MARGULIES: VULNERABILITY AND SENSORY DEVELOPMENT OF WHITE SEABASS 



Figure 11. — Composite maps showing the location of free neuromast organs during development of white seabass larvae. Larval 



outlines were modified from those of Moser et al. (1983). 



some of the recruiting neuromasts were being 

 enclosed by canals on the head and trunk; some 

 existing free neuromasts were being incor- 

 porated in canals as well. At juvenile meta- 

 morphosis, the three major branches of the head 

 canals, as well as the lateral hne on the trunk, 

 were fairly distinct (Figs. lOA, 11), forming the 

 precursor to the juvenile and adult lateral Hne 

 system. 



Swimbladder Development 



In yolk-sac larvae, the swimbladder appeared 

 as a collapsed sac dorsad to the yolk sac, while in 

 first-feeding larvae it was still partially flattened 

 and situated dorsad to the anterior digestive 

 tract. Timing of complete swimbladder inflation 

 was variable; most larvae exhibited full inflation 

 at lengths of 4.5-5.5 mm SL. In larvae >4 mm, a 

 pneumatic duct was present in the anterodorsal 

 area of the swimbladder. It was not clear 

 whether swimbladder inflation occurred by way 

 of air-gulping at the surface or by gas secretion 

 internally. 



DISCUSSION 



Neurosensory Basis For Avoidance 

 Responses 



The overall probability of white seabass larvae 

 escaping predatory attacks seems highly depen- 

 dent on the size of the larvae, the type and qual- 

 ity of sensory input being integrated by the 

 larvae, and the type of predator encountered 

 (Fig. 12). Yolk-sac larvae have nonfunctional 

 eyes, a small number of free neuromast organs 

 on the head and trunk, and a noninflated swim- 

 bladder. At first feeding (~3.2 mm SL), the eye 

 becomes functional, but visual acuity is poor; the 

 pure-cone retina limits peripheral vision and mo- 

 tion detection, and no accommodation is possible 

 (since there is no lens retractor muscle). There is 

 strong correlative evidence that increases in 

 numbers of free neuromasts and improvements 

 in the visual system are responsible for the 

 improved avoidance responses observed in pre- 

 dation trials. In larvae <4.5 mm SL, predator 

 detection is most likely a function of mechano- 

 reception by free neuromasts, since visual 



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