Plenotheres, Ng & Ahyong, 2022

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 : 215-216

Plenotheres , new genus

Type species. Pinnotheres coarctatus Bürger, 1895 , by present designation.

Diagnosis. Carapace dorsal surface smooth, with distinct, relatively deep, clearly incised gastric grooves. MXP3 dactylus slender, inserted near proximal one-third of ventral margin of propodus, exceeding tip of propodus; propodus conical; inner margin of widest part of ischiomerus prominently angular; proximal part of exopod very wide. Female P4 asymmetrical, with left or right side distinctly longer (merus and propodus); dactyli of P2–P4 relatively short, subequal; P5 dactylus long, weakly falciform, different in structure from those of P2–P4. Male pleon relatively broadly triangular, telson semicircular. G1 very stout, arcuate, with prominent subdistal projection, dorsal fold well developed; G2 exopod half length of endopod.

Etymology. From the Latin ‘plenus’ for plump and well built, alluding to the size and shape of the type species; in arbitrary combination with a typical ending ‘- theres ’ for pea crabs, from the genus name Pinnotheres . Gender masculine.

Comparative material. Nepinnotheres corbiculae ( Sakai, 1939) , new combination: 1 male (2.1 × 1.8 mm) (KPMNH 106679), from Corbicula japonica (Prime, 1864) (Corbiculidae) , Kumamoto Prefecture, Japan, coll. T. Sakai, 1963.

Remarks. Pinnotheres coarctatus Bürger, 1895 , transferred to Arcotheres by Ahyong & Ng (2007b), is unusual in the genus in that the gastric grooves on the female carapace are relatively deep and clearly incised ( Figs. 64A , 65 ), resembling some species of Fabia Dana, 1851 ; the MXP3 propodus is conical and stout with the dactylus digitiform and extending beyond the tip of the propodus ( Figs. 66I , 68J–L ); and the MXP3 exopod is relatively broader basally ( Fig. 66I ). In addition, males of P. coarctatus , together with A. exiguus and A. rayi , differ from other species of Arcotheres in the G1 having a prominent, triangular subdistal lobe ( Fig. 68N–P ) (absent in other species of Arcotheres ), and in the articulation between the P5 ischium and merus being oblique rather than perpendicular to the segment axis ( Figs. 67A , 68D, H ). As such, a separate genus, Plenotheres , new genus, is here established for A. coarctatus .

The similarities between the G1 of A. exiguus , A. rayi , and Pl. coarctatus , new combination, are, at present, difficult to interpret; whether they are evidence of close phylogenetic proximity or convergence is unclear. All three species are currently subject of wider phylogenetic studies by us.

In the carapace shape, structure of the MXP3, and general P2–P4 leg proportions, Plenotheres coarctatus superficially resembles the ecologically similar species from Japan, Pinnotheres corbiculae Sakai, 1939 ( Figs. 69–71 ). The hosts of Pinnotheres corbiculae are noteworthy, being freshwater and estuarine clams of the family Corbiculidae Gray, 1847 . Plenotheres coarctatus is known from mangrove clams of the families Cyrenidae Gray, 1840 , and Glauconomidae Gray, 1853 , and all three bivalve families belong to the Cyrenoidea Gray, 1840 , with most of the species inhabiting brackish or freshwater habitats.

Pinnotheres corbiculae was first referred to “ Pinnotheres pholadis ” by Urita (1926: 18), but Sakai (1939: 591, textfig. 77a, b) described it as a new species from two female specimens (one measured, 5.5 × 4.5 mm) supposedly collected from Nagasaki from Corbicula japonica (Prime, 1864) (Corbiculidae) , figuring only part of the MXP3 and chela ( Fig. 69B, C ). Sakai (1976: 572, pl. 200 fig. 4) figured the overall animal ( Fig. 69A ), corrected the type locality to “Sendai-gawa, Kagoshima ”, and also recorded additional specimens: five males and seven females from the same host, from Kikuchi-gawa in Kumamoto (see also Silas & Alagarswami, 1967: 1197, 1224; Schmitt et al., 1973: 42; Ng et al., 2008: 250). The whereabouts of the type material is uncertain; Muraoka (1998: 48) records one specimen from Kumamoto in the Kanagawa Museum, but this specimen, collected in 1963, is not a type. This specimen is examined and figured here. The whereabouts of the types is not known.

Sakai’s (1939, 1976) description of P. corbiculae is relatively brief, but nevertheless useful to quote here as it is the only diagnosis of the species: “The carapace is broad and roundish, the lateral borders are subparallel but the shoulders are weakly angular and the posterior border slightly convex. The front is moderately produced, and the eyes may be observed in dorsal aspect. The external maxilliped has the dactylus slender and much exceeding the tip of propodus (in tsingtaoensis , this segment is distally broadened). The immovable finger has four or five denticles near the base, the movable finger has a stout tooth near the base and its apex is strongly curved inwards. The anterior three pairs of ambulatory legs are subequal in length, the last pair conspicuously smaller. The anterior and posterior borders of all pairs are fringed with longish hairs but the carpus is not crossed by an oblique row of hairs contrary to P. pholadis , which is also the nearest kin of this species. I could not examine the male specimen, but its seventh segment seems to be triangular according to Urita, who referred this species erroneously to P. pholadis ” ( Sakai, 1939: 591) .

The only overall habitus figure of P. corbiculae , that of Sakai (1976: pl. 200 fig. 4) (present Fig. 69A ), is rather schematic and does not clearly show the structure of the ambulatory dactyli, but the general appearance agrees well with Pl. coarctatus . The only figure of the MXP3 by Sakai (1939: fig. 77b) does not show the exopod ( Fig. 69B ).

The male specimen of Pinnotheres corbiculae examined ( Figs. 70 , 71 ) is relatively well preserved, and the general structure of the chela, MXP3, and P2–P5 are not very different from that of the male Plenotheres coarctatus . The MXP3 propodus is not as short or conical in shape, but the inner angle of the MXP3 ischiomerus is angular. The male pleon ( Fig. 71K ) is relatively more elongate than that in Pl. coarctatus ( Fig. 68M ). The G1 ( Fig. 71L–O ), however, is very different, being long and slender with the tip tapering to a sharp point, without any folds or projections (cf. Pl. coarctatus ; Fig. 68N–P ). The absence of a female specimen makes a decision on its genus affiliation more difficult, but based on Sakai’s (1976: pl. 200 fig. 4) figure (present Fig. 69A ), the P2–P5 on both sides appear to be symmetrical, with the dactylus of all legs relatively short with none prominently elongated. Significantly, Sakai (1939: 591) commented that Pinnotheres corbiculae was close to Pinnotheres tsingtaoensis Shen, 1932 , and both species have relatively short ambulatory dactyli. Their G1s are also similar ( Fig. 71L–O ; Shen, 1932: text-fig. 94c). On the basis of the available data, both Pinnotheres corbiculae and Pinnotheres tsingtaoensis should now be referred to Nepinnotheres , as N. corbiculae , new combination, and N. tsingtaoensis , new combination: the MXP3 dactylus is slender in both species and inserts sub-basally (rather than basally), the female P2–P4 do not show any obvious asymmetry, and the ambulatory dactyli are all relatively short and not strongly dissimilar in length.

Figures

Fig. 64. Plenotheres coarctatus (Bürger, 1895), female (6.8 × 6.5 mm) (ZRC 2018.246), Peninsular Malaysia. A, overall dorsal view; B, ventral view of cephalothorax; C, frontal view of cephalothorax; D, outer view of right chela.

Fig. 65. Plenotheres coarctatus (Bürger, 1895), overall dorsal view. A, neotype female (13.1 × 11.4 mm) (ZRC 2020.13), Indonesian Papua; B, female (11.5 × 10.7 mm) (ZRC 2018.766), Indonesian Papua; C, female (13.9 × 12.4 mm) (ZRC 2003.681), Singapore; D, female (13.3 × 11.9 mm) (ZRC 2012.1223), Singapore; E, female (8.0 × 7.6 mm) (ZRC 2018.254), Peninsular Malaysia; F, female (11.9 × 10.6 mm) (ZRC 2018.254), Peninsular Malaysia.

Fig. 66. Plenotheres coarctatus (Bürger, 1895). A–I, neotype female (13.1 × 11.4 mm) (ZRC 2020.13), Indonesian Papua; J, K, female (14.1 × 13.1 mm) (ZRC 2001.2237), Singapore. A–D, left P2–P5, respectively; E–H, right P2–P5, respectively; I, left MXP3; J, distal part of left P5 dactylus (ventral view, soft setae omitted); K, distal part of right P5 dactylus (ventral view, soft setae omitted). Scales = A–H, 1.0 mm; I, 0.5 mm; J, K, 0.25 mm.

Fig. 68. Plenotheres coarctatus (Bürger, 1895), male (3.3 × 3.4 mm) (ZRC 2018.765), Indonesian Papua. A–D, left P2–P5, respectively; E–H, right P2–P5, respectively; I, outer view of right chela; J, left MXP3; K, palp of left MXP3 (outer view); L, palp of left MXP3 (inner view); M, pleon; N, left G1 (ventral view); O, distal part of left G1 (ventral view, setae omitted); P, left G1 (dorsal view, setae omitted); Q, left G2. Scales = A–I, M, 0.5 mm; J–L, N–Q, 0.2 mm.

Fig. 67. Plenotheres coarctatus (Bürger, 1895), male (3.3 × 3.4 mm) (ZRC 2018.765), Indonesian Papua. A, overall dorsal view; B, frontal view of cephalothorax; C, outer view of right chela.

Fig. 69. Nepinnotheres corbiculae (Sakai, 1939). A, female, overall dorsal view; B, left female MXP3; C, outer view of left female chela. A, after Sakai (1976: pl. 200 fig. 4); B, C, after Sakai (1939: text-fig. 77).

Fig. 70. Nepinnotheres corbiculae (Sakai, 1939), male (2.1 × 1.8 mm) (KPM-NH 106679), Japan. A, overall dorsal view; B, frontal view of cephalothorax; C, outer view of left chela.

Fig. 71. Nepinnotheres corbiculae (Sakai, 1939), male (2.1 × 1.8 mm) (KPM-NH 106679), Japan. A, right MXP3 (outer view); B, right MXP3 (inner view); C, D, propodus and dactylus of right MXP3 (outer view); E, outer view of left chela; F–I, left P2–P5, respectively; J, right P3; K, pleon; L, left G1 (ventral view); M, left G1 (dorsal view); N, distal part of left G1 (ventral view, setae omitted); O, distal part of left G1 (dorsal view, setae omitted); P, right G2; Q, distal part of right G2. Scales = A, B, E–M, P, 0.2 mm; C, D, N, O, Q, 0.1 mm.