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The niches invaded by exotic species are generally not entirely vacant, and one possible factor affecting the success of the invader to establish a permanent large population may be the ability of the former to outcompete native species. Eudiaptomus gracilis, which was not present in Italy before the 1980s, is becoming established in an increasing number of Northern Italian lakes, ultimately replacing the endemic E. padanus. Coexistence of the two species in Lake Candia lasted only 7 months, suggesting that species replacement was determined either by environmental changes or by strong competition. To assess whether the potential for interspecific competition existed and to identify species' traits which could explain the competitive superiority of E. gracilis, we examined field seasonal patterns, reproductive parameters and body size of the two species. E. padanus abundance was probably reduced by predation and parasitism, which favoured the displacement of the species shortly after invasion by E. gracilis. The temporarily underexploited niche provided an opportunity for the successful establishment of the invader. The reproductive patterns of the two species were found to be similar in most of their features, with the exception of a markedly larger clutch size and a smaller egg volume in the resident species. By contrast, the invader showed a higher adult:egg ratio and a lower death rate. Thus, despite the greater fecundity of E. padanus, the competitive success of the invader might be attributable to interspecific differences in developmental rates and/or juvenile mortality.

The seasonal variation in length of the most abundant copepod species in the lagoon of Venice were compared and the relative influence of temperature, chlorophyll concentration and salinity examined. Temperature seems to be the primary factor influencing copepod body length in the lagoon of Venice, but the different species vary markedly in their response. Calanoid copepods (Acartia clausi, A. tonsa, Paracalanus parvus and Centropages ponticus) showed a more definite trend of size variation with temperature than the cyclopoids (Oithona nana and O. similis) and the harpacticoid Euterpina acutifrons. The size of the poecilostomatoids Oncaea media and O. subtilis was not affected by temperature, and was almost constant over time. Differences in the metabolic rates, longevity and specialization of calanoid, cyclopoid and harpacticoid copepods could explain their different responses to temperature.