Toole et al. 



Otolith microstructure, microchemistry and early life history of Microstomus paaficus 



747 



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DISTANCE (MICRONS) 



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Figure 20 



Relationship between Sr/Ca ratios and distance from central primordium 

 towards edge in Dover sole, Microstomus pacifieus, otoliths. Distances are 

 not comparable between otoliths because transects were not oriented iden- 

 tically. Approximate dates are indicated, based on counts of increments 

 between microprobe marks and the otolith edge. Dashed lines represent 

 2-o measurement error, calculated as in Toole and Nielsen ( 1992 1. lAl 65.7-mm 

 Stage-4 larva collected on 17 March 1990. (B) 88.6-mm Stage-5 juvenile 

 collected on 17 March 1990. This fish had one annulus forming at the 

 otolith edge, corresponding to the second Sr/Ca peak. 



associated with a habitat transition from warm sur- 

 face waters to colder midwater depths during the lar- 

 val to juvenile transition. Dover sole could experience 

 a similar temperature change at the time of AP forma- 

 tion, as discussed below. 



Because growth from the central primordium was 

 enclosed by growth from AP in otoliths of most Stage-3 

 larvae and because new AP were never observed distal 

 to the point of enclosure, the last AP generally formed 

 during either Stage 2 or Stage 3. The presence of very 

 recently formed AP (<7 days) in some Stage-3 larvae 

 and the possibility that additional AP could form in 

 22% (benthic) to 297c (pelagic) of unenclosed Stage-3 

 larval otoliths suggest that formation of the last acces- 

 sory primordium may have occurred during Stage 3, 

 rather than during Stage 2, in some individuals. Be- 

 cause Stage-3 larvae with unenclosed otoliths had lower 

 SINT/BD1A ratios than Stage-3 larvae with enclosed 

 otoliths, it appears that when the last AP does not 

 form during Stage 2, it forms early in Stage 3. Thus, 



completion of AP formation may occur dur- 

 ing the final stages of eye migration, when 

 Dover sole larvae are still pelagic, or after 

 completion of eye migration, shortly after 

 Dover sole first settle to the bottom. In vir- 

 tually all cases, it occurs before the intesti- 

 nal loop begins to extend into the second- 

 ary body cavity. 



Occasional formation of the last AP af- 

 ter settlement in Dover sole corresponds 

 to observations of AP formation in other 

 species. Alhossaini et al. (1989) described 

 unenclosed otoliths of benthic plaice lar- 

 vae, suggesting that AP formation may not 

 be complete at settlement in that species. 

 Karakiri et al. (1989) noted that the for- 

 mation of accessory primordia "accompa- 

 nies" the transition to a bottom-dwelling 

 mode in plaice. If the formation of AP in 

 California halibut "after metamorphosis" 

 (Kramer, 1991) means "after eye migration 

 is complete," then AP formation likely con- 

 tinues after settlement, because settling 

 behavior of California halibut begins prior 

 to initiation of eye migration (Gadomski et 

 al., 1992). 



Increased prominence of stress checks 

 following enclosure may have been caused 

 by a change in the otolith growth plane 

 relative to the sectioning plane or by in- 

 creased intensity of environmental cues 

 that may induce check formation. 

 Campana (1984b) correlated 15-day cycles 

 of increment width and contrast in otoliths 

 of intertidal starry flounder with similar 

 cycles of tidally-induced temperature variation. 

 Rosenberg (1982) described 14-day cycles of check 

 formation in English sole iParophrys vetulus), which 

 first formed "at the beginning of the metamorphic 

 period." An examination of Figure 2 of Rosenberg 

 ( 1982) suggests that these checks were most obvious 

 proximal to accessory primordia, as in Dover sole. 

 The mean 15.3-day cycle of check formation observed 

 in Dover sole may also be related to tidally induced 

 temperature variation and larvae may be more 

 strongly affected by these patterns following settle- 

 ment, resulting in more prominent checks distal to 

 the point of enclosure. However, the complex nature 

 of rotational tidal currents in the vicinity of Dover 

 sole nursery grounds precludes an evaluation of this 

 hypothesis at present. 



After accessory primordium formation, enclosure of 

 growth from the central primordium, and formation of 

 prominent checks, subsequent changes in larval otolith 

 morphology were continuous, rather than discrete. 



