when they are morphologically hardly
distinguishable ‘‘cryptic species’’ [1]. More and more molecular
studies reveal that cryptic species are common and not restricted
to certain taxonomic groups or ecosystems [2]. Several studies
found divergence between cryptic species in ecological, physiological
or behavioural aspects [1,3,4]. This suggests that the
identification of cryptic species and differences between them
may be essential for understanding population dynamics and
ecosystem functioning.
Multiple cryptic species complexes have been identified in
freshwater vertebrates (e.g. [5,6]) as well as invertebrates (e.g.
[7,8]). Several ecologically important ‘‘species’’ have thus been
shown to consist of several reproductively isolated entities (e.g. [9]),
the biological differences between which are often unknown.
Amphipoda is an order of mostly aquatic crustaceans in which
cryptic species have been found many times. Several amphipods