Diel changes in swim bladder inflation of the larvae of the northern anchovy, Engraulis mordax.

Abstract

Laboratory and field studies demonstrated that larval anchovy 10 mm standard length and larger inflate their swim bladders each night and deflate them in the day. Maximum night levels of inflation were attained 2 h after the onset of dark and typical day levels occurred about 2 h after the onset of light. Laboratory experiments indicated that larvae fill their bladders at night by swallowing air at the water surface and the vertical distribution of sea-caught larvae suggested that they migrate to the surface each night to fill their swim bladders. Gas is released by passing bubbles through the pneumatic duct into the alimentary canal. The diel rhythm of inflation was viewed as an energy sparing mechanism. Measurements of sinking speed of larvae with and without inflated bladders suggested that the energy saved at night by inflation of the swim bladder would exceed the cost of vertical migration to the surface and that the migratory range over which energy savings are possible would be greater as larvae increased in length. Northern anchovy, Engraulis mordax Girard, are more vulnerable to starvation in the larval stage than at any other time of life, consequently, energy sparing mechanisms may be critical to their survival. In a recent paper Uotani (1973) showed that the larvae of several clupeoid fishes, Engraulis japonicus (Houttuyn), Sardinops melanosticta (Temminck and Schlegel), and Etrumeus teres (DeKay) have expanded swim bladders when captured at night in the sea and deflated ones when captured during the day. Energy conservation is certainly one of the possi­ ble adaptive advantages of such behavior, but the energy saved must be evaluated in terms of the energy cost of daily filling the bladder. This requires that the mechanism of filling be known. The object of the present study was to determine if the larvae of the northern anchovy display a similar rhythm and to evaluate this behavior as a possible energy sparing mechanism. The swim bladder in adult northern anchovy is a tubular vesicle that extends the length of the body cavity. It is connected to the alimentary canal by a pneumatic duct which originates from the dorsal wall of the cardiac stomach; no anal duct exists as it does in some clupeoids (O'Connell 1955). Two tubules on each side of the body extend from the anterior end of the bladder into the cranium where they expand into two pairs of capsules, termed