Arcotheres Manning, 1993a

Ng, Peter K. L. & Ahyong, Shane T., 2022, The pea crab genus Arcotheres Manning, 1993 (Crustacea: Brachyura: Pinnotheridae) from Singapore and Peninsular Malaysia, with a reappraisal of diagnostic characters and descriptions of two new genera, Raffles Bulletin of Zoology 70, pp. 134-248 : 144

Genus Arcotheres Manning, 1993a

Type species. Pinnotheres palaensis Bürger, 1895 , by original designation.

Diagnosis. MXP3 dactylus slender (digitiform), inserted on proximal one-third or near midlength of ventral margin of propodus, variable length; propodus linear, usually spatuliform, 2.5–3.0 times longer than wide; inner margin of widest part of ischiomerus rounded to gently angular; exopod relatively slender with proximal part of exopod not especially wide. Female P4 asymmetrical, with one side distinctly longer (usually merus, propodus, and dactylus); longer P4 and both P5 dactyli longer, different in structure than those of P2 and P3; P5 dactylus elongate, linear; ischiomeral articulation perpendicular to segment axis. Male pleon broadly triangular, telson subhexagonal, subpentagonal to broadly triangular or semicircular, widest basally near midlength. G1 relatively stout, arcuate, with or without subdistal ventral projection; G2 with exopod.

Remarks. Arcotheres must be attributed to Manning (1993a) even though the name was first used in 1880 by Nauck in an unpublished note and later cited by Bürger (1895: 361), although he did not recognise the genus. Manning (1993a: 524) diagnosed the genus in a short note, attributed the authorship to Bürger (1895) and assigned Pinnotheres palaensis Bürger, 1895 , as the type species (see also Manning, 1993a: 127, 128). Campos & Manning (2001) clarified the nomenclature of the genus (see also Ng et al., 2008: 252).

Arcotheres is a large genus with over 30 species, with Ahyong & Ng (2021) having recently transferred another five to the genus. Ng et al. (2017) questioned the monophyly of Arcotheres (see also Ahyong & Ng, 2007b) given the diversity in form in the large genus, and singled out A. borradailei ( Nobili, 1906a) and A. ridgewayi ( Southwell, 1911) as being different in several features of the pereopods. Both species are here synonymised with Magnotheres globosus (Hombron & Jacquinot, 1846) (see later). The carapace, MXP3, and pereopods of A. coarctatus ( Bürger, 1895) are also quite different from those of typical Arcotheres , so A. coarctatus is also referred here to its own genus, Plenotheres , new genus. As redefined in this study, Arcotheres now contains 23 valid species ( Table 1).