Themisto, Oken, 1815

Zeidler, Wolfgang, 2004, A review of the families and genera of the hyperiidean amphipod superfamily Phronimoidea Bowman & Gruner, 1973 (Crustacea: Amphipoda: Hyperiidea), Zootaxa 567, pp. 1-66 : 28

Key to the species of the genus Themisto

1. Pereopod 5 not longer than P6 or P7 ........................................................................... 2

­ Pereopod 5 much longer than P6 or P7 ........................................................................ 5

2. Maxilliped without row of setae on distal margin of basal plate (arctic and subarctic of Atlantic) ............................................................................. T. abyssorum ( Boeck, 1871)

­ Maxilliped with row of setae on distal margin of basal plate ..................................... 3

3. Pereopods 5 & 6; dactylus smooth. Antenna 1 of female hooked, stout (colder waters of Australia and New Zealand) T. australis ( Stebbing, 1888)

­ Pereopods 5 & 6; dactylus with setae at base. Antennae 1 of female straight, tapering (North Pacific) ............................................................................................................. 4

4. Antennae 2 of females longer than A1. Adult females reach 9–17 mm in length. Pereopod 3 with 6–8 (usually 7) robust setae on posterior margin of carpus ...................... .......................................................................................... T. japonica ( Bovallius, 1887)

­ Antennae 2 of females equal in length to A1. Adult females reach 5–9 mm in length. Pereopod 3 with 3–5 (usually 4) robust setae on posterior margin of carpus ................ ........................................................................................... T. pacifica ( Stebbing, 1888)

5. Pereopod 7 usually longer than P6, but shorter than P5. Gnathopod 2; merus with relatively straight distal margin,anterodistal corner produced into small tooth(North Atlantic) ................................................................................................. T. compressa Goës, 1865

­ Pereopod 6 & 7 subequal in length. Gnathopod 2; merus with rounded distal margin 6

6. Pereopods 3–7; dactylus with setae on basal part (arctic and subarctic) ........................ ....................................................................... T. libellula (Lichtenstein in Mandt, 1822)

­ Pereopods 3–7; dactylus smooth (colder waters of southern oceans) ............................. ......................................................................................... T. gaudichaudii Guérin, 1825