On the current and possible future status of the neustonic isopod Idotea metallica Bosc in the North Sea: a laboratory study

In the 1990s. a period of extraordinarily mild winter temperatures in the G erman Bight (North Sea). the oceanic, neustonic isopod Idotea metallica Bos e was found for the first time off the island of Helgoland. The species was recorded in subsequent summer periods, reproducing successfully in the are a, while its whereabouts in winter remained uncertain. The question arises whether a continuing increase in mean winter water temperature, as predicte d in most scenarios on Future climate, might enable I. metallica to overwin ter in the North Sea and to become a permanent resident there. Experiments on laboratory-cultured I. metallica were performed for the first time. Popu lation dynamics was studied at different temperatures in microcosms, Furthe rmore, the temperature effects on reproductive output, mortality and durati on of embryonic development were studied in individually reared animals. Th e results suggest that at current temperature conditions (mean winter water temperature of about 5 degreesC) I. metallica is unable to overwinter in t he German Bight, and that even an increase to about 8 degreesC probably wou ld nut change this situation, The recently observed summer populations of t he species in the German Eight obviously originate from individuals introdu ced each year anew by water currents from the Atlantic Ocean. Nevertheless, I. metallica may be useful as a sensitive indicator: in a warming North Se a the species may become a regular summer immigrant, developing more and mo re conspicuous populations in this area as the annual period with water tem perature above the critical level (13 degreesC) for reproduction extends. ( C) 2001 Elsevier Science B.V. All rights reserved.