Coexistence and Reproductive Adaptations of Amphipods: The Role of Environmental Heterogeneity

In the Oslofjord (Norway) distribution and life cycle traits of four Gammaridae species (Crustacea, Amphipoda) were studied at two localities. Three species occurred at both shores and showed some segregation to different intertidal levels. A temporal microhabitat utilization is suggested to lessen the possibility of interference competition among species living at similar intertidal levels. The importance of microhabitat segregation is supported by the fact that competitive exclusion occurred in coexistence experiments, and that differences were measured in resistance to desiccation. Environmental heterogeneity may explain the considerable overlap in breeding periods, periods of main juvenile release and animal size. Reproductive effort (total egg volume per brood) and egg size varied seasonally within species. Individual females produced small broods of large eggs in winter and larger broods of smaller eggs in spring; a reverse trend was observed from summer to autumn. These changes were coupled with increased survival probability of newly hatched juveniles in spring and summer. A graphical model allowing seasonal variation in reproductive effort and juvenile survival is presented to formalize an explanation of the evolution of reproductive traits of boreal and polar Gammaridae. The significance of the relationship between female size and offspring size is questioned. An evolutionarily stable but seasonally variable egg size is predicted in boreal waters, while large eggs are expected in polar waters. However, further quantification and testing of the model is limited by the lack of demographic data. /// В фьорде Осло распределение и характеристики жизненных циклов 4-х видов гаммарид (Crustacea, Amphipoda) исследованы исследованы в двух местообитаниях. 3 вида встречались на обоих берегах и обнарыжили известную сегрегацию в отношении разных литоральных уровней. Установлено, что использование временных микробиотопов снижает возможности интенжеренционной конкуренции между видами, обитаюцими на одинаковых уровнях. Эначение сегрегации микробиотопов подтверждается жактом наличия конкурентного исключения в опытах по сосуществованию и различий устойчивости у высыханию. Гетерогенность среды может обьяснить значительное перекрывание в периодах размножения, основного выхода молоди, размеров животных. Репродуктивный зффект (обций объем яиц в кладке) и величина киц варьирует и разные сезоны внутри вида. Отдельные самки дают небольшие кладки крупных яиц зимой и больши