ii2 DISCOVERY REPORTS 



from the upper iooo m., but what of the abyssal species? Their small to moderately large eyes are 

 equipped with wide pupils and large lenses, and they have presumably retained their visual powers 

 because there is luminescent light to be seen (some deep-sea bacteria are luminescent). Furthermore, 

 at least some of them start life in the upper, lighted layers of the sea. (The same is true of the deep- 

 dwelling pelagic species.) A flash of light may mean a euphausiid meal for a Cyclothone, or signify 

 a predator. While the eyes of some of these fishes do not properly fulfil the second requirement, they 

 have gone some way to meet it. However, the loss of the eyes in some pelagic fishes, that exist below 

 the iooo-m. level, is compensated for by a marked development of the lateral-line organs on the head. 

 Aphyonus has large canals and so has Ditropichthys storeri, which may well belong to this fauna (see 

 also Marshall, 1954). 



These considerations may seem far removed from swimbladder problems. But they have been set 

 down because I once thought that the regression of the eyes and swimbladder might be linked to the 

 disappearance of the pseudobranch. Myctophids, which have well-developed eyes and swimbladders 

 also have large pseudobranchs. In Gonostoma bathyphihim and Aphyonus gelatinosus, with small or 

 regressed eyes and no swimbladders, the pseudobranchiae are absent. Now Copeland (1952) found 

 that removal of these organs in Fundiilus inhibited the inflation of the swimbladder. Furthermore, the 

 blood to the choroidal gland of the eye passes through the pseudobranch in teleosts, although the 

 functional relationship is not clear (Barnett, 1951). However, despite a thorough search under the 

 skin of the gill-chambers of Astronesthes niger and Gonostoma denudation, fishes with swimbladders 

 and large eyes, I was unable to find any trace of pseudobranchiae. 



We have seen that the loss of the swimbladder in pelagic fishes of the deeper, mid-oceanic waters 

 is not related to the pressure factor, but rather to their harsh biological environment, one that seems 

 to be lacking in the potential energy required to maintain the extra tissues associated with an internal 

 float. Is the regression of the eyes in such fishes as the Aphyoninae, the Monognathidae and Cyclo- 

 thone obscura also to be explained on this basis? Is this a further economy of living tissue? 



In dealing with blind cave fishes, Walls (1942) points out that the elimination of the eyes is but 

 a small economy, most of the energy released from food going into motor and secretory activity. 

 However, there is an associated saving of tissue in the central nervous system in the form of a much 

 reduced optic tectum (see Marshall and Thines, 1958). On the other hand, Heuts (1951) maintains 

 that the slow growth-rates, low metabolism and regressed tissues (including the eyes) in Caecobarbus 

 are adaptations to the limited food resources of the environment. He goes on to suggest that this is 

 true of all cave fishes. But this is too sweeping a generalization, as Breder (1953) and Cahn (1958) have 

 shown. Some cave fishes, such as Anoptichthys jordani, may have good supplies of food and they are 

 most active animals. An Anoptichthys, which has a well-developed swimbladder, carries much more 

 muscle than a Cyclothone microdon of the same size. Apart from these considerations, caves differ 

 from the deep mid-waters of the ocean in being totally dark. 



To return to blind deep-sea fishes, Heuts's concept of the regression of the visual tissues seems 

 more applicable to their case than to the condition found in some cave fishes. While economy in the 

 form of reduced visual and nervous tissue is a small item in the energy balance sheet of an Anopt- 

 ichthys, it might well be critical for an Aphyonus. As if to compensate for its loss of vision, Aphyonus 

 has a well-developed ' Ferntastsinn ' sense in the form of neuromast organs on the head. Do these 

 require less energy to maintain than eyes? At all events, they require smaller centres in the brain. 

 Here the problem may be left with the thought that the food-factor may well be critical, but it may 

 not be the only factor involved. 



We are left with certain concepts, which are partly summarized in Text-fig. 47. Living in the 

 ocean at depths above 1000 m. are several hundred species of pelagic plankton-feeding fishes with 



