Key to adult males of Japanese species of the Maera -clade

1. Gnathopod 2, anterior margin of dactylus bearing many setae.................................................. 2

- Gnathopod 2, anterior margin of dactylus with single seta..................................................... 3

2. Body very large (maximum BL:> 50 mm), gnathopod 2 palm defined by blunt tooth bearing strong robust seta...................................................................................... Maera loveni (Bruzelius, 1859)

- Body small (maximum BL: about 10 mm), gnathopod 2 palm defined by acute tooth........... Maera sagamiensis sp. nov.

3. Pereopods 3–7, dactyli bifid............................................................................. 4

- Pereopods 3–7, dactyli simple........................................................................... 7

4. Gnathopod 2 palm with 2 excavations in middle.............................. Quadrimaera quqdrimana (Dana, 1853)

- Gnathopod 2 palm with single excavation in middle.......................................................... 5

5. Excavation on gnathopod 2 palm quadrate................................... Quadrimaera gotoensis Ariyama, 2019

- Excavation on gnathopod 2 palm rounded.................................................................. 6

6. Pereopod 6, posterior margin of basis heavily setose............................ Quadrimaera setibasis Ariyama, 2019

- Pereopod 6, posterior margin of basis not setose........................... Quadrimaera pacifica (Schellenberg, 1938)

7. Uropod 3, tips of both rami pointed......................................... Austromaera ariakensis Ariyama, 2019

- Uropod 3, tips of both rami truncate...................................................................... 8

8. Gnathopod 2 basis wide; uropod 3 rami long, about 3 times length of peduncle........ Meximaera mooreana (Myers, 1989)

- Gnathopod 2 basis ordinary; uropod 3 rami relatively short, 1 – 2 times length of peduncle............................ 9

9. Gnathopod 2 palm transverse or slightly oblique, without robust setae; mandibular palp article 1 with distal tooth........ 10

- Gnathopod 2 palm slightly or moderately oblique, with many robust setae; mandibular palp article 1 lacking distal tooth.. 11

10. Uropod 3 longer than uropod 2, basis of pereopod 5 subovate.................... Maeropsis okinawensis Ariyama, 2018

- Uropod 3 shorter than uropod 2, basis of pereopod 5 subrectangular.............. Maeropsis serratipalma (Nagata, 1965)

11. Palm of gnathopod 2 moderately oblique (angle of palmar corner: 115–130°)..................................... 12

- Palm of gnathopod 2 slightly oblique (angle of palmar corner: 90–105°)......................................... 13

12. Coxa 1 anteroventral corner produced acutely, distal robust setae of telson short (ca. 20% of telson length).................................................................................... Orientomaera obliquua Ariyama, 2018

- Coxa 1 anteroventral corner rounded, distal robust setae of telson long (ca. 45% of telson length)......................................................................................... Orientomaera rotundicoxa Ariyama, 2018

13. Palm of gnathopod 2 with two distinct teeth.......................................... Orientomaera incisa sp. nov.

- Palm of gnathopod 2 with several blunt teeth.............................................................. 14

14. Posterior margin of pereopod 6 basis almost straight, body color bluish pale pink................................................................................................... Orientomaera brevispina (Kim & Kim, 1991)

- Posterior margin of pereopod 6 basis convex, body color light crimson............ Orientomaera decipiens Ariyama, 2018