A novel low salinity switch hypothesis is proposed to account for the speciation of an obligate estuarine (oligohaline) amphipod, Orchestia aestuarensis, from a closely-related one, Orchestia mediterranea, found in both estuarine and marine conditions (euryhaline). The underlying genetic mechanisms could involve: 1. A dimorphic allele, or linked set of alleles, carried by the euryhaline amphipod which controls the ability to breed in low salinity conditions in estuaries and which is selected for in these conditions, producing the oligohaline amphipod. 2. A genetically-assimilated gene or genes, controlling the ability to breed in low salinity conditions in estuaries, which is/are “switched on” by low salinity conditions. 3. Allopatric speciation from a euryhaline to an oligohaline amphipod species where low salinity conditions is the selective switch. It is possible that other estuarine, sibling, amphipod pairs have evolved by salinity switching. In the North Atlantic coastal region, this could include: Gammarus tigrinus/G. daiberi and G. salinus/G. zaddachi (Amphipoda, Gammaridae).