Hemigrapsus takanoi

H. takanoi has been introduced to the coasts of Europe by human agency, and has become an invasive species there. Although originally reported as Hemigrapsus penicillatus, that species has since been divided into two species, and it is H. takanoi, not H. penicillatus which occurs on the coasts of Europe.[2] It is thought to have arrived in 1993, having been discovered in La Rochelle, France, in 1994. A study of hull fouling animals on ships in Hamburg Dockyard and Bremerhaven at the time found six specimens of H. takanoi on the car-carrying ship SPICA, which travelled between the Asian ports of Tokyo, Osaka, Kobe and Pusan, and Europe. The ship was known to have passed the French coast in 1993, and it is thought that the introduction is likely to have occurred when crabs fell from the hull of the SPICA at that time.[4]

By 1997, its range in Europe extended from Fromentine in France (adjacent to the Île de Noirmoutier) to Laredo, Spain, covering a coastline of 700 kilometres (430 mi) around the Bay of Biscay.[5] By 1999, it had reached Le Havre on the English Channel, and by 2005, it had reached the French Opal Coast at the entrance to the North Sea.[6] Based on the native climate, it was predicted in 1999 that H. takanoi would spread across much of the North Sea and western parts of the Baltic Sea.[4] This was later confirmed, with first documented records in the Dutch Wadden Sea in 2004, in the German Wadden Sea in 2007 and the Danish Wadden Sea in 2011.[7][8] Its northward spread along the western coast of Jutland has continued, it now also being present in the Limfjord.[9] It was first confirmed in Great Britain in 2014 when discovered on the southeastern coast at the Medway Estuary.[10] The first record in the Baltic region was in 2014 when individuals were caught in the Kiel Fjord in Germany, likely having arrived via the many ships that pass through the Kiel Canal (which connects the North Sea and Baltic Sea region),[7] and in 2018 it was first recorded in Denmark's South Funen Archipelago.[9]