Niche differentiation in habitat and current preference corroborates taxonomic distinction of Jassa falcata and Jassa marmorata (Amphipoda, Crustacea)

In an investigation of the fauna using the red algae Delesseria sanguinea, Phyllophora pseudoceranoides and Ceramium rubrum as substrata, samples were analysed from two subtidal sites near Helgoland (North Sea), one (semi-) exposed to currents, and the other protected from strong water movement. There were clear differences in the abundances of the gammaridean amphipods Jassa falcata and Jassa marmorata between these sites suggesting strong preference for water movement for J. falcata and a preference for more sheltered habitats for J. marmorata. In laboratory experiments both species preferred current to still water, but J. falcata was stricter in its choice than J. marmorata. Both species differed in their choice of habitat algae in the field and in the laboratory. These results emphasise the importance of taxonomic distinction in ecological studies of the congeners that often have been confused or subsumed in former ecological research.