus gelatinosus, the eyes are much reduced. These apparent 

 exceptions will be discussed at a later point (pp. 111-112). 



If we compare a black Cyclothone, an angler fish or a gulper eel with an abyssal bathypteroid, 

 macrourid or a brotulid, the difference in the relative development of the eyes is not the only con- 

 trasting feature. Actually the bathypteroids have small eyes, but their myotomes and skeleton are 

 far better developed, being almost as firm and well-knit as those of their shallow-water relatives, the 

 lizard fishes (Synodontidae). A 3 - f t . Nematonurus armatus is far better equipped with all manner of 

 tissues than a 3-ft. gulper-eel (and it has kept its swimbladder). The small brotulid Mixonus caudalis 

 is similarly 'better off' than a black Cyclothone or an angler fish. Many other such contrasts could 

 be made, but here we need only add that the gill-surface of the benthic fishes is far better developed 

 than that in the bathypelagic species. 



But the deep-sea floor supports a larger standing crop of animal-life than do the deeper mid-waters 

 of the ocean. In the Kurile-Kamchatka Trench region, which lies under a productive stretch of 

 surface-water, the biomass of bottom-dwelling animals in a square metre was measured at 6-94 g., 

 between depths of 950 and 4070 m. Between 5070 and 7230 m. the quantity fell to 1-22 g./m. 3 

 (Zenkevitch and Birstein, 1956). Bottom samples taken by a Petersen grab from the 'Galathea' 

 indicate that the deep-sea floor supports an average of ten animals per square metre with a total 

 weight of about 1 g., and this must be a minimum value. Having given these figures, Sparck (1956) 

 concludes thus: 'This surprising density right down to between 5000 and 8000 m. suggests that food 

 conditions in the abyss are not so poor as we have been inclined to think, and this in turn leads us 

 to suppose that abyssal water currents must be stronger than formerly believed.' 



The key-organisms in the sediments, perhaps those at the base of the food pyramid, are the bacteria 

 (Zobell, 1954; Zobell and Morita, 1956). They can exist on the dissolved organic matter in seawater 

 and convert intractable substances such as cellulose and chitin into their own substance. The authors 

 just mentioned suggest that the bacteria may be used directly as food, or may aid digestion by living 

 as symbionts in the gut of bottom-dwelling animals. Whatever may be the actual conditions, it is 

 clear that large standing-crops of mud-eating holothurians can exist at depths exceeding 6000 m. 

 (Hansen, 1956). While these animals may be largely free from the attacks of predators, holothurians 

 may still form 30-50 per cent of the biomass of benthic organisms in depths from 3000 to 7000 m., 

 in regions where predatory groups such as the asteroids are particularly abundant (see Zenkevitch, 

 1954, fig. 2). Finally, at depths down to at least 2500 m., conditions of life at the bottom are favour- 

 able enough to attract detachments of active pelagic organisms, such as euphausiids, sergestids and 

 lantern fishes (pp. 87-88). 



It would thus seem that there is sufficient food on or near the bottom to satisfy the day-to-day 

 requirements of the relatively highly organized bodies of bathypteroid, macrourid and brotulid fishes. 

 As we have seen, species belonging to the second and third groups have retained a gas-filled swim- 

 bladder. There is structural and biological evidence that this organ is functional, even at great depths, 

 but because of its decreased buoyant properties at high pressures some reduction of tissue density is 

 necessary if the animals are to be in hydrostatic equilibrium with their environment. Furthermore, 

 there is evidence that the muscular and skeletal tissues have been somewhat reduced. Even so, the 

 level of development of these and other tissues is much superior to that possessed by the bathypelagic 

 fishes poised above them in the deep, nutrient-poor waters of the ocean. In the endless struggle 



1 Moreover, Nematonurus armatus has a wide depth range (2S0-4600 m.) extending from regions receiving sunlight to 

 unlit depths. In Lionurus filicauda, which seems to be restricted to the abyssal plain (2515-4846 m.), the eyes are propor- 

 tionately smaller. A 500 mm. Nematonurus armatus has orbits measuring 18 mm. in diameter (Nybelin, 1957) while in a 

 400-mm. Lionurus filicauda the eyes are about 10 mm. in diameter. 



