Photoperiod and temperature regulation of growth and reproduction inStreblospio benedicti(Polychaeta: Spionidae)

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Abstract
Three experiments were performed to examine responses of the polychaete Streblospio benedicti Webster from North Carolina to combinations of both constant and seasonally changing daylength and temperature. S. benedicti collected in early and late fall were reared under constant (fall) and fall-winter transition daylength and temperature conditions. S. benedicti collected in spring were reared under constant (winter) and winter-spring transition daylength and temperature conditions. In a third experiment, S. benedicti was kept at constant temperature (20°C) and two fixed daylength treatments. Survivorship and maturation of S. benedicti were unaffected by daylength, but body size (setiger number and length), brooding activity and brood size were subject to photoperiod influence. Decreasing daylength and temperature each led to slower growth in the fall-winter transition experiment. Increasing daylength and temperature led to more rapid growth in the winter-spring transition experiment. Long fixed daylength had the same effect. The proportion of females brooding declined under decreasing daylength and/or temperature, but was unaffected by most other treatments. Photoperiod had a significant and unexpected effect on fecundity in all experiments. Brood sizes were larger under conditions of decreasing or increasing daylength relative to fixed treatments. Fixed long daylength produced larger broods of larvae than fixed short daylength. We conclude that photoperiod and temperature act together to regulate growth, reproductive activity and fecundity of S. benedicti and contribute to spring and fall recruitment peaks observed in the field. Episodic events that decouple photoperiod from normal temperature regimes may cause significant fluctuations in reproductive timing and output.