Striacoeloma tubur, Ng & Ahyong & Shane T., 2024

Ng, Peter K. L., Ahyong, & Shane T., 2024, Redefining Lophoplax Tesch, 1918: recognising Myopilumnus Deb, 1989, and the establishment of three new genera, Hosekia, Takedaplax, and Striacoeloma (Crustacea: Brachyura: Pilumnidae), Raffles Bulletin of Zoology 72, pp. 335-355 : 351-354

Striacoeloma tubur , new species

( Figs. 10 , 11 )

Material examined. Holotype: AM P107758 , female (8.6 × 6.3 mm), Middle Arm , Darwin Harbour, Northern Territory, Australia, 12°35′17″S 130°52′17″E, 5–7 m, coll. Cardno Pty Ltd.

Etymology. Named tubur for the tubercle-like protrusions on the extensor margins of the ambulatory legs; used as a noun in apposition.

Diagnosis. As for genus.

Description of holotype female. Carapace subhexagonal; frontal and anterolateral margins with scattered tufts of short and long setae, not brush-like; surface appearing irregularly corrugated, partly eroded; gastric regions prominently raised, forming 2 transverse swellings extending to branchial region, with posterior one reaching to lateral margin; gastro-cardiac region depressed, with grooves visible; cardiac region with transverse swelling, extends almost to posterolateral margin ( Fig. 10A–C ); surface adjacent to posterior carapace margin raised, with prominent wide rounded transverse ridge adjacent to the posterior carapace margin, forming channel extending to sub-branchial region ( Fig. 10A, B ). Subhepatic and suborbital regions slightly rugose; pterygostomial region with numerous fine granules ( Fig. 10D ). Frontal margin bilobed, almost straight, lined with granules, with shallow median notch, lateral lobe small but distinct, not visible in dorsal view ( Fig. 10A–D ). Supraorbital margin sinuous, with low lobe on posterior third, with 2 short fissures ( Fig. 10B, C ). Orbit submarginal, supraorbital margin appearing almost confluent with general carapace outline, eyes partially visible in dorsal view; eyes scarcely movable, ocular peduncle short, filling orbit, scarcely movable ( Fig. 10C, D ). Anterolateral margin with 4 distinct teeth (including external orbital tooth), first 2 broad, lobiform, with surface adjacent to them eroded, each with median oblique depression; last 2 dentiform ( Fig. 10A, B ). Posterolateral margin with 2 distinct tubercles, margins distinctly converging towards posterior carapace margin ( Fig. 10A, B ). Antennules rectangular, relatively large; antennules folding laterally ( Fig. 10D ). Basal antennal article subquadrate; flagellum entering orbital hiatus ( Fig. 10D ). Epistome with shallow biconcave posterior margin, median lobe subtruncate with short median fissue ( Fig. 10D ). Endostome short, recessed posteriorly into buccal cavern oblique to plane of adjacent pterygostomial surface, endostomial ridges low, short.

Third maxilliped ischium short, quadrate, length 1.2× width; merus with anteroexternal angle distinctly auriculiform; exopod stout, tip reaching distal edge of merus ( Fig. 10E ).

Thoracic sternum surface smooth; sternites 1 and 2 completely fused, lateral margins sinuous; sternites 3 and 4 fused with only lateral notches demarcating sternites; suture between sternites 2 and 3 almost straight ( Fig. 11A ).

Adult chelipeds symmetrical or almost so, not enlarged ( Fig. 10A ). Basis-ischium fused but suture visible, flexor margin with short sharp granules ( Fig. 11A ). Merus short, surface rugose; flexor and extensor margins with low sharp tubercles and granules; inner distal angle sharp but not elongate, with subdistal tooth ( Figs. 10A , 11A ). Carpus rugose; inner distal angle with strong sharp tooth ( Fig. 10A ). Chela with dorsal surface rugose and covered with granules, median part smoother; ventral surface near pollex covered with small granules; inner surface smooth; margins of chela with scattered long and short setae, not dense or brush-like; fingers relatively slender, shorter than palm, pigmented brown along distal ⅔; occlusal margins with low teeth, forming small gape when closed ( Figs. 10A , 11C ).

Ambulatory legs relatively short, P4 longest, about 1.3× carapace width; outer surface gently rugose; coxa with distal angle slightly raised, no visible serrated flange; P2–4 with outer surface slightly raised medio-longitudinally, indistinct on P5; merus relatively slender, extensor margin not cristate, with evenly, widely spaced row of blunt, prominent nonsetose nodules; P4 merus length about 4× height, length half carapace width; P2–4 carpus with outer submarginal ridge, absent on P5; propodus laterally flattened, elongate in P2–4, short in P5; P2–4 dactylus falciform, unarmed, P5 dactylus shorter, gently upturned; dactylo-propodal lock visible ( Figs. 10A , 11D ).

Female pleon longitudinally ovate; with 6 somites and telson, all free; not completely covering thoracic sternum; somite 1 broadly rectangular, just reaches base of P5 coxae; somite 2 transversely narrower than somite 1, trapezoidal; somites 3–6 trapezoidal, gradually decreasing in width, somite 6 longest; telson semicircular with convex lateral margins, distinctly longer than somite 6 ( Fig. 11A ).

Sternopleonal cavity shallow ( Fig. 11B ); vulva large, obliquely ovate, covering most of space on sternite 6, without vulvar projections ( Fig. 11B ).

Male. Not known.

Remarks. Striacoeloma tubur , new genus and species, is currently known only from the female holotype. Nothing is known about the ecology other than it was collected from soft, muddy substrates. The carapace and pereopods were partially encrusted with what appear to be light-brown, possibly ferric, precipitates, suggestive of reducing or anoxic environs.

Figures

Fig. 10. Striacoeloma tubur, new genus and species, holotype female (8.6 × 6.3 mm) (AM P107758), Northern Territory, Australia. A, dorsal habitus (right side denuded); B, dorsal view of carapace (right side denuded); C, subfrontal view of cephalothorax (right side denuded); D, frontal view of cephalothorax (right side denuded); E, right third maxilliped.

Fig. 11. Striacoeloma tubur, new genus and species, holotype female (8.6 × 6.3 mm) (AM P107758), Northern Territory, Australia. A, buccal cavity, anterior thoracic sternum and pleon; B, sternopleonal cavity and vulvae; C, right chela; D, right P4 and P5.