Histopathology of swimbladder noninflation in walleye ( Stizostedion vitreum) larvae: role of development and inflammation

Abstract

Noninflation of the swimbladder is a major obstacle in culture of many fish larvae including walleye ( Stizostedion vitreum). 79 walleye larvae, 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, coinciding with the time of yolk sac depletion and initiation of feeding. From 7 to 11 days posthatch, macrophages infiltrated 11 of 18 (61%) noninflated and 3 of 16 (19%) inflated swimbladders. From 12 to 19 days posthatch, macrophages infiltrated 16 of 16 (100%) noninflated and 0 of 14 (0%) inflated swimbladders. Organic debris and large numbers of bacterial rods filled the noninflated swimbladder of a 13 day old larvae examined by electron microscopy. In larvae with noninflated swimbladders, the pneumatic duct was patent and its diameter remained fairly constant (25–45 μm) through 19 day posthatch, but the pneumatic duct atrophied in larvae with inflated swimbladders. During the interval of swimbladder inflation, 6–12 day posthatch, ostia of the common bile duct and pneumatic duct occupied the same lumen of the undifferentiated foregut; we hypothesize that surfactant-like secretions from the common bile duct affected fragmentation of large ingested air bubbles for transfer 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 dorsal wall of the stomach. We conclude that swimbladder noninflation in walleye results from ingestion of bacteria and organic debris into the swimbladder 6–11 day posthatch, and that differentiation of the foregut prevents inflation after 12 day posthatch.