Exagorium, Naruse & Chung & Tangah, 2015

Naruse, Tohru, Chung, Arthur Y. C. & Tangah, Joseph, 2015, Description of a new genus and a new species of the family Camptandriidae Stimpson, 1858 (Crustacea: Decapoda: Brachyura) from Lower Kinabatangan-Segama Wetlands, Sabah, Malaysia, Raffles Bulletin of Zoology 63, pp. 327-333 : 327-328

Exagorium , new genus

Type species. Exagorium fidelisi , new species, by present destination. Gender neuter.

Diagnosis. Carapace hexagonal, CW 1.24–1.40 times CL. Regions poorly defined, epigastric region with one pair of weak convexities, H-shaped gastric groove shallow. Frontal, supraorbital, anterolateral margins connected by rounded, granular ridge; similar ridges present on posterolateral margins (short, oblique), metabranchial regions (short, horizontal), and on intestinal region (long, transverse) just anterior to posterior margin with either end reaching posterolateral corner of carapace. Orbit laterally bordered by low, thin ridge on upper half, lined with long setae, lower half open. Infraorbital crista sinuous, lined with granules, convex dorsally at middle, mesial end forming high triangular inner orbital tooth. Suborbital ridge gently arcuate, lined with small granules, granules becoming longer laterally, ridge lateral to orbit directed downwards, without granules. Posterior margin of pterygostomial region rimmed, lined with granules, arcuate anteriorly, forming distinct Milne-Edwards aperture. Antennule folded obliquely. Antenna entering into orbit, with long flagellum. Eyes weakly compressed dorsoventrally. Third maxillipeds filling entire buccal cavern; ischium wider than long, short, about third length of merus; carpus, propodus, dactylus articulating end to end; exopod narrow, reaching distal third of merus. Thoracic sternum relatively wide. In male, sterno-abdominal cavity deep, long, reaching posterior border of buccal cavern. In female, vulvae horizontally elongated, oblong, entire, placed close to each other at mesial to suture 5/6. Male abdomen with all somites free, somite 1 short, very wide, laterally reaching and partially covering P5 coxae. Female abdomen with all sutures distinct, somites 2–6 functionally fused; telson triangular, proximal portion embraced by distal margin of abdominal somite 6. G1 q-shaped, bent 180°. Distally with two terminal processes, tips pointed. G2 shorter than terminal processes of G1.

Etymology. The genus name “ Exagorium ” is derived from an arbitrary combination of hexagonal shape of the carapace of the type species (L. exagonum) and a suffix for the type genus of the family Camptandriidae “ -rium ”.

Remarks. Exagorium , new genus, is distinct from other known camptandriid genera by the combination of the following characters: hexagonal carapace outline ( Fig. 1 ), male abdomen that covers the G1 completely when closed ( Fig. 2b ), male abdominal somite 1 that reaches and partially covers P5 coxae ( Fig. 3a ), and the G1 with two terminal processes.

Manning & Holthuis (1981) observed that the male abdominal somite 1 of Cleistostoma De Haan, 1833 , and Ilyogynnis Manning & Holthuis, 1981 , reaches the P5 coxae. Close examination of these genera revealed that the male abdominal somite 1 of Exagorium reaches and even partially covers the P5 coxae ( Fig. 3a ), whereas there is a slight space between somite 1 and the P5 coxae in Cleistostoma ( Fig. 3b ) and Ilyogynnis ( Fig. 3c ). Among the genera that were not treated by Manning & Holthuis (1981), the male abdominal somite 1 does not reach the P5 coxae in Nasima Manning, 1991 ( Manning 1991: 304) . Of the genera examined in the present study (e.g., Moguai Tan & Ng, 1999 , Mortensenella Rathbun, 1909 , Paramoguai Ahyong, 2014 , and Takedellus Tan & Ng, 1999 ), the male abdominal somite 1 does not reach the P5 coxae as well. The male abdominal somite 1 of Moguai , Paramoguai and Takedellus is concealed under the posterior margin of the carapace ( Fig. 3d ; posterior margin of the carapace is lifted up dorsally) and only the distal margin of the somite 1 is exposed in situ, but the lateral end of the somite 1 falls short of P5 coxae.

Four camptandriid genera are also known to possess two long processes at the distal end of the G1: Camptandrium Stimpson, 1858 , Moguai , Paramoguai and Takedellus ( Tan & Ng, 1999: figs. 2A–C, 4F, 6G, H; Kishino et al., 2014: fig. 2G; Ahyong, 2014: fig. 1H). Exagorium is, however, easily distinguishable from the above four genera by its distinct hexagonal carapace outline, the relatively smooth dorsal surface of the carapace, the general shape of the third maxilliped, and the wide male abdominal somite 1 that reaches the P5 coxae.

Figures

Fig. 1. Live colouration of Exagorium fidelisi, new genus & species. Holotype, IPMB-Cr 08.1, 7.6 × 10.6 mm.

Fig. 2. Exagorium fidelisi, new genus & species. Holotype, IPMB- Cr 08.1, 7.6 × 10.6 mm.

Fig. 3. Posterolateral portion of male thoracic sternite and abdomen. a, Exagorium fidelisi, new genus & species, holotype, IPMB-Cr 08.1, 7.6 × 10.6 mm; b, Cleistostoma dilatatum (De Haan, 1833), ZLKU11072, 13.4 × 19.1 mm; c, Ilyogynnis microcheirum (Tweedie, 1937), RUMF-ZC-357, 6.6 × 8.2 mm; d, Takedellus ambonense (Serène & Moosa, 1971), IPMB-Cr 08.5, 4.1 × 4.2 mm (Posterior margin of the carapace is lifted up). Arrow indicates where the posterior margin of the carapace is originally placed. a1–3, abdominal somites 1–3; p5c, coxa of pereopod 5; s7, 8, thoracic sternites 7, 8.