The Influence of Photoperiod on the Timing of Reproductive Cycles inGammarusspecies (Crustacea, Amphipoda)

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Abstract
SYNOPSIS. Gammarus species time the release of their young by using ovarian diapause and the duration of embryonic development. During diapause vitellogenesis is arrested and oostegite setae are lost. The onset of diapause is influenced by photoperiod under experimental conditions. Two Gammarus species were studied. Gammarus lawrencianus is typical of the majority of Gammarus species that produce sequential broods through the summer and have a diapause in the autumn. Gammarus setosus is typical of those which have an obligatory diapause lasting most of the year that follows the annual autumn brood. Experimental short day photoperiods (4L to 12L) induce vitellogenesis in Gammarus setosus, and have the opposite effect on Gammarus lawrencianus. Long day photoperiods promote vitellogenesis in Gammarus lawrencianus and prolong the duration of diapause in Gammarus setosus. It is concluded that these two Gammarus species have their own endogenous rhythms and that the role of photoperiod is to time the reproductive cycle. Short days near the time of the fall equinox ensure that this timing is synchronized over a range of latitudes. The effect of photoperiod is modulated by the influence of temperature on embryonic development over the broad range of geographic latitude inhabited by the species. Short photoperiods inhibit but do not prevent sexual maturation of the young. Short days also result in a slower average growth rate and greater size at maturity.

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