The Influence of Photoperiod on the Timing of Reproductive Cycles in Gammarus species (Crustacea, Amphipoda)

SYNOPSIS. Gammarus species time the release of their young byusing ovarian diapause and the duration of embryonic development.During diapause vitellogenesis is arrested and oostegite setaeare lost. The onset of diapause is influenced by photoperiodunder experimental conditions. Two Gammarus species were studied.Gammarus lawrencianus is typical of the majority of Gammarusspecies that produce sequential broods through the summer andhave a diapause in the autumn. Gammarus setosus is typical ofthose which have an obligatory diapause lasting most of theyear that follows the annual autumn brood. Experimental shortday photoperiods (4L to 12L) induce vitellogenesis in Gammarussetosus, and have the opposite effect on Gammarus lawrencianus.Long day photoperiods promote vitellogenesis in Gammarus lawrencianusand prolong the duration of diapause in Gammarus setosus. Itis concluded that these two Gammarus species have their ownendogenous rhythms and that the role of photoperiod is to timethe reproductive cycle. Short days near the time of the fallequinox ensure that this timing is synchronized over a rangeof latitudes. The effect of photoperiod is modulated by theinfluence of temperature on embryonic development over the broadrange of geographic latitude inhabited by the species. Shortphotoperiods inhibit but do not prevent sexual maturation ofthe young. Short days also result in a slower average growthrate and greater size at maturity.