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



727 



ing) vacuoles with age. There was probably lipid in 

 the midgut cells earlier than 23 DAH but in such 

 small amounts so that it could not be identified with- 

 out special staining. The larval walleye pollock hind- 

 gut appears to function in protein digestion like the 

 hindgut of other species offish larvae (Govoni et al., 

 1986). Watanabe (1984) demonstrated that protein 

 is pinocytotically moved into hindgut epithelial cells 

 for intracellular digestion. Iwai and Tanaka (1968) 

 stated that the intracellular protein appears as eosi- 

 nophilic granules (also referred to as eosinophilic 

 vesicles or inclusions) within the apical portion of 

 the hindgut cells. At 13 to 14 DAH (5 to 7 days after 

 FF), eosinic vesicles appear in the apical cytoplasm 

 of the hindgut epithelial cells of walleye pollock lar- 

 vae. For Atlantic cod, G. morhiia, larvae (Kjorsvik et 

 al., 1991), a close relative of walleye pollock, inclu- 

 sions were not observed until 2 to 5 days after FF. In 

 other studies eosinophilic inclusions have been ob- 

 served soon after FF (northern anchovy, E. mordax, 

 O'Connell, 1981; whitefish, C. fera, Loewe and 

 Eckmann, 1988; turbot, S. maximus. Segner et al., 

 1994). Trypsin is present in walleye pollock larvae 

 at hatching, its activity increases with age, and it is 

 not supplemented by prey in the gut (Oozeki and 

 Bailey, 1995). Eosinophilic vesicles appeared at the 

 time the gut began to coil, and both coiling and incre- 

 ased trypsin would allow increased digestion of prey 

 making more protein available to the hindgut, possi- 

 bly explaining why vesicles were not apparent until 

 some time after FF. The appearance of lipid vacuoles 

 and eosinophilic vesicles after the first week of feeding 

 provides evidence for improved digestive capability. 



Eye 



Sight is probably the most important sense walleye 

 pollock larvae use for feeding. Walleye pollock rely 

 on vision to search for prey (Paul, 1983) and do not 

 respond to chemosensory cues (Davis and 011a, 1995). 

 Laboratory experiments show that a group of wall- 

 eye pollock larvae, age 21 DAH, remains aggregated 

 when rotifers are introduced into the group, but when 

 only the scent of rotifers is introduced, the group dis- 

 perses (Davis and 011a, 1995). Because they are vi- 

 sual predators and because light below 0.006 )i mol 

 photon/m^/s limits their ability to capture prey (Paul, 

 1983), walleye pollock larvae probably have a pure- 

 cone retina like many other species of fish larvae. 

 Blaxter and Staines (1970) examined the retina of 

 12 species offish larvae including haddock, Melano- 

 gramnuis aeglefinus (which belongs to the same fam- 

 ily as walleye pollock, Gadidae), they showed that had- 

 dock and seven other species have a pure cone retina. 

 011a and Davis ( 1990) stated that the retina of walleye 



pollock larvae most likely does not contain rods. It is 

 unknown when rods begin to appear in walleye pol- 

 lock, but for herring, Clupea harengus (Blaxter and 

 Jones, 1967), plaice, Pleuronectes platessa (Blaxter, 

 1968), and sole, Solea solea (Sandy and Blaxter, 1980), 

 rods begin to appear at metamorphosis. 



The lack of eye pigmentation at hatching has been 

 found for many teleosts (Blaxter, 1986), including 

 walleye pollock (Bailey and Stehr, 1986; our study), 

 and eyes are probably nonfunctional at this time 

 (Blaxter, 1986). Ocular motor muscles developed just 

 as walleye pollock eyes became fully pigmented (3 

 DAH). The development of eye pigmentation and 

 ocular motor muscles should provide a FF larva with 

 sight and the ability to move its eyes. The lens re- 

 tractor muscle did not develop until after FF; it was 

 considered functional beginning at 15 DAH. The lens 

 retractor muscle develops after FF in other species 

 of fish as well (northern anchovy, E. moi'dax, 

 OConnell, 1981; white seabass, A^rac^osczo/! nobilis, 

 Margulies, 1989). This muscle allows a larva to fo- 

 cus on objects at different distances, thereby incre- 

 asing the field of vison (Munz, 1971). Because this 

 muscle was not functional until 15 DAH, for about a 

 week after FF a larval walleye pollock's field of view 

 is restricted because it is unable to change the focus 

 of its eyes. Thereafter, visual acuity improves, allow- 

 ing walleye pollock larvae to detect both prey and 

 predators more easily. 



In Shelikof Strait, Gulf of Alaska, the proportion 

 of starving walleye pollock larvae decreases dramati- 

 cally after the first week of feeding (Theilacker and 

 Porter, 1995; Theilacker et al., 1996) and the physi- 

 ological condition of walleye pollock larvae improves 

 as they grow; that is, fewer lai-vae are found in poor 

 condition (Theilacker et al., 1996). Improvements to 

 vision (the lens retractor muscle) and a combination 

 of morphological changes to the gut (folding and coil- 

 ing) as well as an increase in digestive enzymes 

 (Oozeki and Bailey, 1995) contribute to walleye pol- 

 lock larvae becoming less vulnerable to starvation 

 after the first week of feeding. Additional contribut- 

 ing factors may include developmental changes oc- 

 curring to other organ systems (e.g. development of 

 trunk musculature and lateral line system as has 

 been shown for other species offish larvae; Blaxter, 

 1986; O'Connell, 1981), and larvae becoming better 

 predators as they grow. For northern anchovy, E. 

 mor-dax. larvae, feeding success rapidly improves 

 during the first week of feeding (Hunter, 1972). At 

 hatching walleye pollock larvae lack functional eyes 

 and mouth, and have a straight-tube gut; the devel- 

 opment of these between hatching and FF allows lar- 

 vae to begin feeding, and their continued development 

 after FF improves the larvae's chance of survival. 



