Polyonyx sinensis, Stimpson, 1858

Osawa, Masayuki & Sato, Taigi, 2022, A distinctive new species of the genus Polyonyx Stimpson, 1858 (Crustacea Decapoda: Anomura: Porcellanidae) from Okinawa, southwestern Japan, Zootaxa 5091 (4), pp. 587-597 : 595-596

Key to the Indo-West Pacific species of the Polyonyx sinensis group

1. Meri of ambulatory legs with rows of spines on ventral margins (spines occasionally obsolete on ventrolateral margin of second pereopod)........................................................................................... 2

- Meri of ambulatory legs unarmed on ventral margins......................................................... 5

2. Carapace with broad V-shaped median cleft on posterior margin; median lobe of rostrum not visible in dorsal view........................................................................................... P. pedalis Nobili, 1906

- Carapace without distinct V-shaped median cleft on posterior margin; median lobe of rostrum clearly visible and anteriorly produced in dorsal view................................................................................ 3

3. Branchial margins of carapace convex. Basal article of antennular peduncle with small projection mesially on anterior margin.......................................................................... P. kumejima Osawa & Ng, 2016

- Branchial margins of carapace subparallel, nearly straight. Basal article of antennular peduncle without projection mesially on anterior margin....................................................................................... 4

4. Carapace with blunt, elevated protogastric ridges, making anterior gastric region inflated. Merus of second pereopod comparatively more slender, 2.6–2.9 times longer than distal height........................ P. heok Osawa & Ng, 2016

- Carapace with weak or obsolete protogastric ridges. Merus of second pereopod 2.5 times longer than distal height...................................................................................... P. planus Osawa & Ng, 2016

5. Median branchial margins of carapace bluntly angular and produced laterally. Dorsal surfaces of carapace and carpi and palms of chelipeds with distinct protuberances......................................................... P. deezi n. sp.

- Median branchial margins of carapace not angular. Dorsal surfaces of carapace and chelipeds without protuberances...... 6

6. Branchial margins of carapace strongly convex. Carapace and carpi of chelipeds with relatively long transverse striae on dorsal surfaces. Merus of each cheliped with broad lobe on dorso-anterior margin........................................ 7

- Branchial margins of carapace subparallel, nearly straight (maybe slightly convex in P. maccullochi ). Carapace and carpi of chelipeds with at most delicate, short transverse striae on dorsal surfaces. Merus of each cheliped with narrow lobe on dorsoanterior margin (comparatively broader lobe in P. maccullochi )................................................ 11

7. Median lobe of rostrum faintly developed, nearly transverse in dorsal view. Merus of each cheliped with subrectangular lobe on dorso-anterior margin............................................................ P. boucheti Osawa, 2007

- Median lobe of rostrum broadly triangular, distinctly produced anteriorly. Merus of each cheliped with rounded lobe on dorsoanterior margin....................................................................................... 8

8. Anteromedian margin of third thoracic sternite slightly concave............................. P. angustus Osawa, 2018

- Anteromedian margin of third thoracic sternite broadly convex................................................. 9

9. Carapace 1.1–1.2 times as broad as long. Merus of second pereopod 1.5 times as long as maximum high.......................................................................................... P. socialis Werding & Hiller, 2019

- Carapace proportionally broader, 1.2–1.4 times as broad as long. Merus of second pereopod more than 2.1 times as long as maximum high...................................................................................... 10

10. Carpus of each cheliped with distinct row of plumose setae on dorso-anterior margin. Merus of second pereopod 2.5–2.7 times as long as maximum high........................................... P. pilosibrachium Osawa, Naruse & Ng, 2018

- Carpus of each cheliped with no or only a few shorter setae on dorso-anterior margin. Merus of second pereopod proportionally stouter, 2.1–2.2 times as long as maximum high.......................................... P. utinomii Miyake, 1943

11. Carapace with scattered long setae on dorsal surface. Propodi of ambulatory legs each with 2–4 irregular, longitudinal rows of spines on ventral surface............................................................. P. haigae McNeill, 1968

- Carapace without long setae on dorsal surface. Propodi of ambulatory legs each with single longitudinal row of spines or sparse some spines on ventral surface.......................................................................... 12

12. Carpus of each cheliped with row of dense, long plumose setae on dorso-anterior margin. Propodi of ambulatory legs each with more than 12 spines on ventral margin.............................................. P. transversus ( Haswell, 1882)

- Carpus of each cheliped with no or row of short plumose setae on dorso-anterior margin (setae most numerous in P. thai ). Propodi of ambulatory legs each with less than 11 spines on ventral margin, excluding pair of distoventral spines........ 13

13. Carapace slightly broader than long; median lobe of rostrum acute, strongly produced........... P. maccullochi Haig, 1965

- Carapace distinctly broader than long; median lobe of rostrum weakly or bluntly produced.......................... 14

14. Carpus of larger cheliped with broad concavity or slope on proximal half of dorso-anterior margin.................... 15

- Carpus of larger cheliped with short concavity proximally on dorso-anterior margin................................ 16

15. Carpus of larger cheliped weakly or moderately concave on proximal half of dorso-anterior margin, fixed finger of larger chela with tip somewhat turned outwards........................................... P. vermicola Ng & Sasekumar, 1993

- Carpus of larger cheliped nearly straight on proximal half of dorso-anterior margin, fixed finger of larger chela with tip not turned outwards.................................................................. P. sinensis Stimpson, 1858

16. Carpus of larger cheliped strongly bulging on dorso-anterior margin, except for distinct proximal concavity............. 17

- Carpus of larger cheliped weakly convex on dorso-anterior margin, except for shallow proximal concavity............. 18

17. Propodi of ambulatory legs slender (e.g., 2.4–2.7 times as long as maximum high on fourth pereopod); ventral margins each with usually 1 subproximal and 1 subdistal spines, in addition to pair of distoventral spines.............................................................................................. P. sasekumari Osawa, Naruse & Ng, 2018

- Propodi of ambulatory legs proportionally stouter (e.g., 2.3 times as long as maximum high on fourth pereopod); ventral margins each with row of 4–9 spines, in addition to pair of distoventral spines........................ P. thai Werding, 2001

18. Third thoracic sternite with median lobe on anterior margin. Propodi of ambulatory legs each with 1 subdistal spine on ventral margin, in addition to pair of distoventral spines........................................ P. loimicola Sankolli, 1965

- Third thoracic sternite without median lobe on anterior margin. Propodi of ambulatory legs each with 3–5 spines on ventral margin, in addition to pair of distoventral spines.......................................... P. tulearis Werding, 2001