PELAGIC AND BENTHIC FISHES, SWIMBLADDER, ECONOMY OF DEEP-SEA LIFE 101 



microdon, acclinidens and obscura) are centred at levels of iooo m. and below. Of the forty-four 

 recorded specimens of obscura, only three came from nets fished above a depth of 2000 m. (Grey, 

 1956), while C. microdon is common between levels of 1000 and 2000 m. (Murray and Hjort, 1912). 

 We may thus compare the three species braueri, microdon and obscura, as they seem to occur at 

 increasingly deeper levels. 



During the larval phase, which is passed in the surface waters, a gas-filled swimbladder is present 

 in the species braueri and microdon, but the organ regresses and becomes invested with fat after 

 metamorphosis (pp. 65-66). Knowledge of the life-history of obscura matches its trivial name. 



C. braueri and C. microdon grow to about the same size (70 mm.), but there is some evidence that 

 the regression of the swimbladder occurs at a later stage in the former. At a length of from 30 to 

 35 mm., the swimbladder of braueri is beginning to regress but a cavity is still present, while in 

 C. microdon of this size regression is almost or quite complete. 



Besides this difference, which requires closer study, the degree of development of the eyes and 

 light-organs forms a graded series in the three species. This was first appreciated by Brauer (1908) and 

 also considered by Hjort (Murray and Hjort, 1912). 1 It will be seen that the eyes are largest in 

 braueri, as compared with microdon, which has medium-sized eyes, while those of obscura are extremely 

 small. The same is true of the photophores. It should be added that Brauer regarded the light organs 

 of C. obscura to be rudimentary and found some of them to be absent. 



Having no specimens of C. obscura, I have only been able to compare the brains of braueri and 

 microdon, which came from individuals of about the same size (25 mm.). Drawings of these appear in 

 Text-fig. 43. Measured from the tip of the forebrain to the end of the cerebellum, both brains have 

 much the same length (2-0 mm.). But the optic lobes and, evidently, the cerebellum of braueri, are 

 considerably larger, the width of the former being i-i mm. in braueri and 07 mm. in microdon. 



Comparison of the gill-system of these two specimens is also revealing. In both species, filaments 

 are borne on the lower arms of the first, second and fourth arches, but on only the forward half of 

 this part of the third arch. In C. braueri the filaments are both larger and more numerous, there being 

 12 per mm. on the first arch, compared to nine in microdon. Reference to Text-fig. 43 will show the 

 difference in filament size. 



To summarize, in these three species of Cyclothone, the deeper the living-space, the less the develop- 

 ment of the eyes, photophores, brain and gills. Since the greater part of the populations of C. braueri 

 live above the threshold of light, while both microdon and obscura are most common below this level, 

 the relatively large eyes and photophores of braueri might seem to be related to an existence in the 

 twilight zone. But this factor cannot be invoked to account for the marked difference in eye and 

 photophore development of microdon and obscura. Some factor (or factors) related to the deeper 

 living-space of obscura would seem to be involved. 



To conclude these studies of related fishes, two species of the family Gonostomatidae will be con- 

 sidered, Maurolicus muelleri and Cyclothone microdon. Maurolicus is most abundant above a level of 

 500 m. (Koefoed, 1958) while C. microdon, as already indicated, is mostly to be found below 1000 m. 

 The first species has a well-developed gas-filled swimbladder, while in Cyclothone this organ is 

 obliterated in the adult phase. 



Drawings of these two fishes may be found in Text-fig. 44. The much greater development of the 

 eyes and light-organs of Maurolicus will be immediately obvious. The myotomes of this fish are also 

 much more compact and extensive. In a well-nourished Cyclothone microdon, the myotomes appear 

 at first sight to be more voluminous than they actually are. Much of the space between the skin and 



The species that Brauer took to be Cyclothone signata Garman proved to be a separate species, which was called 

 C. braueri, by Jespersen and Taning (1926). 



