PELAGIC AND BENTHIC FISHES, SWIMBLADDER, ECONOMY OF DEEP-SEA LIFE 107 



Since diurnal migrations are an integral part of the lives of so many fishes existing in the upper 

 oceanic reaches, it is clear that they could not survive and reproduce their kind without drawing on 

 the rich food-supply in the surface-waters. But this feeding-level is beyond the direct reach of the 

 species living below 1000 m. Evidently the most potent environmental stress, against which they 

 have evolved, is the sparse supply of food. But one favourable factor must have been the coolness of 

 their surroundings : the lower the temperature the less the food required for maintaining life (Brown, 

 1957)- On the other hand, at high hydrostatic pressures the pace of life, measured by the rate of 

 oxygen consumption, seems to be increased (p. 90). And, while they do not have to contend with 

 turbulent waters like the fishes that migrate to the surface-mixed layer, the laminar viscosity of the 

 cool seas (5 C. or less) through which they must move is nearly double that at the surface in the 

 warm ocean. 



The conclusion is inescapable that the paring-down of the tissues, particularly the reduction of 

 muscle and bone substances, is the most striking of these fishes' adaptations to their living-space. As 

 we have seen, there is every indication that the black Cyclothones, ceratioid angler fishes and gulper- 

 eels (Lyomeri) are much less active than the fishes living above them in the upper 1000 m. of the 

 ocean. With the reduction of these tissues they have not only acquired an economy of life to match 

 their food-supply, but have also reduced, more probably eliminated, the effort required to maintain 

 their level in the ocean. 1 If Gonostoma elongatum, which also lacks a gas-filled swimbladder, comes 

 'close to neutral buoyancy (Denton and Marshall, 1958), this must surely be true of these fishes. In 

 a well-fed Cyclothone microdon, the fat stored around the regressed swimbladder, attached to the 

 mesenteries, and deposited in the subcutaneous sinuses must take up more than 10 per cent of the 

 volume of the fish. This light tissue (with a density of about 0-9), together with the reduced muscular 

 tissues and flimsy skeleton, may well make these fishes neutrally buoyant. In a letter to me, dated 

 8 November 1958, Dr Peres has recorded his daytime bathyscaphe observations of Cyclothone, 

 Gonostoma and Chaidiodas. He writes: 'Ces animaux sont toujours immobiles, paraissant Hotter; ils 

 semblent attendre la proie sans la poursuivre, et il m'a ete impossible de voirs quelles sont les nageoires 

 dont le mouvement aide a leur equilibre. En revanche les petits Myctophum sont presque toujours 

 en mouvement.' 



Cyclothone feeds on copepods and chaetognaths (Giinther and Deckert, 1953), while Nusbaum- 

 Hilarowicz (1920) found fish-scales and eye-lenses in the gut of C. braueri. In three individuals of 

 C. microdon taken at St. 395 (48 26f S., 22 10' W. to 48 z6\ S., 22 06J' W. 13. v. 1930; N 450 H; 

 1 500-1400 m.), I found the remains of copepods, chaetognaths, euphausiids, together with detrital 

 material and faecal pellets. Female ceratioid angler fishes feed on organisms ranging from copepods 

 to squids and fishes, while in the dwarf, free-living males, copepods, chaetognaths and Phronima 

 have been found (Bertelsen, 1951). Saccopharynx takes fishes (including benthic species); 

 Eurypharynx, crustaceans, worms and fishes (Bertin, 1934). The angler fishes lure their prey 

 and certain species can master fishes several times their own length. Perhaps Saccopharynx, 

 which is also a giant swallower, attracts prey by means of the spongy luminous tissue on its wisp of 

 a tail. 



Thus all these fishes, including Cyclothone, are predatory, and they possess jaws that are adapted 

 for taking a very wide size-range of food organisms. If we again make the same comparisons of species 

 from the upper and lower mid-waters, this adaptation is strikingly revealed. 



1 Certain of the deeper-living cephalopods (e.g. Vampyroteuthis, Cirroteuthidae, Amphitretus and other octopods) ' . . .have 

 lost most of their firmness and muscular power. They develop a thick coat of jelly-like subcutaneous tissue and the muscles 

 themselves are sometimes degenerate and invaded by this jelly' (Morton, 1958). This parallel in the most active group of 

 nektonic invertebrates is most striking. 



14-2 



