EVIDENCE FOR A PROGRAMMED CIRCANNUAL LIFE-CYCLE MODULATED BY INCREASING DAYLENGTHS IN NEANTHES-LIMNICOLA (POLYCHAETA, NEREIDAE) FROM CENTRAL CALIFORNIA

Timing of parturition, fecundity, and life span were determined in laboratory cultures of the semelparous, self-fertilizing, viviparous polychaete Neanthes limnicola. Worms were exposed to fixed daylengths (short-8h light: 16h dark; neutral-12h:12h; long-16h:8h), switched between different fixed daylengths, and switched from fixed daylengths to increasing or decreasing daylengths. Timing of parturition was synchronized when under neutral daylength, but became asynchronous under both short and long daylength, as well as when any of the fixed daylength was followed by decreasing daylengths. Worms under neutral daylength had the highest fecundities and shortest life spans, while those under long days had the lowest fecundities and longer life spans. When fixed daylength (short, neutral, long) was followed by increasing daylengths, timing of parturition was synchronized, fecundity was high, and life span shortened. These and earlier published experiments on the influence of seasonally changing photoperiods indicate that the life cycle of the estuarine N. limnicola is programmed to be completed in somewhat less than a year, and that seasonally changing photoperiods modulate it to determine the optimal time of parturition.