RICHARDSON ET AL.: EGGS AND LARVAE OF BUTTER SOLE 









21.1 mm SL 



Figure 5. — Recently transformed benthic juvenile /sopserta isolepis, 21.1 mm SL. 



be due to the different environmental conditions of 

 the reared and planktonic larvae. With develop- 

 ment the gut continues to coil and the hindgut, 

 which is initially directed posteriorly, comes to 

 rest in an anteriorly directed position, by about 17 

 mm. 



Notochord flexion begins by the time larvae are 

 about 9 or 10 mm long and the notochord is fully 

 flexed by about 14 mm. After flexion is complete, 

 the tip of the urostyle continues to extend beyond 

 the hypural plate, sometimes until larvae are 17 

 mm long. When larvae are about 12 or 13 mm long, 

 near the end of notochord flexion, the left eye be- 

 gins to migrate to the right side of the head. The 

 left eye is visible on the ridge of the head, when 

 viewed from the right side, in some larvae as small 

 as 15 mm and consistently in specimens >17 mm. 

 The left eye eventually migrates to the degree that 

 it is directed upward, the most advanced position 

 before complete transformation to benthic juve- 

 nile. This was the most advanced stage of eye mi- 

 gration of specimens taken in plankton tows and 

 was observed in some, but not all, specimens >20 

 mm. The largest specimen collected in the plank- 

 ton was 23.6 mm, but the most developed plankton 

 specimen in terms of eye migration and increased 

 pigmentation was 21.9 mm. The smallest speci- 

 men collected in a beam trawl was 18.0 mm, but its 

 eye was on the dorsal ridge of the head directed 

 upward and it had not completed transformation. 



The smallest benthic juvenile, in which the left 

 eye had crossed over the middorsal ridge and 

 juvenile pigment had intensified on the eyed side, 

 was 18.5 mm. 



With development (Tables 2, 3), relative head 

 length increases considerably from mean values of 

 13-15% SL in preflexion larvae to 25% SL in post- 

 flexion larvae. Snout to anus length remains es- 

 sentially constant with respect to standard length 

 with mean values of 29-32% . Relative body depths 

 at the pectoral fin base, at the anus, and behind the 

 anus increase dramatically through the larval 

 period with mean values nearly doubling in most 

 cases between preflexion and flexion stages and 

 doubling again between flexion and postflexion 

 stages. The greatest rate of increase occurs in the 

 depth behind the anus. Depth of the caudal pedun- 

 cle also increases from 3 to 10% relative to stan- 

 dard length. Relative eye diameter is largest in 

 preflexion larvae (29-36% HL (head length)) and 

 decreases in flexion (24% HL) and postflexion (22% 

 HL) larvae as does the relative length of the upper 

 jaw (37-25% HL). 



Ossification of Meristic Structures 



Descriptions, based on cleared and stained 

 specimens, depict only general trends of develop- 

 ment because the size at which bones begin to 

 ossify may vary among specimens and the uptake 



409 



