io4 



DISCOVERY REPORTS 



It will now be clear that the body-tissues of Cyclothone are much reduced by comparison with those 

 of Maurolicus. A striking indication of the reduced economy of life in Cyclothone is (again) found in 

 the form of the kidneys, which have been studied by Owen (1938). One of his drawings is reproduced 

 in Text-fig. 45. The entire system consists of two tubules only, 

 which run side by side down the body-cavity and unite at the 

 bladder. This must surely be one of the simplest kidneys in 

 any adult fish of this size. In Maurolicus the kidneys are much 

 more voluminous and have a compact appearance like those of 

 shallow-water teleosts. 



These detailed studies of related species may now be used 

 as a guide (see Table 8) to a more general survey of the 

 pelagic fishes living in the upper and lower reaches of the 

 deep-sea. Here the fishes with swimbladders, species belonging 

 to the families Gonostomatidae, Sternoptychidae, Astronesth- 

 idae and Myctophidae and having centres of concentration 

 between depths of about 200 and 1000 m. will be compared 

 with those of the groups Lyomeri and Ceratioidea, which have 

 no swimbladder and are most common well below the 1000-m. 

 level. 



When these two assemblages are contrasted, the differences 

 between them prove to be very like those between the closely 

 related species that have just been considered. The facts may 

 best be summarized in Table 8. The observations under the 

 Lyomeri and Ceratioidea are based on the work of Nusbaum- 

 Hilarowicz (1923), Bertin (1934) and Tchernavin (19470,6), 

 Waterman (1948), and Bertelsen (1951) and also on personal 

 observation. 



A few observations may be added concerning the ceratioid 

 angler fishes. I have looked at females of Melanocetus murrayi, 

 Neoceratias spinifer and Danaphyrne. In all three, the first gill- 

 arch is devoid of gill-filaments. Drawings of filaments from 

 the second arch are shown in Text-fig. 46. In Neoceratias 

 and Danaphryne there are no more than 50-60 gill-filaments 

 in each gill chamber. 



In the deeper living fishes, every possible economy of tissue has been developed. Compared with 

 the species that live above them and within swimming distance of the productive surface-waters, they 

 seem degenerate. It might be argued that the most trenchant economy has been effected by the 

 evolution of dwarf-males in the Ceratioidea. During the free-living existence it must take much less 

 food to maintain a male than its partner. However, there are other factors to be considered (Bertel- 

 sen, 1951). 



The ceratioid angler fishes, Cyclothone spp., and probably the Lyomeri (Eurypharynx has a lepto- 

 cephalus larva), spend their larval life in the plankton-rich surface-waters. Evidently there is not 

 sufficient food of a suitable nature to sustain the larvae at the depths occupied by the adults. How- 

 ever, a good start in life must counterbalance the hazards of this type of life-history. 



It is now well-known that the waters below 1000 m. support sparse populations of planktonic 

 animals in comparison with the waters near the surface. The relevant papers and additional evidence 



V-B 



Text-fig. 45. Diagrammatic reconstruction 

 of one of the two tubules forming the kidney 

 of Cyclothone (after Owen, 1938). ( x 7-5.) 

 B, bladder; G, glomerulus; Pi, Pz, first 

 and second parts of proximal convolute; 

 T, terminal segment. 



