HISTOPATHOLOGY OF SWIMBLADDER NONINFLATION IN WALLEYE (STIZOSTEDION-VITREUM) LARVAE - ROLE OF DEVELOPMENT AND INFLAMMATION

Noninflation of the swimbladder is a major obstacle in culture of many fish larvae including walleye (Stizostedion vitreum). 79 walleye larv ae, 4-19 days posthatch, were serially sectioned for histologic study of swimbladder development. All walleye had a swimbladder and vascular rete, Inflation of the swimbladder began at 6 days posthatch, coincid ing with the time of yolk sac depletion and initiation of feeding. Fro m 7 to 11 days posthatch, macrophages infiltrated 11 of 18 (61%) nonin flated and 3 of 16 (19%) inflated swimbladders. From 12 to 19 days pos thatch, macrophages infiltrated 16 of 16 (100%) noninflated and 0 of 1 4 (0%) inflated swimbladders. Organic debris and large numbers of bact erial rods filled the noninflated swimbladder of a 13 day old larvae e xamined by electron microscopy. In larvae with noninflated swimbladder s, the pneumatic duct was patent and its diameter remained fairly cons tant (25-45 mu m) through 19 day posthatch, but the pneumatic duct atr ophied in larvae with inflated swimbladders. During the interval of sw imbladder inflation, 6-12 day posthatch, ostia of the common bile duct and pneumatic duct occupied the same lumen of the undifferentiated fo regut; we hypothesize that surfactant-like secretions from the common bile duct affected fragmentation of large ingested air bubbles for tra nsfer into the relatively small-diameter pneumatic duct. After 12 day posthatch, however, the pyloric sphincter developed and separated the common bile duct in the intestine from the pneumatic duct in the dorsa l wall of the stomach. We conclude that swimbladder noninflation in wa lleye results from ingestion of bacteria and organic debris into the s wimbladder 6-11 day posthatch, and that differentiation of the foregut prevents inflation after 12 day posthatch.