Tacitotheres, 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 : 345-346

Tacitotheres , new genus

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

Diagnosis. Female carapace rounded, slightly wider than long, frontal margin visible from dorsal view; male carapace rounded, slightly wider than long, with distinct front. MXP3 with ischium and merus completely fused, elongate; carpus subovate, small, articulating on subventral margin; propodus elongate, cylindrical, larger than carpus or dactylus; dactylus slender, relatively short, inserted subterminally or terminally on outer surface of propodus; exopod slender, long, adjacent to ischiomerus in situ, with short flagellum. P2–P5 short, bilaterally asymmetrical from left to right; merus relatively elongate; P4 and P5 propodus clavate with distal part wider than proximal part; P5 shortest, dactylus short, gently curved; P3 and P4 dactylus with ventral surface convex in crosssection, not excavated. Male pleon with 6 free somites and telson; G1 arcuate; G2 exopod distinctly longer than endopod.

Constituent species. Pinnotheres glaber Bürger, 1895 , Pinnotheres laevis Bürger, 1895 , and Pinnotheres longipes Bürger, 1895 .

Etymology. The name is derived from the Latin, “tacitus”, for hidden and secret, in combination with a typical pinnotherid generic suffix “-theres”, alluding to the fact that the three species have not been collected for over 100 years! The gender of the genus is masculine.

Remarks. This new genus is established for three species formerly placed in Orthotheres : Pinnotheres glaber Bürger, 1895 (type species), P. laevis Bürger, 1895 , and P. longipes Bürger, 1895 . Although P. laevis and P. longipes are currently known only from their type description ( Bürger, 1895), they are included in Tacitotheres , new genus, based on the form of their MXP3 and the assumption that the female P4–5 propodus is clavate (these appearing, at most, weakly clavate in Burger [1895: pl. 9 figs. 25, 31]). The P4 and P5 propodi of T. glaber are clavate ( Fig. 10A, D ) but were not clearly figured as such by Bürger (1895: pl. 9 fig. 23) and we suspect a similar situation pertains to the type figures of P. laevis and P. longipes (see Burger 1895: pl. 9 figs. 25, 31).

Of Pinnotheres laevis Bürger, 1895 , Campos (1989) already commented that it is not a species of Orthotheres because of its carapace proportions and relative lengths of the articles of the third maxilliped palp, and suggested it be referred to Pinnotheres s. lato instead. The transfer of P. laevis and P. longipes to Tacitotheres addresses some of these concerns. Nevertheless, the correct taxonomy of these species is seriously compromised by the loss of type specimens of both species. Until P. laevis and P. longipes can be evaluated based on specimens, they are tentatively placed in Tacitotheres .

The MXP3 of Tacitotheres , including the presence of a welldeveloped exopod with a short flagellum, closely resembles that of Orthotheres s. str. (cf. Ng & Ho, 2016: figs. 2B, C, G, 5B–E, 8D–F). The ambulatory legs of Tacitotheres , however, are distinctively more elongate and slender ( Fig. 10 A, F ) (versus short and stout in Orthotheres s. str., cf. Ng & Ho, 2016: figs. 3A, B, D, F, H, J–M, 6A–I, K, 9A–D, F–I) and the ventral surface of the P3 and P4 dactylus is convex in cross-section (versus distinctly concave in Orthotheres s. str., cf. Ng & Ho, 2016: figs. 3E, G, 6J, K, 9E). In addition, the G2 of T. glaber has a well-developed and very long exopod ( Fig. 10L ) (exopod four times endopod length versus as long as or shorter than the endopod in Orthotheres s. str.; Ng & Ho, 2016: fig. 5M). The female P4 and P5 propodus is distinctly clavate in shape (i.e. distal part wider than proximal part) in T. glaber ( Fig. 10A ), a feature not present in other species that have been associated with Orthotheres s. str. or Mesotheres , new genus, and that we have examined (except possibly O. laevis and O. longipes , see below). Species of Orthotheres s. str. have a more symmetrical P4 and P5 propodus (cf. Ng & Ho, 2016: figs. 3A, B, L, M, 6C, D, G, H, 9–I) than Tacitotheres as does Mesotheres ( Figs. 5H, I , 6G, H ).

The general carapace shape and left-right symmetry of the ambulatory legs of Tacitotheres resembles those of Pinnotheres Bosc, 1802 , and Nepinnotheres Manning, 1993a , but the structure of the MXP3 differs significantly. In Tacitotheres , the MXP3 dactylus is short and inserted clearly subterminally on the outer surface of the propodus, with the propodus the longest segment, and the carpus short and not enlarged ( Fig. 10E, H, M ).

The MXP3 of Tacitotheres also resembles that of Abyssotheres Manning & Galil, 2000 (see also Komatsu & Ohtsuka, 2009) but differs in possessing a larger palp with the propodus larger than the carpus ( Fig. 10E, H, M ), instead of a short palp with the carpus larger that propodus as in Abyssotheres (cf. Manning & Galil, 2000: fig. 1a; Komatsu & Ohtsuka, 2009: fig. 3F). Tacitotheres further differs in the eyes of the female that are not dorsally visible ( Figs. 9A, B , 10A ) (versus clearly well visible in dorsal view in Abyssotheres ), the propodi of P2–P5 are clavate ( Fig. 10A ) (subquadrate in Abyssotheres ) and the ambulatory legs are asymmetrical ( Figs. 9 , 10A ) (symmetrical in Abyssotheres ) (cf. Manning & Galil, 2000: 66; Komatsu & Ohtsuka, 2009: figs. 1A, 2D–G).

The American Bonita and Enigmatheres also have a clavate P5 propodus ( Campos, 2009: figs. 1E, 2C), but have a proportionately more elongate MXP3 with an enlarged carpus. Another American genus, Raytheres Campos, 2004 (type species Pinnotheres clavapedatus Glassell, 1935 ) also has clavate ambulatory propodi, but these are on P2 and P3 instead (see Campos, 2002: fig. 5E).

Figures

Fig. 10. Tacitotheres glaber (Bürger, 1895) new combination, A–E, lectotype female (7.8 × 6.5 mm) (USNM 32431), Palau; F–I, paralectotype male (4.0 × 3.6 mm) (SMF-ZMG 173); J–M, paralectotype male (4.6 × 4.2 mm) (SMF-ZMG 173). A, F, dorsal overall habitus; B, sternum, anterior margin; C, anterior cephalothorax; E, H, right MXP3; D, G, right chela, outer view; I, J, male pleon; K, left G1, pleonal view; L, left G2, pleonal view, with long exopod; M, left MXP3. A, D–I, modified after Ahyong & Ng (2007a: fig. 24). Scale: A = 2 mm; B–D, F–G, I, J = 1.0 mm; E = 0.5; H, K, L, M = 0.25 mm.

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.

Fig. 9. Tacitotheres glaber (Bürger, 1895) new combination, A, paralectotype male (4.6 × 4.2 mm) (SMF-ZMG 173); B, C, lectotype ovigerous female (7.8 × 6.5 mm) (USNM 32431), Palau. A, B, overall dorsal habitus; C, ventral surface of cephalothorax.