Activity rhythms of maturing American eels (Anguilla rostrata)

Locomotor activity was recorded from 11 female silver American eels (Anguilla rostrata) receiving hormonal injections intended to induce maturation. Five eels treated with extracts of carp pituitaries became at least partially mature, while 6 eels treated with ovine-luteinizing hormone remained unaffected (immature). Immature eels were more active nocturnally and showed peaks of activity at light-dark transitions. Maturing eels were equally active by day and by night, but remained responsive to light-dark transitions. In constant darkness both groups increased their activity at times corresponding to expected light-off, suggesting an innate timing mechanism. When maturing eels were subjected to constant light and photoperiod phase-shifts, they displayed transients which further exemplified the use of a biological clock. The significance of these findings relative to the natural spawning behavior of eels is discussed.