Revised Morphological Relationships Within the Amphipod Genera<i>Pontoporeia</i>and<i>Gammaracanthus</i>and the "Glacial Relict" Significance of their Postglacial Distributions

The Holarctic coastal marine and freshwater members of the amphipod families Pontoporeiidae and Gammaracanthidae reveal much greater taxonomical, distributional–ecological, and physiological diversity than formerly recognized. Pontoporeia is formally divided into three full genera (comprising more than 20 species), namely, Pontoporeia (Kroyer, 1842; Monoporeia, new genus, and Diporeia, new genus. Gammaracanthus is divided into two full genera (encompassing about a dozen species) namely, Gammaracanthus Bate, 1862, and Relictacanthus, new genus; Gammaracanthus is divided into the subgenera Gammaracanthus Bate, and Pseudacanthus, new subgenus. The genera are transferred from family Gammaridae, superfamily Gammaroidea, to family Gammaracanthidae (newly diagnosed herein) within superfamily Eusiroidea. Two lines of evidence refute previous theories (e.g. Hogbom's "sluicing up" theory) concerning the origin and evolution of the so called glacial and marine–glacial crustacean fauna of the northern hemisphere: (1) the broad taxonomic diversity of pontoporeiid and gammaracanthid amphipods; and (2) the regional endemicity of primary freshwater members of Eurasia and North America, between which no intercontinental exchange apparently took place during Pleistocene and recent times. A more credible explanation for such taxonomic and biogeographical diversity requires a much longer time frame (e.g. late Cretaceous to late Tertiary times).