398 ADAPTATIONS TO MEDIA AND SUBSTRATES 



vertebrate known whose eyes have gone without leaving any trace what- 

 ever : Ipnops murrayi. Even this fish has luminescent areas, lying on the 

 head where the eyes ought to be; and this instance — which could be mul- 

 tiplied — is evidence that the photophores of a given fish are not neces- 

 sarily of the slightest use in facilitating the vision of that particular fish. 



Among the bathypelagic fishes, there are situations from which one 

 can deduce something of the usual history of the eye in a species which 

 invades the depths from the surface. Species which live farther and far- 

 ther down — say, from 300 to 500 meters — tend to have larger and larger 

 eyes and more and better photophores. Such forms are obviously trying 

 to hang onto visual acuity, as well as to increase their sensitivity. Comes 

 a point, however, at which the eye seems to 'quit', and becomes smaller 

 once more. The pupil may continue to increase in relative size, accom- 

 plishing a further increase of sensitivity, but the shrinkage of the eye 

 indicates that these deeply-living fishes have resigned themselves to 

 mere light-sense vision. In some deeply bathypelagic forms such as 

 Cetomimus, Saccopharynx, et al, the eye is vestigial. 



These loose relationships of the eye to depth can be seen among 

 elasmobranchs as well as among the teleosts. The chimaeras of the con- 

 tinental slopes, and Etmopterus, have big eyes with huge pupils and 

 vividly brilliant tapeta lucida. The benthonic shark Laemargus on the 

 other hand has a small eye, and no tapetum; and abyssal rays may be 

 wholly blind. 



Vestigial, blind eyes are more common among the benthonic fishes; 

 and these for the most part have also failed to develop photophores. 

 When abyssal forms do produce light, it is usually only a faint glow due 

 to a special luminosity of the film of slime which covers the body of 

 any fish. 



The deeply benthonic fish is better able to dispense with eyes — and to 

 get along without photophores — than is the bathypelagic one. Life on the 

 bottom is largely life in one plane, and the finding of food by touch and 

 chemoreception is vastly easier. Go far enough along the bottom (if 

 you're a fish), and you're bound to bump into something good to eat. 

 But it does so happen that the most conspicuous of the several benthonic 

 families of deep-sea teleosts, the archaic Coryphaenoidids or grenadiers, 

 have retained their eyes, which are neither exceptionally small nor un- 

 usually large. 



The retention of eyes by the Coryphaenoididae may be of special im- 

 portance — not for these fishes themselves, but for some of their descend- 



