Speodromia platyarthrodes Stebbing 1905
Creators: name: Ng, Peter K. L.
Publisher: name: Zenodo
Year: 2016
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Decapoda
Dromiidae
Speodromia
Speodromia platyarthrodes

description: Speodromia platyarthrodes (Stebbing, 1905)
(Figs. 1–10)
Dynomene platyarthrodes Stebbing, 1905: 59, pl. 17; Balss 1938: 6 (discussion).
Speodromia platyarthrodes — Barnard 1946: 371; Barnard 1950: 334, pl. 64; Kensley 1981: 37; Kensley & Buxton 1984: 190; McLay 1993: 182; Guinot & Tavares 2003: 74, 113; Ng et al. 2008: 35.
Material examined. 1 male (28.1 × 22.1 mm) [shot and drawn] (SAM MB-A 000816), station PF18757, St. Croix Island, NW 0.25 W, Algoa Bay, South Africa, 33.8965°S 25.884°E, 8 m, no other data; 1 male (37.2 × 25.9 mm) (SAM MB-A 000817), station PF92, Mossel Bay, South Africa, 34.0833°S 22.2333°E, no other data; 1 female (15.9 × 13.2 mm) (SAM MB-A 000818), station P478, South Africa, 33.8358°S 25.77°E, no other data; 1 male (31.7 × 23.1 mm) (SAM MB-A 010980), station 19601205, Port Alfred, South Africa, 33.6°S 26.8667°E, coll. 5 December 1960; 1 female (26.4 × 21.1 mm) (SAM MB-A 019135), station 19840201, reef 2 km off Bird Rock, reef 2 km off Bird Rock, Algoa Bay, South Africa, 33°50'S 25°40'E, coll. February 1984; 2 females (27.7 × 22.5 mm, 26.5 × 21.3 mm) (SAM MB-A 039629), station TRA 130V, False Bay, South Africa, 34°19.65' S 18°30.5"E, 55 m, dredge, from University of Cape Town Ecology Collection, February 1957; 1 male (33.5 × 24.6 mm) (SAM MB- A039993), station Lan1989040, Schaapen Island, Langebaan, South Africa, 33.0833°S 18.0167°E, coll. C.L. Griffiths, April 1989; 1 female (38.1 × 28.1 mm) (SAM MB-A 073963), cruise And003, station D00287, South Africa, 34.7935°S 21.204°E, 71 m, coll. demersal trawl, J. Currie, 3 May 2014.
Diagnosis. As for genus.
Description. Overall surface of carapace, chelipeds, ambulatory legs, exposed surfaces of thoracic sternum, pleon (Figs. 3, 4 A–C), covered with clumps of clavate, scale-like setae of various thickness, length, density; bases of setae arranged in clumps; those on margins usually denser, longer. Carapace (Figs. 3, 6 C) transversely subovate, with anterior part produced. Anterodorsal surfaces swollen (Figs. 3, 4 A, B), regions clearly demarcated; surface generally smooth, covered with clumps of setae, with bases grouping together giving surface rugose appearance. Gastric regions (Figs. 3, 4 A, B) prominently swollen; epi-, proto-, metagastric regions separated by shallow longitudinal groove; outer side of each metagastric region with small, deep pit; gastro-cardiac groove distinct, deep; gastric, cardiac regions separated by deep transverse depression, depression adjacent to branchial regions deep, broad; branchial regions prominently swollen, with deep pit adjacent to gastric region on each side. Cardiac region (Figs. 3, 5 F) separated from intestinal region by deep transverse groove. Posterolateral, posterior surfaces of carapace (Figs. 3, 5 E, F) with numerous shallow pits. Branchial regions with large, deep cavity on anterior part of sub-branchial side (Figs. 5 A–D), surfaces of cavity smooth, without any structures; branchiostegal membrane curving into cavity, margins lined with dense short setae; opening to sub-branchial cavity joins shallow grooved area on posterior part of sub-branchial region (Fig. 5 E, F), leads to depressed area between P4, P5 coxae, with small gap (Fig. 6 B–D) even when P2–5 are tightly coapted to carapace.
Front (Figs. 3 B, 4A) trilobate, median lobe short, rounded, lateral lobes low. Supraorbital margin (Fig. 4 A) with median spine, joining lateral part of carapace as low, broad tooth. Orbit (Figs. 4 A, B) ovate; eye mobile, relatively short with round cornea, peduncle short, smooth. Suborbital margin (Fig. 4 B, D, E) short, concave, with sharp submedian tooth. Anterolateral margin (Figs. 3, 4 C) arcuate, not confluent with orbits, anteriorly curving downwards towards pterygostomial region; with 6–8 low, triangular teeth of varying strength, sharpness; meeting pterygostomial region as short triangular tooth. Posterolateral margin (Figs. 3, 6 C) not clearly demarcated from anterolateral margin, distinctly convex, converging to posterior carapace margin. Posterior carapace margin (Fig. 3 A, 5E, F, 6C, D) concave. Pterygostomial region (Fig. 4 B) with low, sharp tooth. Epistome (Fig. 4 B, E) forming triangular plate with proepistome; posterior margin broad, separated into 2 truncate lobes by deep median fissure; each lobe with sinuous margin, separated from rest of margin by distinct fissure. Endostome (Fig. 4 E) with low but distinct sublongitudinal ridge. Antennule (Fig. 4 E) folding obliquely inwards; fossa subtriangular; basal article subquadrate, occupying about half space of fossa. Antenna (Fig. 4 E) distinct; inner margin overlapping outer surface of antennule, not separated by hiatus or ridge; article 1 subrectangular, mobile, with inner margin with prominent cleft, urinary opening at base of cleft; article 2 subquadrate, distal margin with distinct cleft, with subtriangular article 3 at base of cleft; article 4 quadrate; flagellum relatively short, ca. as wide as orbit.
Third maxilliped (Fig. 4 F, G) relatively long, surface covered with clumps of clavate setae, those on inner margins longer, denser. Ischium (Fig. 4 F, G) separated from basis by suture but articles immobile; ischium subrectangular with trigonal cross-section; outer surface without sulcus; mesial surface narrow, outer margin relatively smooth, inner margin lined with prominent pectinate crista dentata; inner surface relatively smooth. Merus (Fig. 4 F, G) subtriangular, medially transversely bent. Palp (carpus, propodus, dactylus) (Fig. 4 F) inserted laterally on merus. Exopod (Fig. 4 F) relatively stout, distal edge reaching to just before anterior edge of merus; flagellum long.
Chelipeds (Figs. 3 A, 7A–C) subequal; relatively long; closely fitting lateral sides of carapace when retracted. Coxa (Fig. 8 A) large, broadly L-shaped; basis fused with ischium, suture distinct, inner surface of ischium with 2 low ridges, lined with setae. Merus (Figs. 5 A–D, 7A, B) short, outer surface covered with dense short setae, inner surface of posterior part with deep concavity, ventral margins with low crista; dorsal margin with large expanded crista on distal half, that on ventral half low, forming C-shaped concavity which is adjacent to sub-branchial cavity when held tightly against carapace. Carpus (Fig. 3 A, 7A, B) short, subtriangular, outer surface covered with dense short setae, with broad inner distal tooth. Chela (Fig. 7 A–C) stout, outer surface essentially smooth but clumps of setae gives illusion of longitudinal ridges; dorsal surface smooth, with prominent oblique crista which forms rounded shelf-like structure; inner surface of chela with dense setae. Fingers (Fig. 7 B, C) stout, gently curved, shorter than palm, stout, proximal part covered with dense short setae, smooth but setae give illusion of ridges present; cutting edges with prominent teeth, inner surface of distal part excavate, bracketed by strong teeth.
P2, P3 similar in form (Figs. 3 A, 6A, 7E, F), P2 longer. Coxa (Figs. 8 A, B, 9A) subquadrate, with short ledgelike projection on inner margin of posterior part which locks onto margin of pleonal somites when closed. Merus (Figs. 6 A, 7E, F) relatively short, subrectangular; ventral surface bicristate, forming distinct concavity between cristae; dorsal crista large, prominent, subdistal part rounded, separated from dentiform distal part by prominent fissure. Carpus (Figs. 6 B, 7E, F) short, subtriangular, with median ridge, well developed dorsal ridge, ventral surface with 2 low ridges forming shallow concavity between them. Propodus (Figs. 6 B, 7E, F, 10A, B) rectangular, with median ridge, well developed dorsal ridge, ventral surface with 2 low ridges forming shallow concavity between them. Dactylus (Figs. 7 E, F, 10A, B) stout, relatively short, gently curved with pectinate tip, ventral margin with 3 pectinate spines. Dactylo-propodal lock (Figs. 7 E, F, 10A, B) well developed, proximal part of dactylus with gently curved tooth that overlaps smooth shelf-like surface on rounded distal margin of propodus, propodus with distinct concavity on flexor part of subdistal margin which tooth of dactylus locks into.
P4 (Figs. 6 D, 7D, 10C) similar in form to P2, P3, but proportionately smaller, without dactylo-propodal lock; dorsal crista of merus without subdistal rounded lobe, with only single dentiform distal lobe; carpus with distal margin relatively more expanded; propodus with dorsal surface cristate, median ridge low; flexor side of distal margin with long fixed spine which opposes strongly recurved, hook-like dactylus.
P5 (Figs. 6 C, D, 7D, 10D) smallest; slender, length of merus> carpus> propodus; with merus slender, elongate, dorsal surface with low dorsal ridge, ventral surface with 2 low ridges; carpus, propodus elongate, unarmed, smooth; distal margin of propodus with long fixed spine on flexor side, relatively shorter spine on extensor side, dactylus hook-like.
Thoracic sternum (Figs. 8 A–C) almost entirely covered by pleon. Sternites 1, 2 (Fig. 8 A) completely fused, sunken below buccal cavity, below sternite 4; sternite 3 fused to sternites 1, 2 but discernible as almost vertical plate between sternites 1+2, 4. Sternite 4 (Figs. 8 A, 9A) semicircular, smooth, median part depressed as part of sternopleonal cavity; episternite 4 visible as small extension beneath coxa of cheliped when pleon closed. Sternites 5–8 (Fig. 9 A) with only lateral parts of suture visible, all medially interrupted, with median sternal surface smooth, gently depressed; episternite 5 just visible as short projection, not visible with pleon closed. Penis (Fig. 9 A) not calcified, long, mobile, protruding from P5 coxa anteriorly to median part of coxa of P3. No epipods or podobranchs present on P1–P5.
Telson, all somites (Fig. 8 A–C) free; surfaces covered with short setae, otherwise smooth; closed pleon almost completely concealing thoracic sternum except for anterior part of sternite 4. Telson (Fig. 8 A, D, F) triangular, tip relatively sharp, lateral margins gently convex, tip relatively sharp; uropod ventral, ovate, mobile, underneath telson, not visible from dorsal view. Somites 3–6 (Fig. 8 A–C) forming subrectangular structure with median part swollen; lateral margin of somites 3, 6 distinctly convex; lateral margins of somites 4, 5 sinuous. Somite 2 (Figs. 5 E, F, 6D, 8C) subtrapezoidal; lateral margin almost straight. Somite 1 (Figs. 5 E, F, 6D, 8C) very wide, medially constricted. Other than G1, G2, no other pleopods visible on somites.
G1 (Figs. 8 H, 10E, F) short, outer margin, distal parts with long, soft setae; tip tapering to sharp pectinate point. G2 (Figs. 8 I, 10G) slender, gradually tapering to sharp tip; no endopod present; outer margin with long setae.
Female characters. Females very similar to males in all non-sexual aspects. Thoracic sternum (Fig. 9 B) similar in form to male except relatively wider. Vulva (Fig. 9 B) on posterior half of coxa of P3. Spermathecal groove (sternal suture 7/8) long, prominent (Fig. 9 B, C), extending anteriorly, almost joining, reaching sternite 4, between coxae of cheliped; aperture to spermatheca on each side of rounded tip. P2, coxae of P3 (Fig. 9 B) smooth, without trace of coxal locking mechanism as in male. Female pleon (Fig. 2 D) ovate; telson, somites free; telson (Fig. 8 E, G) semicircular, uropod relatively more longitudinally ovate (Fig. 8 G) compared to males, hidden from dorsal view.
Colour. Dorsal surfaces of carapace and pereiopods brown to orange, with patches of grey and dirty-white; ventral surfaces of ambulatory legs dirty-white to light brown; fingers of cheliped bright pink with inner surfaces white (Figs. 1, 2).
Remarks. Stebbing (1905: 60) described the species on the basis of a damaged female which measured 26 by 21 mm. Barnard (1946: 371) commented that “The type was not returned to the South African Museum. There are, however, one male from Mossel Bay, and one adult male, one juv. male and one juv. female from Algoa Bay, 20–28 fathoms, which I have examined. In view of these latter localities, Stebbing's locality, "off Cape Point, 650–700 fathoms," is so remarkable that it suggests a misplaced label. It is also not among the types in SAM listed by Kensley (1974); nor is it in The Natural History Museum (London), where some of Stebbing’s material is kept (see Ng & Clark 2010). The provenance of Stebbing’s holotype is not known and may be lost. The recent specimens, including several obtained after Barnard’s (1950) report, indicate that his observations on the habitat are correct. All were collected from relatively shallow waters less than 100 m depth.
Stebbing (1905: pl. 17) depicted the spermathecal groove of sternite 7/8 was distally separated but this is not the case. In all the present females, sternite 7/8 meet medially to form a contiguous structure, although the apertures for the spermathecae are on each side of the tip (Fig. 9 C). Stebbing (1905: 59) described the pleon as “trilobed, but his figure (Stebbing 1905: pl. 17) shows a more triangular structure like those observed here (Fig. 8 G, E).
The possession of modified grooves and channels on the sub-branchial surfaces of the carapace is not unique to Speodromia, and is present on several unrelated groups of Brachyura. For example, genera like Rathbunaria Ward, 1933 (Planopilumnidae) and Glyptocarcinus Takeda, 1973, and Antrocarcinus Ng & Chia, 1994 (both Xanthidae) (see Takeda 1973; Ng & Chia, 1994; Ng 2010) possess them but they do not lead to any chamber in the subbranchial regions like Speodromia. Even in the xanthid Guinotellus Serène, 1971, from Philippines (Serène 1971; Mendoza et al. 2008), which has grooves on the sub-branchial region and the sub-branchial region has a prominent concavity, it is not developed into a semi-closed cavity like in Speodromia. All these taxa also have characteristic chelipeds and cristate ambulatory legs which are coapted against the smooth lateral surfaces of the carapace. As discussed by these and other authors, these structures are almost certainly associated with a more efficient respiration, possibly when the animals are hiding under muddy or sandy habitats. It remains unknown how exactly how these concavities, grooves, and chambers work. When the ambulatory legs are appressed against the carapace, the only space left is a small gap just above P4 (Fig. 6 C, D); the P5 being too slender and small to close it or significantly influence water flow (Figs. 5 E, F, 6C, D).
Biology. Most of examined specimens were collected from soft bottom substrates 8– 71 m. One specimen (SAM MB-A019135) was obtained from a reef in Algoa Bay. A specimen was also observed in a coral reef near Cape Town where it “was sitting on a soft coral under an overhang” (G. Jones, pers. comm.) (Fig. 1).
Live specimens of Speodromia platyarthrodes have not been observed to carry any objects (Fig. 1). The subcheliform structure of P4 and P5 are nevertheless well developed and certainly fully functional, and can certainly carry objects if necessary; they certainly do not appear vestigial. It is possible they function to carry objects to help with camouflage like other dromiids when younger (see Guinot et al. 1995; Guinot & Wicksten 2015), but lose the behaviour on reaching adult size. Certainly the carapace and pereiopods are well calcified.
description: Published as part of Ng, Peter K. L., 2016, The taxonomy of Speodromia platyarthrodes (Stebbing, 1905) (Crustacea: Brachyura), an unusual dromiid crab endemic to South Africa, pp. 261-275 in Zootaxa 4111 (3) on pages 262-274, DOI: 10.11646/zootaxa.4111.3.4, http://zenodo.org/record/271466