726 



Fishery Bulletin 97(3), 1999 



Figure 4 



The lens retractor muscle (LR) in the eye oi' a 15-day posthatching walleye pollock, 

 Theragra chalcogramma. larva. (Bouin's fixative, H and E; standard length=6.65 mm; 

 scale bar=20 uml. L = lens. 



and is functional by 4 to 5 DAH. Histological analy- 

 sis in this study showed that mouth and jaw devel- 

 opment proceeded at a similar rate; the jaw was fully 

 formed and the mouth was functional 5 to 6 DAH. 

 As in Atlantic cod, Gadus morhua, larvae (Kjorsvik 

 et al., 1991), the first feature of the walleye pollock 

 mouth to form was the trabeculum cranii ("roof of 

 the mouth"). 



At hatching, walleye pollock larvae had a straight- 

 tube gut. This simple gut arrangement has been 

 noted for larvae of other species offish as well (north- 

 ern anchovy, Engraulis mordax, O'Connell, 1981; 

 Atlantic cod, G. morhua, Kj0rsvik et al., 1991; tur- 

 bot, Scophthalinus maximus, Segner et al., 1994). At 

 3 DAH, three distinct portions of the gut could be 

 identified: the foregut, midgut, and hindgut. This gut 

 arrangement is typical of larval fish (Govoni et al., 

 1986) and walleye pollock larvae showed no major 

 changes in gut structure (i.e. no stomach, or pyloric 

 caeca) up to 31 DAH. For larval fish, major changes 

 in gut structure happen rapidly at metamorphosis 

 rather than gradually during the larval period 

 (Govoni et al., 1986). However, at 11 to 12 DAH (4 

 days after FF) the larval walleye pollock midgut epi- 

 thelium began to develop large folds, and the gut 

 began to coil 13 to 14 DAH (6 days after FF). For 

 walleye pollock larvae reared at 6''C, Oozeki and 

 Bailey ( 1995 ) noted that gut coiling began on 16 DAH 



and was complete by day 23. Midgut coiling increases 

 the length of the gut, and the residence time of prey 

 in it ( Yamashita and Bailey, 1989). At one week after 

 FF, both epithelial folds and gut coiling enable wall- 

 eye pollock larvae to assimilate nutrients more effi- 

 ciently through increased absorptive surface area and 

 longer residence times. 



Histological evidence suggests the larval walleye 

 pollock gut functions in the same manner as in other 

 fish larvae. The midgut of fish larvae digests and 

 absorbs lipids (Govoni et al., 1986) and this appears 

 to be the function of the larval walleye pollock mid- 

 gut as well. For larval whitefish, Coregonus fera , lipid 

 vacuoles appear as circular "voids" in the apical por- 

 tion of the midgut epithelial cells one day after feed- 

 ing (Loewe and Eckmann, 1988). In other species of 

 fish, similar vacuoles are also reported to contain 

 lipid (goldfish, Carassius aiiratus, Iwai, 1968; tur- 

 bot, S. t7w.xlmiis. Segner et al., 1994). In our study, 

 vacuoles (circular voids) were observed in midgut 

 epithelial cells of one walleye pollock larva at 23 

 DAH. This is later in development than they have 

 been observed in other fish larvae but lipase is 

 present in walleye pollock larvae at hatching; there- 

 fore lipid digestion could occur at FF. Lipase activity 

 increases with age (Oozeki and Bailey, 1995) so that 

 as larvae grow, more lipid can be digested and this 

 could produce larger, more visible (by H and E stain- 



