Fecundity and seasonal changes in reproductive output of females of the gravel beach amphipod, Pectenogammarus planicrurus

Pectenogammarus planicrurus Reid (Crustacea: Amphipoda) is an inhabitant of unstable gravel beaches. The reproductive output of females of this species was studied at Aberystwyth, on the west coast of Wales. There was marked seasonal variation in the size of eggs: the mean volume of eggs at an early stage of embryonic development ranged from 0.023 mm(3) in summer to 0.030 mm(3) in winter. Egg size was negatively related to brood size, but seasonal variation in brood size was greater than could be accounted for by egg size alone. Mean brood size ranged from 1.4 eggs in winter to 5.9 eggs in summer. Brood size was also strongly related to female body size, ranging up to 14 eggs in a large female. These findings are discussed in relation to adaptation to interstitial living in coarse intertidal gravels. A model of seasonal variation in reproductive output is suggested in which the importance of point reproductive output is related to the probability of females surviving to produce subsequent broods.