Geosesarma vicentense Rathbun 1914
Creators: name: Ng, Peter K. L.
name: Lemaitre, Rafael
Publisher: name: Zenodo
Year: 2017
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Decapoda
Sesarmidae
Geosesarma
Geosesarma vicentense

description: Geosesarma vicentense (Rathbun, 1914)
(Figs. 1, 5A–D)
Sesarma (Sesarma) vicentense Rathbun, 1914: 74.
Sesarma (Geosesarma) vicentensis — Serène 1968b: 106.
Geosesarma vicentensis — Serène & Soh 1970: 402 (list); Ng 1988: 119 (list). Geosesarma vicentense —Ng et al. 2008: 221 (list).
Type material. Holotype: male (9.2 × 8.5 mm), Port San Vicente (Luzon side), Palaui Island, off northern Luzon, 18°31′N 122°7′60″E, along beach, coll. 15 November 1908 (USNM 45758).
Comparative material. Geosesarma maculatum (De Man, 1892): 1 male (12.4 × 11.9 mm), station MALU 47, Halmahera, coll. MALU Expedition, L. Deharveng, July 1988 (ZRC 2017.0113).
Diagnosis. Carapace almost quadrate, wider than long, width to length ratio 1.1, lateral margins subparallel (Fig. 1 A, B); dorsal surface with regions weakly demarcated, anterior regions densely covered with low, small rounded, flattened granules (Fig. 1 A, B); front deflexed, frontal lobes broad with subtruncate margins, separated by shallow broad concavity; postfrontal, postorbital cristae moderately low, distinct (Fig. 1 A, B, E); external orbital tooth acutely triangular, directed anteriorly, outer margin almost straight, first epibranchial tooth low, second epibranchial tooth barely demarcated (Fig. 1 A, B). Merus of third maxilliped subovate, subequal in length to ischium; exopod slender, with long flagellum (Fig. 5A). Outer surface of palm of adult male with numerous low granules, shallow pits; inner surface granular but without distinct transverse ridge; dorsal margin of dactylus with 3–5 low conical granules on proximal part, tips not chitinous, distal half uneven but without distinct granules (Fig. 1 F–H). Ambulatory legs each with relatively stout merus having sharp subdistal spine on dorsal margin and lateral, mesial surfaces gently rugose (Fig. 1 A); propodus without brush-like setae on ventral margin. Male pleon triangular, relatively wide; somite 6 with convex lateral margins; telson semicircular (Fig. 1 C, D). G1 slender, distal chitinous part elongated, almost straight, directed upwards, subspatuliform, tip dilated, medially clefted (Fig. 5B–D).
Remarks. The taxonomic position of Geosesarma vicentense has been uncertain, with Ng & Takeda (1992: 81) questioning its placement in Geosesarma, especially since the only known specimen was obtained from a beach. Geosesarma species are otherwise found further inland in freshwater habitats (Ng 1988). Geosesarma vicentense, however, belongs to the same group of species as G. maculatum (De Man, 1892), G. ternatense (Serène, 1968a), and G. hednon Ng, Liu & Schubart, 2004, in which the G1 is slender, straight, with the chitinous distal part subspatuliform and directed upwards. Most of the species in this group have small eggs and almost certainly have planktotrophic larvae that must be released into the sea.
Compared to G. maculatum, the frontal orbital lobes of G. vicentense are more convex (Fig. 1 A, B) (lobes truncate in G. maculatum; Fig. 2 A, B); the external orbital tooth is relatively less prominent and separated from rest of margin by a shallow cleft (Fig. 1 A, B) (tooth more prominent and separated from margin by deep cleft in G. maculatum; Fig. 2 A, B); the ambulatory meri are relatively slender (Fig. 1 A) (meri broader in G. maculatum; Fig. 2 A); and the male pleonal somite 6 is proportionately broader (Fig. 1 D) (somite 6 proportionately longer in G. maculatum; Fig. 2 C) (see also Ng et al. 2004: figs. 1A, B, 2A). Geosesarma ternatense is distinct in that the male pleonal somite 6 is proportionately very broad with strongly convex lateral margins and the telson is relatively wider (Ng et al. 2004 fig. 8B) (somite 6 and telson less wide in G. vicentense; Fig. 1 D). Geosesarma vicentense is closest to G. hednon in general form, but can be distinguished by its external orbital tooth being distinct and separated from the rest of the margin by a small cleft (Fig. 1 A, B) (tooth subparallel to the rest of the lateral margin and separated by a fissure in G. hednon; cf. Ng et al. 2004: figs. 9B, 11B) and the slender chitinous part of the G1 is relatively shorter (Fig. 5B–D) (chitinous part relatively longer in G. hednon; cf. Ng et al. 2004: figs. 13C–G,14C– H).
Geosesarma protos Ng & Takeda, 1992, also described from Luzon, is very different from G. vicentense in possessing an external orbital tooth that is prominent and directed obliquely laterally, a third maxilliped in which the exopod has no flagellum and the G1 is stout with a short chitinous part (Ng & Takeda 1992: fig. 1A, B, G–I)
description: Published as part of Ng, Peter K. L. & Lemaitre, Rafael, 2017, On the taxonomy of two poorly known species of Geosesarma De Man, 1892 (Crustacea: Brachyura: Sesarmidae) from the Philippines, pp. 475-483 in Zootaxa 4250 (5) on page 476, DOI: 10.11646/zootaxa.4250.5.5, http://zenodo.org/record/495550