Geosesarma De Man, 1892

Manuel-Santos, Marivene, Ng, Peter K. L. & Freitag, Hendrik, 2016, Two new species of Geosesarma De Man, 1892 (Crustacea: Brachyura: Sesarmidae) from Palawan, the Philippines, Raffles Bulletin of Zoology 64, pp. 335-342 : 335-336

Geosesarma De Man, 1892

Type species. Type species Sesarma (Geosesarma) nodulifera De Man, 1892 , subsequent designation by Serène & Soh (1970).

Remarks. Geosesarma was last revised in part by Serène (1968) and Serène & Soh (1970) but is still heterogeneous in regard to several important diagnostic characters: (1) the egg size and developmental pattern: representatives with large eggs that practice completely abbreviated development (e.g., G. notophorum Ng & Tan, 1995 ), reduced larval development (e.g., G. peraccae ( Nobili, 1903) and species described in this study); as well as inland forms with small eggs that presumably return to the sea for larval release (e.g., G. maculatum (De Man, 1892) ; G. hednon ; G. ternatense ( Serène, 1968)) ; (2) the shape of the distal chitinous part of the G1, which can either be long and straight (e.g., G. maculatum ; G. hednon ); long and bent (e.g., G. krathing Ng & Naiyanetr, 1992 , G. peraccae ( Nobili, 1903)) ; or short, bent and sometimes flattened (e.g., G. notophorum , G. sabanum Ng, 1992 ); and (3) the presence of a long flagellum of the exopod of the third maxilliped (e.g., G. krathing ; G. rathbunae ; G. lawrencei ); the flagellum is reduced (e.g., G. hednon ); or is absent ( G. malayanum group, e.g., G. sabanum ; G. protos ; G. aurantium Ng, 1995 ) ( Serène, 1968; Serène & Soh, 1970; Ng & Takeda, 1992; Ng & Naiyanetr, 1992; Ng, 1992, 1995a; Ng et al., 2004; Manuel-Santos & Yeo, 2007). The dorsal margin of the adult male dactylar finger of the cheliped in many species is usually lined with a distinct row of prominent pectinate tubercles along most of the length (e.g., G. malayanum group, G. peraccae , G. noduliferum , G. bicolor Ng & Davie, 1995 , G. peraccae , G. hagen Ng, Schubart & Lukhaup, 2015 , G. dennerlei Ng, Schubart & Lukhaup, 2015 ) (see Ng, 1988; Ng & Lim, 1987; Ng et al., 2015), but in several species, such tubercles are absent. In some species (e.g., G. maculatum , G. hednon , G. aurantium , G. sabanum , G. danumense , G. lawrencei ), the proximal part of the dactylar finger has some sharp tubercles but these are not arranged in a discrete row or pectinate at the tips, and they are reduced or absent towards the distal half of the finger (Ng, 1992, 1995a, 2002; Ng et al., 2004; Manuel-Santos & Yeo, 2007).

Geosesarma batak , new species, and G. tagbanua , new species, together with G. lawrencei , share the following unique combination of characters: a squarish carapace in which the external orbital tooth is triangular in shape; the postfrontal lobes are prominent, project distinctly anteriorly, and are separated by deep, wide grooves; the ambulatory legs are relatively longer; the exopod of the third maxilliped has a long flagellum; the proximal part of the dactylar finger of the cheliped has only scattered non-pectinate tubercles; the male abdominal somite is distinctly wide; and the G1 is slender with the chitinous distal part long and gently or distinctly bent. In this combination of features, they are easily distinguished from other congeners in the Philippines or Southeast Asia. In features like larger adult size, longer ambulatory legs and arboreal habits, G. batak and G. tagbanua are superficially similar to species of Labuanium Serène & Soh, 1970 , or Scandarma Schubart, Liu & Cuesta, 2003 . Unlike Labuanium , these Palawan species do not have a short ambulatory dactylus; and compared to Scandarma , they lack the distinct granulated crest on the dorsal margin of the adult chela, and there is also no transverse ridge on the inner surface of the chela (see Schubart et al., 2003; Naruse & Ng, 2007; Ng, 2012, 2013). Their G1 structures are very different. In species of Labuanium and Scandarma , the G1 is relatively stout with the distal chitinous part short and truncate. In the two Palawan species, the G1 is distinctly more slender and the distal chitinous part is elongate and spatulate in form. In this respect, their G1s are more like those of G. krathing and G. peraccae (see Ng, 1988; Ng & Naiyanetr, 1992; Schubart et al., 2003; Naruse & Ng, 2007; Ng, 2013).

Geosesarma batak and G. tagbanua are also distinct from all congeners by their comparatively large adult size (carapace dimensions at least 22.8 × 22.4 mm for G. batak and at least 21.8 × 21.8 mm for G. tagbanua ). Other Geosesarma species are adults at sizes of 7–13 mm in carapace width (see Ng, 1988); and the largest species now known include G. rathbunae (13.0 × 12.0 mm, from Panay in the Philippines), G. insulare ( Ng, 1986) (13.9 × 13.6 mm, from Anambas Islands, Indonesia), G. katibas Ng, 1995 (13.5 × 13.5 mm, from central Borneo), G. danumense (14.8 × 14.6 mm, from Sabah, Borneo) and G. sabanum (13.1 × 13.6 mm, from Sabah, Borneo) (cf. Serene, 1968; Ng, 1986, 1995a, b).

Some species of Geosesarma do appear to be related to Labuanium Serène & Soh, 1970 , and Scandarma Schubart, Liu & Cuesta, 2003 . As several taxonomic characters in the genera Labuanium and Geosesarma , are either overlapping (e.g., basal antennular segment subglobular and swollen), inconsistent (presence of salient postfrontal lobes; carapace shape) or might have been neglected in previous studies (e.g., type of larval development; shape of the G1; presence/ absence of an exopod flagellum at third maxilliped), this group of genera needs revision. This is now in progress by T. Naruse and the second author.