FISHERY BULLETIN: VOL. 87, NO. 3, 1989 



Ontogeny of Mechanoreceptors 

 Neuromast Development 



Staining with Janus Green provided somewhat 

 variable results, in that not all white seabass 

 larvae took up the stain equally well. Some 

 neuromast organs stained better than others, 

 but the use of 6-8 fish per larval size class 

 seemed to provide good composite patterns of 

 neuromast formation. 



At hatching, larvae had 5 or 6 pairs of neuro- 

 masts on the head and 5 or 6 pairs extending 

 along the trunk (Fig. lOA, B). All of these 

 organs appeared to be free neuromasts (not en- 

 closed by canals). Each free neuromast consisted 

 of a basal, naked neuromast organ attached 

 apically to a cyhndrical, gelatinous cupula. The 



only other stained structures were the nasal 

 pits. 



The total number and patterned formations of 

 free neuromasts increased with ontogenetic de- 

 velopment (Figs. 10, 11). At first feeding, larvae 

 had 6-8 neuromasts on the head and 6 or 7 on the 

 trunk. During development, new head neuro- 

 masts appeared first in the supraorbital and in- 

 fraorbital areas, while on the trunk they re- 

 cruited midlaterally (Fig. 11). The period of most 

 rapid addition of free neuromasts was in the 

 larval size range of 4-9 mm SL. During the early 

 postflexion stage (—7-10 mm SL), free neuro- 

 masts began to form in distinct supraorbital, in- 

 fraorbital, and preopercular (hyomandibular) 

 rows on the head and in a single midlateral row 

 on the trunk (Figs. 10, 11). By 12.5 mm SL, 



Figure 10. — Numbers of free neuromasts developing on larval white 

 seabass as a function of larval length (A) and age (B). En'or bars are 2 

 X SE. I.O.'s are infraorbital and supraorbital rows on the head; P.O. is 

 the preopercular or hyomandibular row on the head; and L.L. is the 

 lateral line. Values are based on staining and should be considered 

 appro.ximate only. 



546 



