PELAGIC AND BENTHIC FISHES, SWI MBL ADDER, ECONOMY OF DEEP-SEA LIFE m 



But Hjort's Paradox is still unresolved. I have mentioned that certain of the brotulids are ex- 

 ceptional in having very regressed eyes. Aphyonus gelatinosas is such a species and it lacks a swim- 

 bladder. The tissues of its body are also much reduced by comparison with another brotulid of about 

 the same size, Mixonns laticeps, a species with a swimbladder. This brotulid has an elongated tapering 

 body, sheathed with scales and equipped with well-knit myotomes. The skeleton is quite firm. The 

 eyes are small, but well-formed, with a wide pupil and a large lens. In the type specimen, which 

 measures 125 mm. in standard length, the horizontal width of the eye-ball is about 4-0 mm. 



As indicated by its specific name, Aphyonus has a gelatinous appearance. It is semi-transparent 

 and colourless, except for deep-set points of pigment that represent the remains of the eyes. The skin 

 is loose and scaleless, while the skeleton and myotomes are much reduced, the notochord being 

 persistent. 



While the type-specimen of Aphyonus gelatinosus is bulkier and somewhat longer (standard length 

 130 mm.) than that of Mixonus laticeps, it has a much simpler gill-system. On the first arch there are 

 about fifty filaments with an average length of about 1-5 mm., the second arch bears about forty 

 filaments, while there are only two to three on the fourth arch. Mixonus laticeps has somewhat shorter 

 filaments but there are at least a hundred on the first arch and about eighty on the second. Moreover 

 the filaments carry about twenty-five lamellae per millimetre, about twice as many as those developed 

 in Aphyonus. 



These contrasts are very like those already shown to exist between pelagic fishes from the upper 

 and lower oceanic reaches. Perhaps Aphyonus is one of the deeper-living pelagic fishes. This genus, 

 together with Barathronus, Sciadonus and Leucochlamys are placed by Nybelin (1957) in a new sub- 

 family Aphyoninae, ' ... as they show many mutual similarities and in many respects differ from other 

 Brotulids. As common characteristics may be mentioned a comparatively small body size, a thin 

 loose scaleless skin without or with only very feeble pigmentation, strongly reduced eyes and bone 

 tissues, and a more or less persistent notochord; an opercular spine, characteristic of the typical 

 Brotulids, is lacking.' Moreover, they have very reduced gills and from Nybelin's figures of Bara- 

 thronus erikssoni and Sciadonus kullenbergi, it is clear that a swimbladder is absent. As the cleft of the 

 mouth runs obliquely upward in Barathronus, Nybelin has suggested that this genus is pelagic in 

 habit. The only direct evidence is the rather young specimen of Barathronus parfaiti that was found 

 by Legendre (1934) in the stomach of a long-finned tunny (Thunnus alalunga). 



I suggest that the entire subfamily consists of deep-dwelling, bathypelagic fishes. If this proves to 

 be so, the common characters might not be indicative of genetic affinity, but rather of convergent 

 adaptation to their food-poor environment. {Barathronus and Sciadomus are viviparous, while the 

 ovaries of the type of Aphyonus gelatinosus contain many eggs, suggesting oviparity.) The deeper 

 mid-waters could even be regarded as a refuge for species that have been edged-out of the two main 

 feeding grounds of the ocean, the waters under the interface between sea and atmosphere and those 

 near the interface between sea and sediments. 



If the Aphyoninae are not benthic fishes, the abyssal fauna consists almost entirely of species with 

 small to moderately large eyes. 1 Now Denton and Warren (1957) suggest there are two main factors 

 related to the size and development of the eyes in deep-water fishes. A fish living in the twilight zone 

 is exposed to a large field of light, and here the size of the eye matters less than the relative proportion 

 of the dioptric parts, a wide pupil being necessary. To perceive spots of luminescence efficiently a 

 large collecting pupil is required; the bigger the eye the better. These conditions are met in fishes 



1 Typhlonus nasus is a blind brotulid and might be an exception, but it has a small, thin-walled swimbladder. The bones 

 are soft and flimsy and the myotomes excessively thin. The eyes of Ipnops are not regressed but curiously modified. Leucicorus 

 htsewsus, which looks more like a bottom-dwelling brotulid, seems to be an exception (see also Marshall, 1954). 



