SWIMBLADDER INFLATION OF THE ATLANTIC MENHADEN BREVOORTIA-TYRANNUS

Atlantic menhaden Brevoortia tyrannus larvae inflate their swimbladder s at night and deflate them during the day. The present study consider ed the relationship of inflation to light intensity, the time-course o f inflation, and the presence of an endogenous rhythm in inflation. Th e percentage of laboratory-reared larvae that inflate their swimbladde rs increased upon sudden exposure to a decrease in light intensity. Pe rcentage inflation was maximal at an intensity of 10(13) photons cm-2 s-1 and lower. For any specific size of larvae, the inflation volume d id not vary significantly with light intensity, but volume increased w ith total larval length. Inflation began within 5 min of introduction into darkness, and maximum percent inflation was evident by 20 min. Th ere was a rhythm in which darkness induced a low percent inflation dur ing the day phase and a high percentage at the beginning of the dark-p hase. This dramatic increase in inflation at sunset may function for p redator avoidance.