The first time the crab was brought to Europe was most likely by commercial vessels. Ships must fill their ballast water tanks and during one of these filling events, it could have been the spawning time for the mitten crab. Since the larvae are free floating and in size, it would have been easy for them to be swept into the ballast water tank. Once the ship reached Europe and emptied its tank, the crab larvae were released. Over time, this repetition would allow for a prominent mitten crab population in Europe. The crab has spread and can be found in Continental Europe, Southern France, United States of America (the San Francisco Bay), and the United Kingdom. A 15-year period in Germany when the crabs were gradually entering Europe is known as the "establishment phase".