Using only the photograph it is quite difficult to identify the species. However, I can say that it is the most probably the representative of the family Pontogeneiidae (possibly genus Gondogeneia, Paramoera or Prostebbingia). The books by Barnard & Karaman can help for sure in genus identification. Also the papers by Thurston 1974 (The Crustacea Amphipoda of Signy Island, South Orkney Islands. British Antarctic Survey Scientific Reports, 71: 1-133 and Crustacea Amphipoda from Graham Land and the Scotia Arc, collected by Operation Tabarin and the Falkland Islands Dependencies Survey, 1944-59. British Antarctic Survey Scientific Reports, 85: 1-89) could be useful. As far as I know there are no keys dedicated specifically to these three genera.
Maybe one of these articles can help you identify the species? Or, if you still have the sample, you can use the identification key to sort out the genus/sp. I hope this helps.
Using only the photograph it is quite difficult to identify the species. However, I can say that it is the most probably the representative of the family Pontogeneiidae (possibly genus Gondogeneia, Paramoera or Prostebbingia). The books by Barnard & Karaman can help for sure in genus identification. Also the papers by Thurston 1974 (The Crustacea Amphipoda of Signy Island, South Orkney Islands. British Antarctic Survey Scientific Reports, 71: 1-133 and Crustacea Amphipoda from Graham Land and the Scotia Arc, collected by Operation Tabarin and the Falkland Islands Dependencies Survey, 1944-59. British Antarctic Survey Scientific Reports, 85: 1-89) could be useful. As far as I know there are no keys dedicated specifically to these three genera.