The first occurrence of the Ponto-Caspian invader, Hemimysis anomala G. O. Sars, 1907 (Mysidacea) in the U.K

An invasive Ponto-Caspian mysid, Hemimysis anomala G. O. Sars, 1907, was recorded in England for the first time in 2004. Usually a deep water species, in England H. anomala has been observed in shallow waters, in which it shelters under or within anthropogenic structures during daylight. This behaviour renders traditional, net-based survey methods ineffective. Therefore, a distribution survey of the English East Midlands was conducted by searching for individuals by torchlight after dark. H. anomala was found to be widespread within the study area, occurring at 24 out of 51 sites surveyed. However, the geographical limits of its distribution were not determined. The species occurred at low densities in canals and in backwaters of the River Trent, whilst dense swarms were observed in September 2005 in a regatta lake connected to the River Trent. H. anomala has the potential to spread through England's canal network and could colonize the lower reaches and estuaries of rivers including the River Thames and River Severn. Habitat preference analysis indicated that flowing water and absence of shelter prevented population establishment, although the species' U.K. distribution suggests that it can migrate through such areas of unsuitable habitat.