The marine isopod Idotea metallica is a recent
immigrant in the German Bight (North Sea).
Extraordinarily mild winter temperatures and the
resulting extension of the annual reproductive period
enable this warm adapted species to develop
conspicuous summer populations in the North Sea
which, however, go extinct when temperatures are too
low for reproduction.

In laboratory experiments, the immigrant proved to be
competitively inferior to indigenous species. The
competitive interaction with the ecologically similar
Idotea baltica is based on intensive interference with
juveniles being preyed upon by both conspecific and
heterospecific adults. As I. baltica usually occurs in
much higher densities, juvenile I. metallica are exposed
to extremely high predation pressure leading to local
extinction of the immigrant. Consequently, the
occurrence of I. metallica in the North Sea is unlikely to
have a significant impact on the local species
composition in contrast to observations on several
other neozoa.

An obligatory association of I. metallica with floating
objects allows for efficient, passive dispersal with
surface currents. Most probably, the species occurs in
the North Atlantic as a yet undescribed persistent
network of small local populations exhibiting all
features of a marine metapopulation. North Sea
summer populations might originate from small
populations permanently introduced by the eastward
flowing North Atlantic Drift.

In contrast to floating macroalgae abiotic flotsam is
resistant to natural decay but has only low nutritional
value for colonizing organisms. Populations of the
rather herbivore I. baltica grew significantly slower
when fed only with animal food. The rather carnivor I.
metallica was only little affected by the absence of plant
food. Thus, I. metallica proved to be better adapted to
long-term persistence on abiotic objects where animals
feed mainly on the zooplankton from the surrounding
water. Additionally, floating algal substrates are
destroyed much slower by I. metallica than by I. baltica
because of the lower demand for plant food.
Furthermore, Idotea