The recent arrival of the oceanic isopod Idotea metallica Bosc off Helgoland (German Eight, North Sea): an indication of a warming trend in the NorthSea?

In 1988 a long-term study was started of the isopod fauna associated with s urface drift material off Helgoland (German Eight, North Sea). In the summe r of 1994 specimens of Idotea metallica Bose were recorded for the first ti me. There is no evidence that this species has ever been present in the Ger man Eight before. The samples contained males, both gravid and non-gravid f emales, and juveniles, indicating that the species reproduced successfully in the Helgoland region. Interbreeding of specimens from Helgoland and the western Mediterranean produced fertile offspring. As a neustonic species, I . metallica shows a high natural capacity for dispersal. It thus seems unli kely that the arrival of the species in the North Sea resulted from an acci dental introduction by man. We are probably witnessing an extension of the species' geographical range by natural means of dispersal, as a response to recent changes in the ecological conditions of the German Eight. Temperatu re data measured by the Biologische Anstalt Helgoland since 1962 show that the last decade (except 1996) was characterized by unusually mild winters. Following the severe winter of 1996, I. metallica was again absent from the Helgoland region. After the subsequent mild winters (1997 and 1998), howev er, the species reappeared in the summer of 1998 with higher numbers than e ver before. This suggests that the observed phenomena are closely connected with the recent temperature anomalies. I. metallica can be regarded as a p otential immigrant to a warmer North Sea, and may be useful as a sensitive indicator of the predicted long-term warming trend.