108 DISCOVERY REPORTS 



Reference to Text-fig. 43 will show that C. microdon has much longer jaws than Manrolicus , the 

 suspensoria of which are more or less vertical in position. It is clear that the diet of Mauroliciis must 

 be restricted, probably to organisms the size of large copepods. However, it lives nearer to good 

 supplies of such food. 



Again, Gonostoma bathyphilum possesses longer jaws hinged to more backwardly carried sus- 

 pensoria than its relative G. denudation (see Text-fig. 42). This tendency is carried to an extreme in the 

 Lyomeri in which the suspensoria and jaws are many times longer than the neurocranium. More- 

 over, the premaxillae and maxillae have been lost, the teeth in the upper part of the mouth being 

 carried on the palatopterygoid elements (Tchernavin, 1947a). The skeleton is also reduced in many 

 other ways. But the most fantastic regression of bone and other tissues occurs in the Monognathidae, 

 fishes that may well live in the same oceanic layers as the Lyomeri (Bertin, 1938). 



The jaws, and especially the buccal cavity, of most female ceratioid angler fishes are also large. 

 ' The length of the premaxillary and maxillary is usually more than 50 per cent of the distance from 

 snout to end of cranium and reaches 100-150 per cent in Caulophryne and many Linophrynids ' 

 (Bertelsen, 1951). 



The degree of adaptation of the jaw-mechanisms of these fishes is perfectly evident, a structural 

 emphasis fitting them to an environment with poor supplies of food. In such surroundings it is an 

 advantage to be able to take the largest possible meal that comes along, and at the same time, not to 

 turn aside from a copepod. As Thorson, quoted by Moore (1958) has pointed out, the swallowing 

 capacity of a predator must not only be proportional to the sparseness of the prey, but also be inversely 

 related to its own speed of movement. (Moreover the copepod (or euphausiid) may at least be partly 

 a carnivore and thus be well down the food-chain that begins with the plants of the surface-waters. 

 Bogorov (1958) mentions that deep-sea copepods, such as Pareuchaeta, and Bathycalanus feed 

 on radiolarians and other small animals. Some deep-sea decapods and amphipods are also 

 carnivores.) 



Despite their reduced economy of life, or rather because of it, these fishes are not to be regarded as 

 ' misfits '. Cyclothone microdon is perhaps the most ubiquitous fish in the ocean, its range extending 

 from subarctic to antarctic regions, although, the catches of mid-water nets are likely to give a ' false ' 

 impression of its abundance owing to its reduced activity. Off Bermuda, Beebe (1937) took eight 

 times as many Cyclothone as lantern fishes, but the ocean must support greater numbers, certainly 

 a greater weight, of the latter. (Cyclothone spp. are less easily recognized from a bathyscaphe than 

 lantern fishes ; thus experienced observers usually see more of the latter.) 



Besides the regression of the muscular and skeletal systems, we have seen that the eyes and light- 

 organs of these deeper living fishes are small in comparison with those of the species living above 

 them. I have also implied that this economy of tissue must be linked to reduced development of the 

 nervous and excretory tissues, a correlation which will also apply to the circulatory system. (A study 

 of the alimentary system would also be of interest, for Nusbaum-Hilarowicz (1920) found the in- 

 testine of Cyclothone to be particularly simple in structure.) 



Considering for the moment the eyes, if submarine sunlight is the only controlling factor in their 

 development, the benthic fishes of the abyssal plain would be expected to have markedly regressed 

 visual organs, but this expectation is by no means realized. As Hjort (Murray and Hjort, 191 2) wrote: 

 ' But if it be the case that the size of the eyes in pelagic fishes decreases vertically with the decreasing 

 intensity of light, how can we explain the fact that the bottom fishes, like Macrurus ( = Nematonurus) 

 armatus, living in abyssal depths possess large and apparently well-developed eyes.' Perhaps this 

 should be called Hjort's Paradox. However, the eyes of the abyssal macrourids and brotulids do tend 

 to be relatively smaller than those of their relatives living over the continental slopes (Marshall, 



