Borneosa Ng & Grinang 2022, gen. nov.
Creators: name: Ng, Peter K L
name: Grinang, Jongkar
Publisher: name: Zenodo
Year: 2022
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Decapoda
Gecarcinucidae
Borneosa

description: Borneosa gen. nov.
urn:lsid:zoobank.org:act: 583B9824-3E1F-4D14-BBA6- EA2A23F0FED3
Type species: Sundathelphusa tenebrosa Holthuis, 1979, by present designation.
Diagnosis: Carapace transversely ovate; dorsal surface gently granulated, striated, rugose or pitted, convex, not distinctly swollen or inflated; cervical grooves deep, relatively broad; epigastric cristae slightly to distinctly anterior of postorbital cristae, never fused into single ridge; frontal median triangle distinct, with dorsal margin cristate, fused with lateral margins or separated only by small gap; epibranchial tooth low but distinct with shallow cleft to confluent with anterolateral margin; lateral striae on antero- and posterolateral margins strong, prominent; larger chela never inflated or prominently enlarged even in large specimens; male thoracic sternites 3 and 4 proportionately wide, tip of sternopleonal cavity reaches or almost reaches suture between sternites 3 and 4; male pleon distinctly T-shaped; adult male chelipeds symmetrical or only slightly asymmetrical; tubercle of male pleonal locking mechanism on anterior third of sternite 5; G1 terminal segment subcylindrical, tip rounded wide; dorsal and ventral surfaces with numerous long setae; G2 subequal or longer than G1, distal segment flagelliform, about half to one-third length of basal segment; vulvae on submedian part or anterior third of sternite 6 large, round, without sternal cover, posterior margin slightly rimmed.
Etymology: The name is derived from the name of the island the species are endemic to, Borneo; in arbitrary combination with the genus name Thelphusa. Gender feminine.
Remarks: The taxonomic concept of Sundathelphusa Bott, 1969, remains unsatisfactory and a revision is needed. Ng and Sket (1996) treated Archipelothelphusa Bott, 1969 (type species Thelphusa grapsoides H. Milne Edwards, 1853) as a junior subjective synonym because the characters used by Bott (1969 1970) to separate the two genera are not discrete and overlap. Ongoing studies suggest that Sundathelphusa is actually made up of several discrete species groups with well demarcated distributions, which should be recognised as separate genera in the future (see also Ng 1990; Klaus et al. 2009), and Archipelothelphusa may be a good genus after all. One of the groups which is endemic to Borneo is here described as a new genus, Borneosa gen. nov.
There are several key characters that distinguish Sundathelphusa sensu lato from Borneosa. Many species of Sundathelphusa lack a frontal median triangle of any sort, and those that possess it invariably have the dorsal margin wide, subcristate and spanning the entire frontal margin, but does not fuse with the lateral margins at all (Fig. 1A). In Borneosa, the structure of the frontal median triangle is different, being more distinct and occupies only the median part of the frontal margin, with the dorsal margin cristate and fuses with the lateral margins or nearly so (Fig. 1B). In Sundathelphusa, male thoracic sternites 3 and 4 are proportionately narrower transversely and the tip of the sternopleonal cavity reaches to midway between sternites 3 and 4, with a distinct gap visible between it and the suture between sternites 3 and 4 (Fig. 1C). In Borneosa, male thoracic sternites 3 and 4 are proportionately wider and the sternopleonal cavity reaches distinctly more anteriorly, with the tip of the cavity reaching the suture between sternites 3 and 4 (Fig. 1D). Although thoracic sternites 3 and 4 are fused in both genera, there remains a shallow groove or ridge that marks the boundaries of the two sternites. In addition, the G1 terminal segment of Sundathelphusa is cone-shaped with only the ventral margin distinctly lined with long setae and the tip is sharp with the opening small (cf. Chia and Ng 2006: figs. 40A–D, 41A–D, 43A–D; Ng and Anker 2016: fig. 4A–C; Schubart and Ng 2008: fig. 4A–D; Husana and Ng 2019: figs. 2A–D, H–M, J–S, 5A–D, I–L) whereas in Borneosa, it is subcylindrical to subconical in shape, with the overall surface covered with short and long setae and the tip is rounded with a relatively large opening (e.g., Fig. 20I–K). The dorsal carapace surface of Sundathelphusa species is relatively smooth, and striae, when present, and generally low and rounded (cf. Chia and Ng 2006: figs. 39, 42; Ng and Anker 2016: fig. 2; Schubart and Ng 2008: fig. 1A; Husana and Ng 2019: figs. 1A–C, 3E, 4A, B, 7A); in Borneosa the surface is always prominently rugose, with the striae on the lateral margins strong (e.g., Figs. 2B, 4B, 6B, 8B, 10B, 12B, 15B, 17B). The same is true for the outer surface of the chelae; being relatively smooth or at most uneven in Sundathelphusa species (cf. Ng and Anker 2016: fig. 3C; Schubart and Ng 2008: fig. 2; Husana and Ng 2019: figs. 1F, 4F, 7E, F) but prominently granulated and rugose in Borneosa (e.g., Figs. 5D, E, 13D, E, 16D). In large males of Sundathelphusa, one of the chelae is always prominently enlarged and swollen, often at least twice the size of the minor one (cf. Chia and Ng 2006: fig. 42A; Ng and Anker 2016: figs. 2, 3C; Schubart and Ng 2008: fig. 1A; Husana and Ng 2019: fig. 7A, E, F). In Borneosa, even in large males, the major chela is only slightly larger than the minor one and never prominently swollen (e.g., Figs. 5D, E, 16D).
description: Published as part of Ng, Peter K L & Grinang, Jongkar, 2022, Establishment of A New Bornean Genus of Gecarcinucidae (Crustacea: Brachyura), with Descriptions of Five New Species., pp. 1-35 in Zoological Studies 61 (13) on page 2, DOI: 10.6620/ZS.2022.61-13, http://zenodo.org/record/8074520