To determine whether alpine gammarids also differ morphologically from Gammarus lacustris, the biologists sought the assistance of an experienced taxonomist from Slovenia. “Specialists like this are indispensable for our research,” says Altermatt, “but unfortunately they’re increasingly rare in this country.” Guided by the Slovenian expert, the biologists dissected the gammarid specimens (around two centimetres long), comparing the antennae, mouthparts and various other appendages – and made an interesting discovery. Alther explains: “The most noticeable feature distinguishing the alpine gammarid is that, on its second pair of feeding limbs, it only has one group of bristles. In Gammarus lacustris, there are two.” On the basis of the genetic and morphological differences (Fig. 3), the biologists assigned the alpine populations to a new species, which they named Gammarus alpinus.
Scientists who describe a new species have to follow certain rules. The first description must appear in a scientific publication, indicating the relevant (“diagnostic”) characteristics which enable the new species to be identified and distinguished from related species. Alther and Altermatt’s description has been published in the renowned Zoological Journal of the Linnean Society. The description must be based on a type specimen, known as the holotype, deposited in a public scientific collection. In the case of Gammarus alpinus, the holotype is a specimen from Lake St Moritz, held by the Lausanne Museum of Zoology. Additional specimens are deposited in Eawag’s collection at Dübendorf. The new species name has been recorded in the official registry of zoological nomenclature (ZooBank).
Speciation promoted by glaciation
Analysis of the phylogenetic tree suggests that Gammarus alpinus began to develop as a separate species around seven million years ago. According to the authors, the speciation process was probably promoted by glaciation of the Alps during the subsequent Pleistocene epoch. The ice sheets served as barriers to reproduction, isolating the alpine populations from other gammarid populations and preventing gene flow. The authors believe that, during glacial maxima, Gammarus alpinus withdrew to a southern refugium, spreading northwards again as the glaciers retreated. This explains why the species now occurs almost exclusively in areas that drain into the Mediterranean or the Black Sea and are absent in most of the Rhine drainage basin (Fig. 4).
The distribution of Gammarus alpinus appears to be restricted to the Alps. As Altermatt points out, “This means the Alpine countries have a special responsibility for the conservation of this endemic species.” Certain alpine lakes are exposed to nutrient inputs associated with agricultural land use, and gammarid populations may be adversely affected as a result. In the future, climate change could also pose an increasing threat to Gammarus alpinus, which is adapted to cold conditions.