I 



SHORT BIGEYE IN WESTERN NORTH ATLANTIC 



117 



dividuals, as discussed earlier. Despite a wide 

 range of eye diameters, particularly in the larv^ae 

 (about 5 nnn. or less), in which they may be 

 influenced by physical distortion of the specimens, 

 the eyes of pelag:ic specimens generally are 

 relatively smaller than those of holtoin-living 

 ones. Transforming bottom forms plot generally 

 lower on the graph than pelagic forms of the same 

 size. Whether this increase in relative eye 

 size with change of habitat is cause or effect is 

 not known, though T favor effect (or the need for 

 a larger eye in the dimly lit zone which the adult 

 usually inhabits as compared with the brightly 

 lit surface waters occupied by the larvae and 

 prejuveniles). It is not clear what processes 

 trigger the descent of tlie fish to the bottom. 



These offshore-caught forms were taken at 

 or near the surface (both the larvae and pre- 

 juveniles), except for a group taken by a mid- 

 water trawl set at 24 fathoms over 1,300 fathoms 

 (table 1). These individuals, well north of the 

 range of the adult (fig. 1), might have been 

 seeking the bottom. Perhaps the stimulus in 

 this case is light — with the fish changing from 

 a positive to a negative phototropic response. 

 This might also initiate the increase in relative 



eye size, to compensate for the reduced illumi- 

 nation. A change in dietary requirements and 

 the pursuit of food also may influence the change 

 of habitat in that the new requirements are met 

 oidy in the bottom habitat. 



The diftVreiitial rate of growth (previously 

 noted) related to habitat is well illustrated by the 

 material examined. The largest actual or in- 

 ferred pelagic forms (up to 58 mm.) were taken 

 in northern Atlantic waters (table 1), which 

 suggests they continued to grow without changing 

 form as they were carried north by the Gulf 

 Stream. The large pelagic form illustrated in 

 figure 2, from Massachusetts, is one of these; 

 while the small, nearly transformed bottom form 

 is from about 10 fathoms, off St. Augustine, 

 Florida, or well within the range of the adult. 



The smaller pelagic forms taken in northern 

 Atlantic waters could easily have been spawned 

 in the northern part of the range of the adult 

 and not yet had time to reach a large size. Pre- 

 juveniles taken in southern latitudes in the 

 Atlantic (tabic 1) are all small (maximum size 

 about 20 mm.), suggesting that they had been 

 spawned from nearby adults or had found their 

 wav to an inshore bottom to transform rather 



STANDARD LtNGTH (MMJ 



Figure 5. — Relation of ratio of eye diameter in standard length to standard length in P.ieudnpriacnnthus altus. (Untrans- 

 forined pelagic larvae and prejuveniles represented by open circles, untransformed bottom-living prejuveniles by 

 open squares, pelagic transforming prejuveniles by half closed squares, and transformed juveniles and adults by 

 closed squares.) 



