Platepistoma seani Davie & Ng, 2012, sp. nov.
Creators: name: Davie, Peter J. F.
name: Ng, Peter K. L.
Publisher: name: Zenodo
Year: 2012
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Decapoda
Cancridae
Platepistoma
Platepistoma seani

description: Platepistoma seani sp. nov.
(Figs 1, 2, 3 B, 4–6)
Material Examined. HOLOTYPE: 3 (45.5 x 34.5 mm) (ZRC 2012.0719), off Durban, KwaZulu-Natal Province, South Africa, 29°50.5'S, 31°36.7'E, bottom trawl, F.V. Ocean Surf, 365– 475 m, coll. S. Fennessy, 25 February 2006.
Description of holotype.—Carapace (Figs 2, 3 B) ovoid; 1.32 times broader than long; convex in both directions. Regions moderately distinct but separated by broad, shallow depressions; 3M moderately well separated from 2M; 2M not partially longitudinally divided; urogastric, cardiac, intestinal regions defined laterally by moderately deep sinuous grooves, but not clearly divided transversely from each other; large sub-triangular region (6L) present on inner part of mesobranchial area lateral to gastro-cardiac region, but demarcated only by low depressions, connected at inner posterior end with rest of mesobranchial region; similar but smaller, narrower subtriangular region on metabranchial area adjacent to intestinal region; lateral hepatic and epibranchial grooves broadly indicated. Carapace surface mostly smooth, finely punctate, except 5 or 6 small sharp spines on anterior part of 2M and anterolaterally on 1L, 3L; longitudinal row of 4 spines on narrow part of 3 M, transverse row across posterior part of 3M, and across cardiac region (1P); other regions with 1 or 2 short spines; spines do not project through tomentum which forms thick covering over entire dorsal surface, dorsally obscuring presence of anterolateral teeth. Anterolateral margins strongly convex, divided into 10 broad teeth, including outer orbital angle; first 9 of similar shape and size, sub-triangular to somewhat truncated, each distally armed with 2–4 accessory spinules on either side of slightly more prominent apical spine; tenth smaller, situated on posterolateral border, followed posteriorly by row of variously sized small sharp spines, with last most prominent and directed slightly forwards. Posterolateral margins strongly convergent posteriorly. Front 0.2 times carapace width; moderately projecting, medially tri-lobed, lateral lobes broad, armed with small sharp spines, medial lobe on lower level, projecting upwards as short spine (tip broken); deep pre-orbital concavity for antenna. Upper orbital border armed with row of prominent, small, sharp spines clustered particularly on inner lateral margin. Inner angle of lower orbital border formed by a spine-tipped triangular tooth; narrow V-shaped notch laterally. Antennal flagellum very long, not entering orbit, peduncle with long setae on inner distal edge of last 2 articles. Basal antennal segment distally armed with spines.
Third maxilliped (Fig. 4 B) with merus distinctly smaller than ischium; 0.75 times as long as maximum width; antero-external angle produced, rounded, bluntly pointed; inner margin with medial angle armed with sharp spinule; 0.7 times length of ischium. Ischium rectangular, 1.7 times longer than wide; inner margin granular.
Chelipeds (Figs 2 A, 5A, B) large, robust, subequal; dorsal surfaces of carpus and chela covered with dense setae that obscure surface and some of spines; merus with posterior border with only low granules, but strong subdistal and terminal spines present; lower and anterior borders bearing only small pointed granules; inner face of carpus smooth, distal margin armed with long ventral and dorsal spine; carpus bordered with spaced sharp spines that extend across upper surface, tips just visible through tomentum. Outer surface of palm microscopically granular, but armed with 6 distinct, evenly spaced transverse rows of pointed granules that increase in size to become short spines across upper face; thick tomentum obscures dorsal spinous rows; superior border similarly armed with row of sharp spines with distal-most spine largest; palm high, 0.55 times length including fixed finger. Inner surface of palm microscopically granular. Immovable finger with ventral and medial ridge; moderately long. Ventral border of chela concave at base of fixed finger. Dorsal surface of dactylus granular near base, but with 2 or 3 larger spinules; becoming microscopically granular to smooth distally. Fingers pointed; with only narrow gape near base.
Walking legs (Figs 5 C–F) relatively long, slender, moderately flattened; first three pairs of similar length; anterior margins of merus, carpus, propodus each armed with row of moderately long spines; carpus with additional row of dorsal spines; posterior margin of meri of first three pairs with row of short spinous tubercles, most prominent on first leg, weakest on third leg. Last pair of legs somewhat more spinous; merus with an extra row of spines on upper half, and more pronounced ventral spines. Dactylus somewhat longer than propodus; with 2 deep longitudinal grooves; terminating in long, acute chitinous recurved tip. Leg segments fringed with numerous long, stiff setae that extend onto outer face of carpus and propodus.
Male abdomen (Figs 4 A, 6A) with third somite 1.33 times wider than somite 2; lateral margins triangular in shape, broadest medially. Somites 3–6 tapering. Somite 6 1.65 times wider than long. Telson longer than preceding somites; 1.3 times longer than wide; reaching about three-quarters distance to suture between thoracic sternites 3, 4.
Male G1 (Fig. 6 B, C) long, relatively evenly tapering from base; sub-apical flange comparatively long, broad, low; tapering to acute tip; aperture opening terminally. G2 (Fig. 6 D) narrow, projecting short distance beyond tip of G1, tapering only slightly; apical portion c. 0.4 total length, separated from basal portion by semicircle of very short setae at point of slight constriction.
Females unknown.
Colour. The tomentum is brownish-orange overall (Fig. 1), with the surfaces of the carapace and pereiopods white. The fingers of the chela are black.
Etymology. The species is named after the collector, Sean Fennessy, who has obtained a number of rare and interesting brachyuran species from South Africa in recent years, including Calappa africana Lai & Ng, 2006, and Pleistacantha ori Ahyong & Ng, 2007 (see Lai & Ng 2006; Ahyong & Ng 2007).
Distribution. Known only from the type locality off Durban, South Africa. Bathymetric range: 365– 475 m.
Remarks. Most species of Platepistoma have extremely similar facies and the major features to readily separate them rely on the density and coverage of the setae, as well as the sharpness of the dorsal tubercles (Davie 1991). Platepistoma seani sp. nov. is similar to the two other Indian Ocean species, P. guezei (Crosnier, 1976) and P. seychellense Davie, 1991, by having the posterolateral margins marked by a row of spinous granules terminating in a moderately large spine laterally above the posterior carapace margin (Fig. 3 B). In P. guezei and P. seychellense, however, the posterolateral margins have only raised sharpened granules rather than prominent spines. Of these two species, P. s e a ni sp. nov. is more closely related to its geographically nearest congener, P. guezei from a depth of 350–720 m off Madagascar and Reunion.
The new species can be separated from Platepistoma guezei by numerous characters. 1) a much thicker setal covering that almost completely hides the anterolateral teeth from dorsal view (anterolateral teeth clearly visible in P. guezei; Crosnier 1976: fig. 7, pl. 1–1; Davie 1991: fig. 1a). 2) less well-defined carapace regions (Fig. 3 B) than in P. guezei; (Fig. 3 A; Crosnier 1976: fig. 7, pl. 1–1; Davie 1991: fig. 1a). 3) more spinular anterolateral and posterolateral carapace margins (Fig. 3 B) than in P. guezei (Fig. 3 A). 4) sparser but more prominent dorsal carapace spines (Fig. 3 B) than in P. guezei; Crosnier 1976: fig. 7, pl. 1–1). 5) more spinular claws with 6 horizontal rows of spines over a microscopically granular surface (Fig. 5 A, B) (3 rows of pointed granules and more coarsely granular surface in P. guezei; Crosnier 1976: fig. 8a, g; Davie 1991: fig. 2a). 6) dorsal surfaces of the carpus and chela are covered with dense setae (Fig. 2 A) (scattered setae in P. guezei; Crosnier 1976: pl. 1–1; Davie 1991: figs 1a, 2a). 7) more prominent spines on the carpus of the claw (Fig. 5 A, B) than in P. guezei (Crosnier 1976: pl. 1–1; Davie 1991: fig. 1a). 8) ventral spinules on the meri of the walking legs (Fig. 5 C–F) (absent in P. g u e z e i Crosnier 1976: pl. 1–1). 9) evenly triangular lateral margin of abdominal somite 3, widest in medial line (Figs 4 A, 6A) (offset triangular with widest point across distal third in P. guezei; Crosnier 1976: fig. 8c). 10) slightly more tapered male telson (Figs 4 A, 6A) than in P. guezei (Crosnier 1976: fig. 8c).
The other Indian Ocean species, P. seychellense Davie, 1991, is known only from the holotype from the Seychelles, western Indian Ocean, from a depth of 420– 430 m. Platepistoma seychellense is easily separated from P. s e a n i sp. nov. and P. guezei by much more strongly defined carapace regions, with the inter-regional furrows always being in all cases relatively broader, deeper, and sharply defined. This is particularly marked in the case of the deep longitudinal groove over the anterior two-thirds of 2M; the strong grooves almost splitting 3M from 4M; the strong definition of the triangular region on the inner part of the mesobranchial area lateral to the gastro-cardiac region, which is strongly demarcated posteriorly by a broad deep groove; and the strongly curved mediolateral groove on the branchial region (Davie 1991: figs 6c, 7c). The first impression of the carapace of P. seychellense is one of strongly defined meandrine areolation whereas the carapaces of P. s e a n i and P. guezei present a much less obvious definition of the regions.
description: Published as part of Davie, Peter J. F. & Ng, Peter K. L., 2012, A new species of Platepistoma Rathbun, 1906 (Decapoda: Brachyura: Cancridae) from deep water off South Africa, pp. 73-80 in Zootaxa 3522 on pages 73-79, DOI: 10.5281/zenodo.282677