The study of the life cycle features that allow species to adapt successfully to new environmental conditions is one of the current directions of ecological evolutionary developmental biology (Eco-Devo). The invasive species Gmelinoides fasciatus (Stebbing 1899) (Crustacea: Amphipoda) is a promising model object for research in this direction. This species is able to form stable populations in the recipient reservoir Lake Onega. Number of degree-days (2269–2626 for the period from the end of May to the beginning of October) sufficient for development contributes to the successful naturalization of G. fasciatus in new conditions on the northern border of species dispersion of the European part of Russia (Petrozavodsk Bay and Povenetsky Bay of Lake Onega). Thus, in the conditions of 62° N (Petrozavodsk Bay)–63° N (Kumsa Bay of Povenetsky Bay) of Lake Onega, the temperature factor does not limit the development of the amphipod population. These temperature conditions determine the 1-year life cycle characteristic of the local population of G. fasciatus with the simultaneous presence of generations of the previous year and the current year. Apparently, it is the plasticity of the life cycle that provides G. fasciatus with the possibility of successful invasion into recipient reservoirs.