Action Spectrum of the Photoperiod Mechanism Controlling Sexual Maturation in the Threespine Stickleback, Gasterosteus aculeatus

Abstract

Experiments in which groups of 80 threespine stickleback (Gasterosteus aculeatus) were exposed to four discrete segments of the visible spectrum (388–466, 455–518, 513–583, and 585–653 mμ) and compared with two control groups (experimental and wild) showed no major differences in seasonal rates of gonad maturation. This result contrasts with the narrower action spectra of the photoperiod mechanisms in some birds (600–750 mμ) and many insects (400–500 mμ). Light energy levels of < 0.37 × 10³ erqs/cm²-sec (equivalent to approximately 5 lux at 420 mμ) induced normal seasonal changes indicating a level of sensitivity for the photoperiod mechanism in this species comparable to that of certain birds and insects (approximately 1 lux). Histological comparisons of ovary development with the brook stickleback (Culaea inconstans) showed closely similar development patterns except for the final stage of yolk deposition. A further stage of yolk differentiation described for the threespine stickleback probably accounts for the larger egg size in this species.