Toole et al 



Otolith microstructure. microchemistry and early life history of Microstoma pxificus 



733 



ment dates and post-settlement growth and mor- 

 tality rates (Alhossaini et al., 1989; Karakiri et al., 



1989). 



In Dover sole (Microstomas pacificus), a commer- 

 cially important northeast Pacific Ocean flatfish, meta- 

 morphosis differs from the rapid process described 

 above. Markle et al. (1992) define five stages in Dover 

 sole development. Stage 1 includes planktonic pre- 

 metamorphic larvae from 6.1 to 58.5 mm standard 

 length (SL). Eye migration begins when Stage- 1 lar- 

 vae are 10-15 mm SL but is arrested about midway 

 through the process (Pearcy et al., 1977; Markle et al., 

 1992). Thus, Dover sole are optically asymmetrical dur- 

 ing most of their planktonic life. Stage 2 includes plank- 

 tonic metamorphic larvae from 42.3 to 60.4 mm SL, in 

 which the eye has migrated past the mid-dorsal ridge 

 and a pronounced shrinkage in body depth has begun. 

 Stage 3 includes transitional metamorphic larvae from 

 40.7 to 74.9 mm SL, which are found in both the plank- 

 ton and benthos. Stage-3 larvae are characterized by 

 attainment of the adult configuration for a number of 

 morphological characters. Stage 4 includes predomi- 

 nantly benthic, metamorphic larvae from 41.7 to 

 79.3 mm SL, which have formed a characteristic intes- 

 tinal loop in the secondary body cavity. Stage 5 in- 

 cludes post-metamorphic juveniles from 48.9 mm SL 

 to sexual maturity, which occurs at lengths greater 

 than approximately 250 mm total length ( Yoklavich and 

 Pikitch, 1989; Hunter et al., 1992). Based on seasonal 

 collections of staged larvae, Markle et al. (1992) esti- 

 mated that duration of the metamorphic period (Stages 

 2, 3, and 4) is approximately 9 months. 



The protracted metamorphic period in Dover sole 

 provides an opportunity to examine changes in otolith 

 structure and chemistry associated with each stage of 

 metamorphosis. It may also be possible to distinguish 

 between otolith landmarks associated with develop- 

 mental processes and those associated with settlement 

 from the water column to the bottom, because these 

 events are not as coincidental in Dover sole as in other 

 flatfish species. The objectives of this study were to 

 1) describe microstructure and microchemistry of 

 Dover sole otoliths collected before, during, and after 

 metamorphosis; 2) identify structural and chemical 

 landmarks representing otolith growth during impor- 

 tant morphological and behavioral transitions; 3) vali- 

 date periodicity of increment formation in otoliths of 

 metamorphic Stage-3 and Stage-4 larvae and post- 

 metamorphic Stage-5 juveniles; and 4) using the otolith 

 landmarks determined in objective 2 and increment 

 counts between those landmarks and the otolith edge, 

 re-examine the chronology of metamorphic events de- 

 scribed in Markle et al. < 1992). 



Methods 



Dover sole collections 



Otoliths of six Stage-1, 103 Stage-3, 82 Stage-4, and 

 220 Stage-5 Dover sole were examined for microstruc- 

 ture and microchemical analysis. Specimens were ob- 

 tained on various dates between 1987 and 1990 from 

 bottom trawl collections off Oregon (Markle et al., 

 1992), opportunistic bottom trawl collections of com- 

 mercial fishermen from Oregon and Washington, and 

 midwater trawl collections off Oregon and central Cali- 

 fornia 1 (Appendix 1). 



The following measurements were taken to the near- 

 est 0.1mm on all specimens: standard length, body 

 depth at anus (BD1A), and distance from snout to pos- 

 terior extent of intestine (SINT). Weight of pat-dried 

 specimens was determined to the nearest 0.1 g. 



Otolith preparation and analysis 

 for microstructure 



Sagittae were removed after the fish were measured. 

 Otoliths not immediately prepared for analysis were 

 stored dry in vials. Terminology of otolith morphology 

 follows Campana and Neilson (1985) and Secor et al. 

 (1991), modified to account for features in Dover sole 

 otoliths (Fig. 1 and Results section below). Left and 

 right otoliths of Stage 3-5 Dover sole differ noticeably 

 in shape (Fig. 1 and Results section below) and were 

 analyzed separately. 



The longest and shortest axes of sagittae from 

 Stage-1 larvae were measured to the nearest 0.01mm 

 prior to mounting on slides in either a toluene-based 

 medium (Histoclad) or an acrylic glue (Super Glue). 

 Otoliths were then ground in the sagittal plane with 

 600-grit paper until growth increments became 

 apparent. 



Sagittae from Stage 3-5 Dover sole were cleaned in 

 ethanol, air-dried, weighed to the nearest 0.01 mg, and 

 measured along the anterior-posterior axis (maximum 

 length) and along the dorsal-ventral axis (maximum 

 width) to the nearest 0.01 mm (Table 1). The number 

 of translucent rings (annuli) outside the clear central 

 growth area (Fig. 1 and Results section below) was 

 determined upon examination of whole otoliths under 

 reflected light against a black background. Most otoliths 

 were mounted on slides in a toluene-based medium 

 (Histoclad) and the lateral face was ground in the 



'Whipple, J. 1991. Progress in rockfish recruitment studies. U.S. 

 Dep Commer NOAA. Natl. Mar. Fish. Serv.. Southwest Fish. Sci. 

 Center. P.O. Box 271, La Jolla. CA 92038. Admin. Rep. T-91-01. 

 57 p. 



