Key to the Dikerogammarus species (based on adult males only)

1. Gnathopod 1 with mid-palmar spine(s)........................................... D. aralychensis (Birstein, 1932)

- Gnathopod 1 without mid-palmar spine(s).................................................................. 2

2. Peduncle segments of A1 short and swollen...................................... D. gruberi Mateus & Mateus, 1990

- Peduncle segments of A1 long and slender.................................................................. 3

3. Dorsal elevations on urosomites obsolescent.......................................... D. fluviatilis Martynov, 1919

- Dorsal elevations on urosomites well developed............................................................. 4

4. Metasome segments carinate and toothed................................................ D. caspius (Pallas, 1771)

- Metasome segments not toothed.......................................................................... 5

5. Inner surface of basal segment of pereopod 7 with setae....................................................... 6

- Inner surface of basal segment of pereopod 7 without setae..................................................... 7

6. Ventral side of the peduncular segment 2 of A1 with few short setae (shorter than the diameter of the segment)............................................................................................. D. istanbulensis sp. nov.

- Ventral side of the peduncular segment 2 of A1 with many long setae (longer than the diameter of the segment)........................................................................................ D. bispinosus Martynov, 1925

7. Pereopods slender, basal segment of pereopod 5 unlobed.................................... D. oskari Birstein, 1945

- Pereopods robust, basal segment of pereopod 5 lobed......................................................... 8

8. Dorsal elevations (humps) with 3 spines.............................................. D. villosus (Sowinsky, 1894)

- Dorsal elevations (humps) with 2 spines.................................................................... 9

9. Ventral side of the epimeral plate 2 with many setae..................................... D. balatonicus Ponyi, 1955

- Ventral side of the epimeral plate 2 with a few setae............................... D. haemobaphes (Eichwald, 1841)