no DISCOVERY REPORTS 



for a living-space, interaction between fish and environment has been such that tissues have evolved 



not only to match the biological environment, but to conform to the physical stresses besides. 



We are now in a better position to consider the deeper dwelling bathypelagic fishes. Presumably 

 the species of Cyclothone once had a gas-filled swimbladder in the adult phase, but this is now only 

 functional during larval life. Gonostoma bathyphilum has lost all trace of this organ, but it is well 

 developed in a related species, G. denudation. The ancestors of the ceratioid angler fishes and the 

 gulper eels may also have had a swimbladder. 



Now it is clear that the loss of this organ is not related to the pressure factor, or indeed, to any 

 physical effect. The foregoing discussion simply suggests that the cause largely resides in the trophic 

 conditions obtaining in the deeper mid-waters of the ocean. To begin with, a pelagic fish with a swim- 

 bladder is able to carry more muscle, attached to a firmer skeleton, than a fish (of comparable dimen- 

 sions) without such buoyant uplift (Denton and Marshall, 1958). In turn, the proper maintenance 

 and functioning of these tissues require well-developed circulatory, respiratory, alimentary, excretory 

 and nervous systems. But with the loss of the swimbladder, the degree of tissue organization could 

 become ' geared-down ' to the low level of potential energy in the biological environment. 1 At the 

 same time neutral buoyancy could be approached or, more likely, achieved. 



Moving into the upper thousand metres of the ocean, it is surely significant that most of the plankton 

 feeders, the small gonostomatids, hatchet fishes and lantern fishes possess a swimbladder (Marshall, 

 1 951). Having this internal float, they are able to carry the muscle necessary to propel them upwards 

 during their daily migrations to the food-rich surface-waters. It is also striking that most of the 

 predatory fishes (Stomiatidae, Melanostomiatidae, Chauliodontidae, Idiacanthidae, Malacosteidae and 

 Alepisauroidea) lack a swimbladder. (The exceptions are Astronesthes spp. and Chiasmodon niger.) 

 We have seen that (pp. 85-86) some of these fishes also undertake vertical migrations. It is probable 

 that they, like Gonostoma elongatum, come close to neutral buoyancy through a reduction of skeletal 

 and muscular tissue (Marshall, 1954, 1955; Denton and Marshall, 1958), but the regression of these 

 tissues is by no means so marked as that in the pelagic fishes living in the deep, underlying 

 waters. 



Chauliodus sloanei is one of these predators living in the upper mid-waters. The younger individuals 

 live fairly close to the surface and are thus well placed for food (Ege, 1948; Haffner, 1952). The 

 deeper dwelling, older fishes grow to lengths of more than 250 mm. 



During the daytime Dr Peres has watched Chauliodus from a bathyscaphe and in a letter dated 

 17 November 1958 has written of his observations. These fishes hover in the water with the long axis 

 of the body at an acute angle to the horizontal plane, the head being above the tail. At the same time 

 the long second dorsal ray, which is tipped with luminescent tissue (Brauer, 1908), is curved forward 

 over the head so that the extremity of the ray lies in front of the mouth. Clearly, the fish is behaving 

 very much like a female ceratioid and is angling for prey. It is also looking obliquely upward, poised 

 in a good position for detecting prey silhouetted against the down-going and scattered rays of sun- 

 light, but towards sunset, it moves upwards to richer feeding grounds. While the density of its bone 

 and muscle is less than that of a myctophid, it is much the larger fish, and what it lacks in unit 

 muscle-power it gains in size and power of movement, which is also true of most of the other pre- 

 datory fishes without a swimbladder. Moreover, it is interesting that the only predatory stomiatoids 

 containing this organ are the species of Astronesthes, which are smaller than their predatory relatives 

 without it (p. 90). Like its myctophid prey, Astronesthes niger, which crosses the upper thermocline 

 during its migrations, has a firm skeleton and compact well-formed myotomes. 



1 If a teleost had both 'light weight' tissues and a capacious hydrostatic organ it would be overbuoyant. 



