The brush-clawed shore crab Hemigrapsus takanoi is a native species from the Western Pacific and an invader of the European Atlantic coast from northern Spain to southern Denmark. Despite the increasing concern about its rapid expansion, little is known about the early stages in its life history. In the present study, the larval morphology of H. takanoi is described and illustrated from specimens obtained in the laboratory from its type locality, Tokyo Bay, Japan. Its larval development follows the pattern of Varunidae, that involves five zoeae and one megalopa. The morphological characters of the larvae of H. takanoi are compared with those of the other known Hemigrapsus species of the North Pacific. In addition, to facilitate an early detection of the invasive species of Varunidae inhabiting European Atlantic waters, a summary of the key characters to identify their larval stages is included.

The duration of brood development in the introduced crab, Hemigrapsus takanoi in the Oosterschelde, The Netherlands, was compared at three different water temperatures. At 12, 18 and 24°C the females took an average of 32, 11 and 8 days respectively to lay eggs, which took 86, 28 and 18 days respectively to complete development. Five stages of development were identified, with each brood stage comprising a similar proportion of the duration time at different temperatures. The duration of each brood stage was also somewhat proportional to the number of females found carrying each brood stage in the field at the beginning of the breeding season. There appears to be a trigger for the breeding season in H. takanoi in the field at around 15°C above which ovary development begins. The results suggest that an increase in water temperature as a result of climate change may result in an increased net reproductive rate in H. takanoi due to earlier onset of the breeding season and increased number of broods per inter-moult period resulting in population growth. Increased temperatures may therefore lead to increased invasiveness of H. takanoi where it is already present, and range extension into locations where its establishment is currently excluded by unsuitable temperature.

The sighting of the brush-clawed shore crab Hemigrapsus takanoi on the western coast of northern Cotentin confirms the extension of this species into the western part of the English Channel. This new sighting remains enigmatic because the Normanno-Breton Gulf circulation patterns do not appear to promote natural transport of crab larvae from the Atlantic or from the eastern part of the English Channel. It has been suggested that the professional oyster transport could be the origin of this species' presence on this part of the French coast.