BOTTOM FISHES 385 



Other fishes have become adapted to live on, rather than in, the sub- 

 strate, most of these being dependent for concealment upon their incon- 

 spicuous shapes and colorations. The bottom elasmobranchs — the skates 

 and rays, the sawfish (Pristis) , the guitar-fish (Rhinobatos) , and the ray- 

 like shark Squatina — have a depressed form. A consequence of their dor- 

 soventral flattening has been an equal rotation of their two eyes so that 

 they look more or less upward — in Squatina, for example, the visual lines 

 slant upward at 45° angles. A number of teleosts have evolved the 

 depressed shape also, the angler-fish Lophius for example, and to a less 

 degree the stargazers (Uranoscopidae) ; but most flat-lying teleosts are 

 among the more than 500 species of the flatfish group : 



In the flatfishes, the laterally-compressed animal has simply lain down 

 on its side (right or left, according to species) during its individual 

 development. The new under surface remains unpigmented and loses its 

 eye, by migration over the top of the head (or even through it) , to the 

 new upper or eyed side. In the more specialized flatfishes the mouth tries 

 its best to twist too, but not very successfully, so that it works largely 

 crosswise. The begirmings of the flatfish habit can be seen in some sea- 

 perches which habitually rest on their sides, the families Serranidae and 

 Labridae particularly. In one primitive tropical flounder (Psettodes 

 erumei) , the eye from the future blind side stops at the crest of the head, 

 never moving completely over onto the eyed side to join its non-migra- 

 tory fellow. Unlike other flounders, individual Psettodes may end up 

 lying on either the right side or the left — that is, either eye may be elected 

 to migrate. The dorsal fin commences behind the head in this species, 

 whereas in a perfected flatfish it waits until the migration of the eye has 

 taken place, and then grows forward — cutting off the eye's retreat, so 

 to say. 



A topside position and approximation of the eyes brings with it an 

 advantage and a disadvantage, to either or both of which various fishes 

 have responded adaptively. The advantage is the opportunity to secure 

 an exceptionally broad binocular visual field, especially in an upward 

 direction, with a consequent improvement of space-perception. The dis- 

 advantage is that the eyes are subjected to dazzlement by the vertically 

 incident sunlight. 



Some upward-looking bottom fishes have met the problem of dazzle- 

 ment by placing the eyes so that they can look horizontally, permanently. 

 The dorsal binocular field may then be largely sacrificed, of course, as 

 in Manta. Others, such as Lophius, are able to swing the eye downward 



