MARGULIES: VULNERABILITY AND SENSORY DEVELOPMENT OF WHITE SEABASS 



as white seabass become more demersal. Growth 

 and stratification of the optic tectum allow for 

 more complex interconnections with other brain 

 centers in teleosts and are essential for the de- 

 velopment of progi"essively more sophisticated 

 behavioral responses (Munro 1984). The torus 

 longitudinalis functions in midbrain integi'ation 

 of proprioceptive information (Groman 1982) and 

 is involved in the control of visual motor patterns 

 (Munro 1984). 



The free neuromasts in fish larvae probably 

 function in detection of differences in velocity 

 between the fish and surrounding water. The 

 incorporation of neuromasts into canals (as oc- 

 curs in older white seabass larvae and early 

 juveniles) probably aids in schooHng or detection 

 of accelerations in water movements caused by 

 other animals, such as predators (Blaxter et al. 

 1983). The increases in numbers and in pat- 

 terned formations of neuromasts with white sea- 

 bass larval size probably improve detection of 

 predator movements and aid in swimming move- 

 ments and proprioception. 



During the early postflexion stage (7.5-10.0 

 mm SL), these improvements in mechanorecep- 

 tive and visual capabihties appear to be directly 

 related to improved detection and escape re- 

 sponses. However, depending upon the type of 

 predator encountered, a significant difference 

 exists in the degree of improvement in avoidance 

 capabihties (Fig. 12). Early postflexion larvae 

 exhibit a modest improvement in evading north- 

 ern anchovy attacks, but display a dramatic im- 

 provement in detecting and escaping the slower, 

 more discontinuous attacks of juvenile white sea- 

 bass. Since some startle responses are elicited 

 even in yolk-sac larvae, it appears that neural 

 motor pathways such as Mauthner-type neurons 

 (Eaton and DiDomenico 1986) are present and 

 functioning during all developmental stages. 

 Just prior to and during the early postflexion 

 stage, larvae undergo notable additions of neuro- 

 mast organs and major improvements in the vis- 

 ual system. Since the outcome of a predator-prey 

 interaction is heavily dependent upon reaction 

 velocity and timing (Webb 1976), the slower, 

 close-range attacks of juvenile white seabass 

 probably allow' more time for detection of sen- 

 sory stimuli from several modalities as well as 

 sensory-motor integration needed for response 

 and escape movements. 



Improvements in visual and mechanoreceptive 

 systems have been implicated in the evasion be- 

 haviors of northern anchovy larvae (Webb 1981; 

 Folkvord and Hunter 1986), while acoustic stim- 



uh detected through the gas-filled otic bullae 

 seem important in the development of startle 

 responses of Atlantic herring larvae (Fuiman 

 1989). The inflation of the otic bullae with gas 

 and the occurrence of a well-developed acous- 

 tico-laterahs system, however, seems to be more 

 characteristic of clupeoid larvae (Fuiman 1989). 

 Although acoustic stimuli or Rohon-Beard 

 (mechanoreceptive) input could also be related to 

 improved evasion responses of white seabass 

 larvae, the observed improvements in the 

 neuromast and visual systems seem to be 

 directly related to the improved avoidance 

 capabilities. 



Larval Vulnerability to Attacks 



Larval size and developmental stage are the 

 most important factors related to larval vulner- 

 abihty in laboratory trials. The type of predator 

 encountered also influences predation rates. 

 However, although white seabass larvae were 

 better able to respond to juvenile white seabass 

 attacks than those of northern anchovy, this did 

 not result in significantly reduced predation 

 rates (in comparisons between predator types) 

 until larvae were >8.5 mm in length. This sug- 

 gests that other factors related to predator de- 

 tection of prey, such as prey morphology, water 

 clarity, and alternative prey abundance, may be 

 as important as predator type in controlling vul- 

 nerability of small white seabass larvae. Physical 

 background and morphological conspicuousness 

 of prey can be important factors controlling pre- 

 dation rates of planktivorous fishes (Vinyard and 

 O'Brien 1976), while relative abundance of al- 

 ternative prey has been shown to have signifi- 

 cant effects on fish consumption rates on small 

 white perch larvae (Margulies in press). 



One disadvantage of laboratory studies is that 

 reahstic encounter rates between larval prey 

 and fish predators are difficult to simulate. Al- 

 though the main purpose of this study was to 

 delineate the developmental basis for larval 

 avoidance behaviors, it is important to recognize 

 the hmitation of predicting total vulnerabihty of 

 larvae based on laboratory trials only. Total vul- 

 nerability to predation is a function of the prob- 

 ability of encounters between predator and prey, 

 the probability of capture of prey and the prob- 

 abihty of attacks by a predator (O'Brien 1979). 

 My data provide reliable estimates of probability 

 of prey capture and, to a lesser degree, probabil- 

 ity of attacks by the experimental fish predators. 

 Encounter rates, however, are affected by a 



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