Toole et al.: Otolith microstructure, microchemistry, and early life history of Microstomas paaficus 



749 



in body depth (at lengths of =10-15 mm; Pearcy et al., 

 1977; Markle et al., 1992), based on radii of otoliths 

 from larvae of known lengths. (The radius of a 17-mm 

 Dover sole larval otolith 2 was 38 um and otolith radii 

 of 20.6 and 30.1-mm larvae collected in the present 

 study were 75 and 50 um, respectively (Table 3].) El- 

 evated Sr/Ca levels in the outer portion of the clear 

 central area appeared to form near the termination of 

 Stage 1, when larval morphology does not change 

 (Markle et al., 1992). 



Metamorphic larvae during Stage 2 and early 

 Stage 3 Sr/Ca ratios declined in opaque otolith ma- 

 terial presumably deposited during Stage 2 and early 

 Stage 3. The decline either levelled prior to formation 

 of the last AP or reached a minimum after enclosure of 

 growth from the central primordium (Fig. 20A). Dover 

 sole probably experienced progressively colder tempera- 

 tures during this period as they moved from offshore 

 waters onto the continental shelf and from the water 

 column to the bottom (Pearcy et al., 1977; Landry et 

 al., 1989; Markle et al., 1992). However, some larvae 

 continue to move between the bottom and the water 

 column, perhaps on a diel basis, throughout the meta- 

 morphic period (Markle et al., 1992), which would add 

 further complexity to this pattern. Larvae appeared to 

 lose mass and either did not grow in length or grew 

 very little during this period (Markle et al., 1992). 

 There was, however, a rapid increase in growth of 

 otoliths, as evidenced by formation of accessory pri- 

 mordia with wide growth increments. 



As with Sr/Ca ratios in otoliths of Stage-1 larvae, 

 underlying causes of the observed pattern are unclear. 

 If Sr/Ca ratios in Dover sole responded as in other 

 species, decreasing temperature and slow somatic 

 growth rate would be associated with increasing, rather 

 than decreasing, Sr/Ca ratios. Otolith growth rate may 

 be a better predictor of Sr/Ca ratio than somatic growth 

 rate, and this may become apparent only when the 

 two are uncoupled, as appears to occur during Stage 2 

 (Fig. 19). 



Metamorphic larvae during Stages 3 and 4 Sr/Ca 

 ratios remained low from formation of the last acces- 

 sory primordium to formation of the first post- 

 settlement annulus (Fig. 20B). Opaque otolith mate- 

 rial formed during Stages 3 and 4, when most Dover 

 sole were primarily, if not exclusively, on the bottom 

 (Markle et al., 1992). Dover sole larvae increased in 

 length and weight (Toole and Markle, unpubl. obser- 

 vations) and otolith length increased (Fig. 19) during 

 this time, while bottom temperature decreased and 



-J. Butler. S.W. Fisheries Science Center, NMFS, P.O. Box 271, La 

 Jolla, CA 92038, pers. commun. 1992. 



nutrient levels increased following the onset of up- 

 welling (Huyer et al., 1979; Landry et al., 1989). The 

 correlation of low Sr/Ca ratios with rapid growth cor- 

 responds to observations of Sadovy and Severin (1992); 

 the correlation of low Sr/Ca ratios with low tempera- 

 ture is contrary to observations of Radtke ( 1984, 1989), 

 Townsend et al. ( 1989), and Radtke et al. ( 1990). Sadovy 

 and Severin (1992) suggest that temperature was not 

 the causative factor of Sr/Ca levels in previous stud- 

 ies; rather, it was an indicator of growth rate, which 

 was positively correlated with temperature. For spe- 

 cies such as Dover sole, which reside in upwelling sys- 

 tems where fastest growth occurs at cold temperatures, 

 growth rate appears to over-ride temperature as a 

 predictor of Sr/Ca ratios. 



Postmetamorphic juveniles Microchemistry of otolith 

 material deposited during Stage 5, prior to formation 

 of the first post-settlement annulus. was identical to 

 that formed following enclosure during Stages 3 and 

 4. The first and subsequent annuli were composed of 

 translucent material deposited during the late fall and 

 winter, with a high Sr/Ca ratio. During this period 

 downwelling occurs, resulting in higher water tempera- 

 ture and lower nutrient concentrations than during 

 the spring-summer upwelling season (Huyer et al., 

 1979; Landry et al, 1989). Kreuz et al. (1982) exam- 

 ined scales of adult Dover sole and noted that fastest 

 growth occurred during the cold upwelling season, 

 rather than during the warmer downwelling season, 

 presumably because of higher productivity. The corre- 

 lation of high Sr/Ca ratios and slow growth rate dur- 

 ing formation of the first and subsequent post- 

 settlement annuli is in agreement with observations 

 of Sadovy and Severin ( 1992), as is the continued depo- 

 sition of low Sr/Ca opaque material during subsequent 

 seasons of rapid growth. 



Summary of otolith landmarks 



Several structural and chemical landmarks were 

 correlated with metamorphic stages of Dover sole 

 (Table 6). These landmarks were highly consistent 

 among otoliths and, when coupled with information 

 from daily growth increments, may be useful in deter- 

 mining timing of developmental and behavioral events 

 associated with metamorphosis. 



We conclude that the most consistent landmarks as- 

 sociated with the initiation event of metamorphosis 

 (Markle et al.. 1992), which includes movement of the 

 left eye beyond the mid-dorsal ridge, are the transi- 

 tion from clear to opaque otolith material and forma- 

 tion of the first accessory primordium. Completion of 

 eye migration apparently occurs between formation of 

 the first and last AP, which encompasses a period of at 



