NOTE Porter and Theilacker: Development of digestive tract and eye in larval Theragra chalcogramma 



723 



period. Eight to 10 larvae were sampled 

 every day or every other day after hatch- 

 ing to 23 DAH in 1991 and to 31 DAH in 

 1992. Larvae were placed into either 

 Bouin's fixative which was replaced with 

 VO'/r ethanol 24 to 48 h later, or Z-fix^' (a 

 solution of 10% formalin with zinc and 

 buffers added). Larvae were dehydrated 

 in a graded series of butyl alcohols, em- 

 bedded in paraffin wax, sectioned sagit- 

 tally into 6-|im serial sections, and stained 

 with hematoxylin and eosin (H and E). 

 Standard lengths (SL, tip of upper jaw to 

 end of notocord) of preserved larvae were 

 measured to the nearest 0.08 mm and ad- 

 justed to live SL (Theilacker and Porter, 

 1995). To describe development of the 

 mouth, digestive tract, and eyes, histologi- 

 cal sections of 5-10 larvae were examined 

 from each sampling day. 



Results 



Larval growth 



In 1991, the mean SL at hatching was 4.27 

 ±0.07mm, whereas in 1992 hatching size 

 averaged 4.66 ±0.15mm (Fig. 1). The de- 

 crease in average size of larvae sampled 

 on day 23 in 1991 (Fig. 1) was probably 

 due to the sampling of slow growing lar- 

 vae in poor condition. A bowl was used to 

 sample larvae from the tank, and we sus- 

 pect that for this sample only slow grow- 

 ing larvae were captured. The overall 

 mean growth rate from hatching to 20 

 DAH was 0.12 mm/d in both 1991 and 

 1992. In 1992, experiments continued un- 

 til 31 DAH and average growth from 

 hatching was 0.10 mm/d. The onset of FF 

 occurred over a period of three days; day 

 of first feeding was defined as the time 

 when 50% of the larvae began feeding. In 

 1991, day of first feeding occuired at 8 DAH, 

 and the mean SL was 5.82 ±0.12 mm. In 1992, larvae 

 began feeding at 7 DAH, and the mean SL was 5.73 

 ±0.25 mm (Fig. 1). 



Mouth and digestive tract development 



At hatching a membrane covered the mouth, and jaw 

 cartilage had not yet formed. At 2 DAH the mem- 



Days after hatching 



Figure 1 



Growth of walleye pollock, Theragra chalcogramma. reared at 6'C in 

 the laboratory from hatching to 2.3 days after hatching in 1991, and to 

 31 days after hatching in 1992. Mean standard length (preserved length 

 adjusted to live length, see text) and standard deviation are shown; 5 

 to 10 larvae were measured at each age. The small size for 1991 at 23 

 days after hatching may be due to sampling slow growing larvae in 

 poor condition (see text). Arrow indicates day of first feeding. 



2 Anatech, Ltd., Battle Creek, MI. 



brane began to degenerate leaving an orifice that was 

 22 to 55 |jni wide. Mouth and jaw cartilage also be- 

 gan forming at this time, and the cartilage in the 

 roof of the mouth, the trabeculum cranii, became 

 most distinct. All jaw cartilage elements were present 

 and almost completely formed at 5 DAH (approxi- 

 mately 2 days before FF); the mouth was considered 

 functional 5 to 6 DAH (Table 1). 



At hatching the gut was a straight tube with a 

 narrow lumen. The foregut could be distinguished 



