Mesotheres, Ng & Ahyong & Campos, 2019

Ng, Peter K. L., Ahyong, Shane T. & Campos, Ernesto, 2019, Two new genera of pinnotherid crabs (Crustacea: Brachyura: Pinnotheroidea) from the Americas and the Western Pacific, Raffles Bulletin of Zoology 67, pp. 337-351 : 338-339

Mesotheres , new genus

Type species. Pinnotheres strombi Rathbun, 1905 , by present designation.

Diagnosis. Female carapace transversely ovate, distinctly wider than long, frontal margin strongly deflexed, not visible in dorsal view; male carapace prominently rounded, enlarged, slightly wider than long, frontal margin just visible from dorsal view. MXP3 with ischium and merus completely fused, very elongate, slender, carpus of palp ovate to round, larger than propodus and dactylus, articulating on subventral margin, propodus short, rounded, dactylus very short, knoblike, inserted terminally on propodus; exopod very short, concealed by ischiomerus in-situ, without flagellum. P2–P5 short, bilaterally symmetrical, merus very short, P4 and P5 propodus broad, subrectangular, P5 shortest, dactylus short, gently curved, P3 and P4 dactylus with ventral surface convex, not excavated. Male pleon with 6 free somites and triangular telson; G1 straight; G2 exopod absent.

Constituent species. Pinnotheres barbatus Desbonne , in Desbonne & Schramm, 1867, Pinnotheres serrei Rathbun, 1909 , Pinnotheres strombi Rathbun, 1905 , and Fabia unguifalcula Glassell, 1936 .

Etymology. The name is derived from the term Meso- America for Central America, where most of the constituent species occur, together with a typical pinnotherid generic suffix “-theres”. The gender of the genus is masculine.

Remarks. The four American species previously referred to Orthotheres and herein transferred to Mesotheres have a unique suite of carapace, P2–P5 and MXP3 characters. Most characteristic is the unusual structure of the MXP3, which has the ischiomerus prominently elongate, with the carpus enlarged and larger than the propodus and dactylus, and the very short exopod, which lacks a flagellum and is completely concealed by the ischiomerus ( Figs. 5A, B , 6 A–C, I, J, 8). In Orthotheres s. str. and Tacitotheres , new genus, the MXP3 ischiomerus is proportionately wider, the carpus is large but still proportionately smaller, and most significantly, the exopod is large and elongate, with a distinct flagellum, and in-situ, is not completely hidden behind the ischiomerus (cf. Ng & Ho, 2016: figs. 2B, G, 5B, D, 8D). Mesotheres also differs from Orthotheres and Tacitotheres in having a straight (versus arcuate G1) and lacks the G2 exopod (versus present in Orthotheres and Tacitotheres ). In addition, Mesotheres differs from Orthotheres in having the ventral surface of the P3 and P4 dactylus gently convex as in most pinnotherids rather than distuinctly excavated (cf. Ng & Ho, 2016: figs. 3E, G, 6J, K, 9E), and also differs from Tacitotheres in the non-clavate female P4 and P5 propodi. The MXP3 of the American Bonita Campos, 2009 (type species Bonita mexicana Campos, 2009 ), Enigmatheres Campos, 2009 (type species Fabia canfieldi Rathbun, 1918 ), and Austinotheres Campos, 2002 (type species Pinnotheres angelicus Lockington, 1877 ), are also similar to that of Mesotheres , new genus, but in these genera, the exopod is elongate, visible in outer view and possess a distinct flagellum ( Campos, 2002: fig. 1D, F; Campos, 2009: figs. 1A, B, 3B, D, E).

We have included O. serrei ( Rathbun, 1909) (from Puerto Rico) and O. unguifalcula ( Glassell, 1936) (from Mexico) in Mesotheres as their characters fit the current diagnosis of the genus. Mesotheres serrei , was well described and figured ( Rathbun, 1909: 69, unnumbered fig.; Rathbun, 1918: 84, text-fig. 41, pl. 19 figs. 1–7) and there is no need to elaborate on its taxonomy here. Similarly, the taxonomy of M. unguifalcula , from Mexico, was treated at length by Campos (1989: 1125, figs. 2, 3) and need not be revisited. Campos (1989) observes that M. unguifalcula is close to M. serrei and M. strombi .

Figures

Fig. 5. Mesotheres strombi (Rathbun, 1905) new combination, male (4.5 × 4.0 mm) (USNM 172221), Florida. A, outer view of right MXP3; B, inner view of right MXP3; C, pleon; D, anterior thoracic sternum, buccal cavity and right third MXP3; E, outer view of right chela; F, right P2; G, right P3; H, right P4; I, right P5; J, K, left G1; L, left G2. Scales: A, B, J–L = 0.5 mm; C–I = 1.0 mm.

Fig. 6. Mesotheres strombi (Rathbun, 1905) new combination, A–H, female (7.3 × 5.3 mm) (USNM 172221), Florida; I–P, male (12.5 × 10.2 mm) (USNM 122770), Florida. A, I, outer view of right MXP3; C, K, outer view of carpus, propodus and dactylus of right MXP3; B, J, inner view of right MXP3; D, outer view of right chela; E, right P2; F, right P3; G, right P4; H, right P5; L, pleon; M–O, left G1; P, left G2. Scales: A, B, D–H, I, J, M–P = 1.0 mm; C, K = 0.5 mm; L = 2.0 mm.