Toole et al Otolith microstructure, microchemistry, and early life history of Microstomus pacificus 



739 



Structural patterns 



Growth from central primordium Two prominent areas 

 were seen within the field of growth emanating from 

 the central primordium. 



Clear central area Stage- 1 larval otoliths were 

 translucent and hemispheric (appearing nearly circu- 

 lar in sagittal section), with all growth emanating 

 from the central primordium (Figs. 1 and 6). The left 

 eye had previously migrated to the dorsal ridge of the 

 cranium in all Stage-1 larvae examined. The diameter 

 of Stage-1 larval otoliths ranged from 0.10 to 0.30 mm 

 and the number of increments ranged from 77 to 229 

 (Table 3). When fully formed in Stage-3 larval otoliths, 

 this clear central area was 0.24- 

 0.49mm (mean=0.39, n=60) and 

 contained 187-230 increments 

 (Table 4). Mean increment width 

 in the clear central growth area 

 ranged from 1.22 to 1.56 um, and 

 increment widths nearer the cen- 

 tral primordium were approxi- 

 mately 0.5 um, suggesting that, 

 when counting increments, under- 

 estimation may have occurred ow- 

 ing to resolution limitations of light 

 microscopy (Campana et al., 1987). 



A structural feature of some 

 Stage-1 larval otoliths was the 

 presence of one or two conical cavi- 

 ties, which radiated towards the 

 anterior and posterior edges 

 (Fig. 7). Each cavity was enclosed 

 laterally and medially and was 

 open at the anterior or posterior 

 end. The anterior cavity may be 

 continuous with the sulcus, which 

 was visible on one Stage-1 larval 

 otolith. 



Opaque central area An opaque area surrounded 

 and was continuous with the clear central area in all 

 Stage 3-5 larval otoliths (Figs. 1 and 8, A-D). Incre- 

 ments in this region were wider than increments in 

 the clear central area ( 1.6-1.8 um) and there was higher 

 contrast between continuous and discontinuous zones 

 within increments. Continuity of increments was in- 

 terrupted by the conical cavities described previously 

 and by accessory primordia. The number of increments 

 in this area ranged from 46 to 95 (Table 4). 



Growth from accessory primordia We recognized two 

 zones of growth from accessory primordia, distinguished 

 by the point at which growth from the central primor- 



Figure 6 



Sagitta from 20.6mm Stage-1 Dover sole, Microstomus pacificus. larva collected on 

 1 April 1990 at NMFS Station 9003-35. Estimated number of increments was 103 

 (Table 3). Magnification = 1000 x. 



