The establishment of many non-indigenous species is primarily controlled by propagule pressure, local environmental conditions, and biological interactions. An introduction is doomed to fail if any one of these factors is unsuitable. A few Atlantic blue crab Callinectes sapidus Rathbun, 1896 specimens have been collected along a limited stretch of the central Portuguese coast since the late 1970s, but a viable population was never detected. However, starting in 2016, a population of the Atlantic blue crab has established and expanded along the southern Portuguese coast. The objective of the present study was to provide insights into the invasion of the Atlantic blue crab in Portugal based on unpublished museum collection records and new records made by citizen scientists on the western coast and to provide a mechanistic explanation for the recent expansion based on observational oceanography data. Citizen science records along with observational oceanography data from 2019 and 2020 suggest that the southern Portugal population is expanding towards the western coast due to warmer coastal countercurrent events that form in the Gulf of Cadiz during the reproductive period of the Atlantic blue crab (summer–early fall). This oceanographic feature facilitates the transport of larvae towards the western coast of Portugal, which increases propagule pressure, while estuaries along the southwestern coast may serve as stepping stones supporting the northwards expansion of the species in tandem with increasing sea temperature. This study also highlights the value of citizen science in detecting the range expansion of invasive species over wide geographical areas.
