Key to adult specimens of known Magelonidae species in the North-Western Pacific (type localities/ confirmed locations given in brackets (modified from Mortimer et al. 2022). Full descriptions of characters can be found in Mortimer et al. (2021a).

1. Prostomium with distinct prostomial horns................................................................. 2

- Prostomium without prostomial horns, or with a squared anterior margin (often referred to as “rudimentary horns”)....... 9

2. Anterior margin appearing smooth (minute crenulations only present under high magnification)...................... 3

- Anterior margin crenulate (degree of crenulation variable).................................................... 5

3. Deep purple pigment band between chaetigers 5 and 8....................... M. japonica (Korean Archipelago, Japan)

- No marked pigmentation of the thoracic region............................................................. 4

4. Prostomium wider than long, no superior dorsal lobes of the thoracic region. Abdominal parapodia sub-equal, those of the neuropodia being somewhat smaller............................................ M. koreana (Korean Archipelago)

- Prostomium longer than wide, superior dorsal lobes from chaetigers 1–8, abdominal parapodia approximately equal.................................................................................. M. boninensis sp. nov. (Japan)

5. Prostomium width approximately similar to length.......................................................... 6

- Prostomium wider than long............................................................................ 7

6. Abdominal hooded hooks bidentate......................... M. crenulifrons (Viet Nam, Hong Kong, Natuna Islands)

- Abdominal hooded hooks tridentate................................ M. cf. cornuta (Hong Kong, Japan, see remarks)

7. Superior dorsal lobes present in the thorax................................................................. 8

- No superior dorsal lobes present in thorax, anterior margin only minutely crenulate............. M. petersenae (Thailand)

8. Small superior dorsal lobes present on chaetiger 9, abdominal hooded hooks tridentate.......... M. lenticulata (Viet Nam)

- No superior dorsal lobes present on chaetiger 9, abdominal hooded hooks bidentate............... M. methae (Thailand)

9. Chaetiger 9 with mucronate chaetae..................................................................... 10

- Chaetiger 9 bearing capillary chaetae only, as in chaetigers 1–8................................................ 13

10. Abdominal hooded hooks tridentate..................................................................... 11

- Abdominal hooded hooks bidentate........................................... M. sachalinensis (Sakhalin Island)

11. Anterior thoracic notopodial lamellae with smooth or lightly crenulated upper edges.............................. 12

- Anterior thoracic notopodial lamellae with pectinate upper edges............................. M. pectinata (Thailand)

12. Abdominal parapodia without triangular processes at the inner margin of chaetal rows (DML, VML). M. parochilis (China)

- Notopodial lamellae of chaetiger 8 distinctly bilobed, abdominal parapodia with triangular processes at the inner margin of chaetal rows (DML, VML).............................................................. M. tinae (Thailand)

13. Abdominal hooded hooks polydentate............................................ M. agoensis (Ago Bay, Japan)

- Abdominal hooded hooks bi- or tridentate................................................................ 14

14. Thoracic notopodia with superior dorsal lobes............................................................. 15

- Thoracic notopodia without superior dorsal lobes.......................................................... 18

15. Prostomium wider than long........................................................................... 16

- Prostomium longer than wide.......................................................................... 17

16. Thoracic notopodial lamellae slender, no superior dorsal lobe on chaetiger 9..................... M. kamala (Thailand)

- Stout species, with distinct armour-like appearance, thoracic lamellae foliaceous, superior dorsal lobe on chaetiger 9 present................................................................................. M. armatis sp. nov. (Japan)

17. Prostomium only marginally longer than wide, anterior margin squared (very rudimentary horns)..... M. noppi (Thailand)

- Prostomium length greater than width, rudimentary horns with a triangular anterior margin....... M. alba sp. nov. (Japan)

18. Anterior thoracic neuropodial lamellae distinctly scoop-shaped........................... M. cf. cincta (Hong Kong)

- Anterior thoracic neuropodial lamellae slender triangular.................................................... 19

19. Moderate to large species with rudimentary horns, broad palps with several rows of papillae..... M. mickminni (Thailand)

- Prostomial margin rounded to straight, minute species with slender palps carrying two rows of papillae.................................................................................................. M. pygmaea (Thailand)