Endogenous activity rhythms in tomcod (Microgadus tomcod) post-yolk-sac larvae

Post-yolk-sac tomcod (Microgadus tomcod) larvae were caught in a spawning river. They were held in the laboratory under three different environmental conditions: (i) constant conditions; (ii) exposure to a simulated tidal cycle; and (iii) exposure to simulated tidal and diel cycles. Swimming activity determinations made under constant conditions revealed significant endogenous rhythms in each group. Chronobiological flexibility affected by simulation of the basic cycles of the estuarine habit was demonstrated. Groups of larvae exposed to simulated tidal signals demonstrated modified rhythmic patterns relative to control groups held under constant conditions. Rhythms of groups exposed to both tidal and diel light signals included a low-frequency component in the 20-h region. Examination of the metabolic effects of feeding on endogenous activity patterns revealed lower activity levels when larvae were deprived of food. However, control groups exhibited the same period length whether fed or unfed, and entrai...