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Marenzelleria sibling species Types I and II are characteristically distributed within the salinity gradient of the Elbe estuary. For the first time F 1 -hybrids between the 2 Marenzelleria species were found. Type I inhabits mainly the isoosmotic/hyperosmotic range, while Type II and the F 1 -hybrids prevail in the hypoosmotic range. A sympatric occurrence could be inferred in salinity ranges from 3.3 to 12.3 ppt. Marenzelleria sibling species Type II has been found for the first time in the North Sea (Elbe estuary). The influence of salinity on the distribution of Marenzelleria spp. was examined by laboratory experiments. In a time-course experiment with salinity changes from 10 to 25 ppt and from 25 to 10 ppt, respectively, enzyme activities and free amino acids were determined. The activities of alanine aminotransferase, aspartate aminotransferase, and glutamate dehydrogenase were always higher in Type I than in Type II. A salinity increase from 10 to 25 ppt resulted in a faster increase of the concentrations of D,L-alanine, glycine, serine, glutamate and threonine in Type I. The biochemical results showed that the capacity of cell volume regulation could be involved in the different spatial distributions of the 2 sibling species.

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