First observations of the freshwater jellyfish Craspedacusta sowerbii Lankester, 1880 in Ireland coincides with unusually high water temperatures

The freshwater hydrozoan Craspedacusta sowerbii was observed for the first time in Ireland at five localities in two separate river catchments (Shannon and Erne) during the summer of 2013. All collected medusae from Lough Derg on the Shannon catchment were female. Analysis of water temperature data for the period 2001-2015 found that water temperatures greater than 21°C were only recorded in 2013. The occurrence of medusae in three distinct areas during this unusually warm summer suggests that it may have been present in its polyp form for some years, or even decades. While it is not known when the species arrived in Ireland, the spread of this species may have involved different pathways. With climate warming, further appearances of the medusa-stage may be expected.