MAEGULIES: VULNERABILITY AND SENSORY DEVELOPMENT OF WHITE SEABASS 



Figure 8. — Cross-section through the optic 

 tectum of a 9.2 mm SL white seabass larva. The 

 tectum is bilayered at this stage, with an inner 

 stratum periventriciilare (SPV) and an outer 

 stratum zonale (SZ). The SZ is thickening and 

 exhibits increasing numbers of neurons migrating 

 from the inner layer. The torus longituduialis (T) 

 is quite prominent by this stage, x 100. 



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2 4 6 8 10 12 14 30 

 LARVAL STANDARD LENGTH (mm) 



length (Fig. 9B). Growth and differentiation of 

 the SZ appeared to involve cell migi-ations from 

 the inner SPV with a progi'essive increase in the 

 SZ:SPV ratio (Fig. 9A). This ratio increased 

 rapidly at first feeding (~3-4 mm SL), stayed 

 constant at lengths of 4.0-7.5 mm SL, and then 

 increased again from 7.5 mm to early juvenile 

 stages. 



Another important event in the ontogeny of 

 the optic tectum involved the development of the 

 torus longitudinalis (TL). This structure first 

 appeared histologically at a larval length of 

 6.8-7.0 mm SL (Fig. 9A). Seen in transverse 

 section, the TL developed as a teardrop-shaped 

 structure with a wide area of contact dorsad with 

 the optic tectum and ventrad with the epithala- 

 mus (Fig. 8). The TL continued to gi'ow with 

 ontogeny, and in older larvae and early juveniles 

 the TL e.xhibited an increasing number of 

 neuronal projections to the cerebellum. 



FiorRE 9. — Development of the optic tectum of larval white 

 seabass as a function of larval length. A. Change in the ratio 

 of the outer layer/inner layer, T.L. is the first appearance of 

 the torus longitudinalis. B. Solid circles represent total 

 thickness of tectum, error bars are 2 x SE, regression equa- 

 tion is Y = -48.6 + 38.8 SL - 1.35 SL^ (n = 64, r^ = 0.97), 

 where Y = total width of tectum (jjim) and SL = larval 

 standard length (mm); open circles represent thickness of 

 outer layer, error bars are 2 x SE, regression equation is Y 

 = -33.8 -I- 20.3 SL - 0..56 SL^ (« = 60, r = 0.97). where 5" 

 = width of outer layer (fim) and SL = larval standard length 

 (mm). Values for a 30 mm SL juvenile are given for compari- 

 son to larvae. 



545 



