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Fishery Bulletin 97(3), 1999 



Table 1 



Mean standard length (tip of upperjaw to end of notoeord) and age at the development of various elements of the mouth, digestive 

 tract, and eye for walleye pollock, Theragra chalcogramma. reared at 6'C in the laboratory in 1991 and 1992. '+', vacuoles were 

 not observed at 23 DAH in 1991, the last sampling day. Preserved standard length adjusted to live standard length (Theilacker 

 and Porter, 1995). SD = standard deviation, n = number of larvae measured. Age = days after hatching. 



Mean standard length mm (SD) 



1991 



1992 



Age (year) 



Mouth and digestive tract 

 Foregut. midgut, and hindgut 



separated by valves 

 Jaw developed and mouth functional 



Folding of midgut epithelium 



Midgut coiling, eosinic vesicles 

 in the hindgut 



Lipid vacuoles in midgut 

 Eye 



Ocular motor muscles apparent 



Eye fully pigmented 



Lens retractor muscle developed 



by its cuboidal epithelium, but no distinction could 

 be made between the midgut and hindgut which were 

 lined with similar columnar epithelium. At 1 DAH, 

 the gut lumen widened and began to separate into a 

 long midgut and short hindgut that were demarcated 

 by a constriction at the future site of the ileocaecal valve. 

 At 3 DAH, the pyloric valve separated the foregut and 

 midgut, and the ileocaecal valve separated the midgut 

 and hindgut (Table 1; Fig. 2). At 11 to 12 DAH (4 days 

 after FF) the gut began to develop large folds, and be- 

 gan to coil 13 to 14 DAH (6 days after FFt (Table 1). 

 Also, at 13 to 14 DAH, eosinic vesicles (also referred to 

 as eosinophilic granules or inclusions), ranging in size 

 from 1 to 3 |.im, began appearing in the apical cyto- 

 plasm of the hindgut epithelial cells (Table 1). Vacu- 

 oles in midgut epithelial cells were first observed at 23 

 DAH in 1992 larvae (Table 1). The vacuoles were not 

 distinct and observed only in one larva out often. Wall- 

 eye pollock larvae showed no other changes in gut sti-uc- 

 ture (i.e. no stomach, or pyloric caeca) up to 31 DAH, 



Eye development 



At hatching, a lens was present, and small scattered 

 patches of pigment were located throughout the 

 retina ( Fig. 3 ). The pigment patches increased in size 

 and joined to form the pigment layer; by 4 DAH the 

 eye was fully pigmented (Table 1). Ocular motor 

 muscles were not developed at hatching, but at 3 



DAH these muscles became apparent (Table 1). At 9 

 DAH, the lens retractor muscle was identified as a 

 thin stioicture that connected the lens to the retina. 

 From 9 to 15 DAH the muscle increased in width as it 

 grew. The size and shape of this muscle changed very 

 little beyond 15 DAH; therefore it was considered func- 

 tional beginning at this time (Fig. 4, Table 1). 



Discussion 



Larval growth 



Growth of walleye pollock larvae in this study was 

 similar between years (0.12 mm/d from hatching to 

 20 DAH for both 1991 and 1992) and to rates found 

 by others for walleye pollock larvae reared at 6°C in 

 the laboratory; 0.14 mm/d from hatching to 19 DAH 

 (Theilacker and Shen, 1993), 0.11 mm/d from hatch- 

 ing to 21 DAH (Yamashita and Bailey, 1989) and 

 0.065 mm/d from hatching to 15 DAH (Nishimura 

 and Yamada, 1984). Growth was slightly lower than 

 the 0.14 to 0.23 mm/d calculated for field-collected 

 walleye pollock larvae (Bailey et al., 1996). 



Mouth and digestive tract 



From observations on live larvae, Bailey and Stehr 

 ( 1986) found that walleye pollock have no mouth at 

 hatching; the mouth begins to develop 2 days later 



