Markle et al : Metamorphosis of Microstomus pacificus 



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40 80 120 160 200 



Standard Length (mm) 



Stage 5 (juvenile), 48.9 mm SL to sexual maturity 



We define the climax event, and Stage 5, as the point 

 at which length of the intestinal loop attains adult pro- 

 portions. The continuous nature of this process is il- 

 lustrated in the logarithm ratios of SINT/SL (Fig. 8). 

 We chose a cut-off ratio by calculating the ratio for 

 2mmSL increments and examining the rate at which 

 the ratio changes over length. The greatest rate of 

 change occurs between 67 and 69mmSL, during which 

 the mean ratio changes from 0.85 to 0.89. We chose 

 the midpoint of these ratios and therefore define Stage 

 5 as those individuals with a ratio of In (SINT)/ln (SL) 

 > 0.87. Coincident with this change is an overall 

 darkening of body color such that Stage 5 specimens 

 look like small adults. 



General features of early development 

 and metamofphosis 



Unlike most flounders, initiation of eye migration in 

 Dover sole is uncoupled from the change in habitat 

 from planktonic to benthic, as well as from the process 

 of metamorphosis (as defined herein). Eyes are sym- 

 metrical up to a maximum size of only 13.4 mm SL, and 

 the left eye can be on the midline in specimens as small 



Figure 8 



Relationship between the In SINT/ln SL ratio and stan- 

 dard length during development of Dover sole Microstomus 

 -pacificus. Symbols represent Stage 3(H), Stage 4 (O), and 

 Stage 5 (A). 



as 9.5mmSL (Pearcy et al. 1977a). Eye migration in 

 Dover sole is arrested during planktonic growth, 

 with the left eye stopping at the dorsal margin of 

 the cranium at 15-20 mm SL (Fig. 4A). It remains in 

 this position until metamorphosis. Thus, during most 

 of their planktonic life, the eyes of Dover sole are 

 asymmetrical. 



There is a complex relationship between body depth 

 and SL (Fig. 2), including (1) an interval of rapid in- 

 crease from about 10 mm to at least 60.4 mm SL in some 

 individuals, (2) a compensatory shrinkage phase over 

 the size range 40.7-74.9 mm SL, and (3) a more typical 

 linear growth phase that may begin in specimens as 

 small as 41.7mmSL. Body depth reduction is a regres- 

 sive process (Youson 1988) in which lengths of neural 

 and hemal spines and pterygiophores are reduced (Fig. 

 9). Two- and three-fold reductions occur in lengths of 

 first caudal neural and hemal spines and their imme- 

 diate anterior and posterior pterygiophores. Conse- 

 quently, metamorphosing Stage-3 specimens 40-50 mm 

 SL have neural and hemal elements comparable in 

 length to those of 20-30 mm SL Stage-1 larvae. Neural 

 and hemal elements and dorsal and anal pterygiophores 

 in Stage-1 larvae are cartilaginous or weakly ossified, 

 and vertebral centra lack zygopophyses. Complete 

 ossification of neural and hemal elements and forma- 

 tion of zygopophyses occurs in Stages 2 and 3. 



During most of metamorphosis, especially in Stages 

 2 and 3, body length appears to be arrested (Fig. 10). 

 Although the sample size of Stage-2 larvae limits our 

 confidence in further analysis, the data show little in- 

 dication of growth between Stages 2 and 3 (Fig. 10). 

 Because metamorphosis occurs over a broad range of 

 sizes, similarity in size minima and maxima between 

 stages also suggests little or no growth in body length. 

 For example, the minimum sizes for Stages 2, 3, and 

 4 are almost identical (42.3, 40.7 and 41.7mmSL, 

 respectively). During Stage 4 there is finally some 

 indication of grovrth because the smallest Stage-5 

 juvenile is 48.9 mm SL, more than 7 mm larger than the 

 smallest Stage-4 larva. Yet, even this juvenile is 26 mm 

 smaller than the largest metamorphosing Stage-3 

 larva. 



There is an apparent loss in body weight during 

 metamorphosis because of a decrease in mean weight 

 from 2.6 to 2.4g from Stage 2 to Stage 4 (Fig. 11). 

 However, the small sample size of Stage 2 and our 

 measuring error preclude attaching significance to the 



