MARGULIES: VULNERABILITY AND SENSORY DEVELOPMENT OF WHITE SEABASS 



floor of the retina and anteriad to the lens by 

 means of an extremely thin ligament. In older 

 larvae and juveniles, the lens retractor became 

 bipartite, presumably aiding in movements of 

 the lens posteriad and/or ventrad. 



The outer nuclear layer (ONL) of the retina, 

 containing the photoreceptive cell nucleii, was 

 single-tiered until larvae reached a length of 

 7.0-7.5 mm SL. At this stage, compact and 

 darkly staining mitotic bodies began to appear in 

 the ONL (Figs. 5, 6A); these mitotic bodies ap- 

 peared to be rod precursors. At a larval length of 

 10.5 mm SL, double cones were present in the 

 photoreceptive layer and the ONL was multi- 

 tiered. At 12.5 mm SL the ONL nucleii were 

 multilayered and a clear retinomotor response 

 was evident, indicating the presence of rods 

 (Figs. 6A, 7). 



During ontogeny the density of cone cells in 

 the photoreceptive layer decreased linearly, 

 while lens diameter increased linearly (Fig. 6B). 



Visual acuity improved nonlinearly with larval 

 size, changing from 91 minutes of arc in first- 

 feeding larvae to 26 minutes in metamorphosing 

 fishes (Fig. 6A). The period of most rapid im- 

 provement in acuity occurred from approxi- 

 mately 4-9 mm larval length. 



Development of the Optic Tectum 



In yolk-sac larvae, the optic tectum of the 

 mesencephalon (midbrain) was composed of 

 undifferentiated matrix cells. During early feed- 

 ing stages, the tectum differentiated into an 

 inner, neuronal stratum periventriculare (SPV) 

 and an outer, fibrous stratum zonale (SZ) (Fig. 

 8). This bilayered configuration persisted 

 throughout the larval stages, with the entire 

 tectum and the outer SZ thickening during on- 

 togeny (Fig. 9A, B). The period of most rapid 

 tectal differentiation during the early hfe stages 

 occurred from approximately 3 to 9 mm larval 



Jt 





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Figure 4. — Sagittal section of the eye of a 4.8 mm SL white 

 seabass larva showing the position of the lens retractor 

 muscle (LR). (A) indicates the anterior direction, x 100. 



Figure 5. — Cross-section of the retina of an 8.0 mm SL 

 white seabass larva showing several dark mitotic bodies (M) 

 in the outernucleai- layer. x250. 



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