Takedaplax sextuberculata ( Takeda & Kurata, 1984 ), 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 : 348-350

Takedaplax sextuberculata ( Takeda & Kurata, 1984) , new combination

( Figs. 8 , 9 )

Lophoplax sextuberculata Takeda & Kurata, 1984: 200 , figs. 14–16; Takeda, 1993: 40; Ng et al., 2008: 143; Takeda & Komatsu, 2018: 176, fig. 6H; Takeda & Komatsu, 2023: 167.

Material examined. 1 male (4.0 × 3.2 mm) (NSMT-Cr S 1253), west of Chichi-jima [= Island], Ogasawara Islands, Japan, 27°04.55′N 142°09.16′E – 27°04.73′N 142°09.31′E, 83– 81 m, R / V Koyo, 2008 cruise, station KY-08-15, 28 October 2008 .

Diagnosis. As for genus.

Description. Carapace rectangular; frontal and anterolateral margins with scattered tufts of short and medium-length setae, not brush-like, not concealing margins; regions well demarcated with 6 anterior areoles always large, distinct: epigastric areole with cristate anterior margin; postorbital areole transversely wide, anterior margin subcristate; hepatic areole rounded with subcristate anterior margin; protogastric, cardiac, mesobranchial and cardiobranchial regions gently convex to barely discernible in smaller specimens, gently raised to low areoles in larger specimens; gastro-cardiac with grooves shallow ( Fig. 8A, B ); intestinal region not prominently raised, no visible intestino-branchial areole; surface adjacent to posterior carapace margin almost flat, not forming ridge, no transverse channel visible ( Fig. 8B ); mesobranchial regions smooth; metabranchial regions covered with granules ( Fig. 8B ). Subhepatic and suborbital regions almost smooth; pterygostomial region with scattered small granules ( Fig. 8D ). Frontal margin distinctly bilobed, convex, with deep median cleft, lateral lobe very low, just visible in dorsal view ( Fig. 8A, B ). Supraorbital margin long, sinuous, with distinct submedian lobe, without fissure or cleft ( Fig. 8B ). Orbit dorsal, supraorbital margin, eyes clearly visible in dorsal view; eyes freely movable, ocular peduncle relatively long, filling orbit ( Fig. 8B, D ). Anterolateral margin with 4 visible teeth or spines (including external orbital tooth), external orbital tooth triangular, next 3 teeth acutely triangular to spiniform, last tooth smallest ( Fig. 8A, B ). Posterolateral margin sinuous, unarmed, margins gently converging towards almost straight posterior carapace margin ( Fig. 8A, B ). Antennules rectangular, relatively large; antennules folding laterally ( Fig. 8D ). Basal antennal article subquadrate; flagellum entering orbital hiatus. Posterior margin of epistome entire, without lateral fissures, median part triangular, lateral parts distinctly concave (8E, 9D). Endostome not obliquely recessed posteriorly into buccal cavern, subparallel to adjacent pterygostomial surface, low, short endostomial ridges present.

Third maxilliped with ischium relatively short, subquadrate, length 1.3× width, with shallow median oblique sulcus; merus subquadrate, anterolateral angle slightly auriculiform. Exopod relatively stout, tip just reaching to before distal edge of merus ( Figs. 8C , 9C ).

Male thoracic sternum surface smooth; sternites 1 and 2 completely fused, wide, lateral margins gently concave to sinuous; sternites 3 and 4 fused with only very shallow median groove and lateral notches visible; suture between sternites 2 and 3 gently sinuous ( Fig. 8F ). Sternite 8 clearly visible as subovate plate adjacent to pleonal somite 2 when pleon closed ( Fig. 9G ). Tubercle of male pleonal locking mechanism on anterior one-third of somite 5.

Adult male chelipeds almost symmetrical, relatively slender ( Fig. 8A, G, H ). Basis-ischium fused but suture visible; extensor margins granulated ( Fig. 1C ). Merus short, surface slightly rugose; flexor and extensor margins with tubercles and granules; inner distal angle dentiform with low subdistal angle ( Fig. 8A, H ). Carpus surface granulate; dorsal surface of carpus with prominent longitudinal areole, separated from swollen lateral fringe by groove; inner distal angle with sharp tooth ( Figs. 8A, H , 9B ). Chela with dorsal surface covered with granules; inner surface almost smooth; median and ventral surfaces covered with tubercles and granules; margins of chela with scattered long and short setae, not dense or brush-like; fingers slender, slightly shorter than palm, pigmented brown along distal half; occlusal margins with low teeth, forming distinct gape when closed ( Fig. 8G, H ).

Ambulatory legs long, P4 longest, about 2.0× carapace width; P4 merus length about 4.3× height, distinctly longer than half carapace width; outer surface almost smooth; coxa rounded; merus elongate, P2–5 extensor margin not armed, entire, lined with short and long setae that does not obscure surface; surface of P2–5 gently convex, without median swelling or ridge; P2–5 carpus relatively smooth, with very low submarginal ridge, lowest or barely visible in P5; propodus laterally flattened, unarmed, relatively long in P2–4, shortest in P5; P2–4 dactylus falciform, unarmed, P5 dactylus shorter, slightly upturned; dactylo-propodal lock visible ( Figs. 8A , 9E, F ).

Male pleon triangular, all somites and telson free; somite 1 broadly rectangular, reaching base of P5 coxae; somite 2 transversely narrower than somite 1, subtrapezoidal; somite 3 broadly subtrapezoidal, as wide as somite 1, somites 4 and 5 trapezoidal, somite 6 and telson missing ( Fig. 9G ).

G1strongly sinuous, distally recurved, tapering to slightly rounded tip, hooked inwards; subdistal surface with 1 long stout seta ( Fig. 9H, I ). G2 short, about ¼ length of G1, sigmoid ( Fig. 9J ).

Remarks. Lophoplax sextuberculata was described from the female holotype (8.2 × 6.2 mm) and two male paratypes (both 5.2 × 3.8 mm) collected from inside a fish stomach from Takinoura Bay, Ani-jima [= Island], Ogasawara Islands, and a paratype male (5.5 × 4.5 mm) from Tanegashima [= Island] (southern Kyushu) ( Takeda & Kurata, 1984; Takeda, 1993). Takeda & Marumura (1995) reported additional specimens from the Kii Peninsula and Ryukyus.

They noted that while the median and posterior surfaces of the carapace were smooth in the type specimens from the Ogasawara Islands (as in the case of the present specimen; see also Takeda & Komatsu, 2018: fig. 6H); the specimens from Tanegashima [= Island] and Kii Peninsula had seven additional areoles in two transverse rows on the following regions: two on the protogastric, one each on anterior parts of the mesobranchials, one cardiac and one each on the cardiobranchials (see Takeda & Marumura, 1995: 89, 91, fig. 1). They noted that the only specimen they had from the Ryukyus was small (4.8 × 4.0 mm) and had the median and posterior parts of the carapace smooth ( Takeda & Marumura, 1995: 91). As such, Takeda & Marumura (1995: 91) observed that “… there may be some individual differences as for the development of the areolets on the posterior part, but it is concluded that Lophoplax sextuberculata is characterised by having 13 areolets, not 6 mentioned in the original description and scientific name”. Examining the present specimen from the Ogasawara Islands, we note that its protogastric, mesobranchial, cardiac, and cardiobranchial regions are only gently convex and smooth, and barely discernible as areolets in dorsal view ( Fig. 8A, B, D ). In the colour figure of this specimen ( Takeda & Komatsu, 2018: fig. 6H), we note that these regions are also slightly more orangish than the surrounding beige surface as described by Takeda & Marumura (1995) for their specimens. As such, it seems likely that these areolets are present but not always discernible, probably becoming more prominent in larger individuals. Ng & Rahayu (2023) reported a similar situation for Myopilumnus sordidus and M. vermiculatus , where the areoles are relatively poorly developed in young specimens.

Figures

Fig. 8. Takedaplax sextuberculata (Takeda & Kurata, 1984), new combination, male (4.0 × 3.2 mm) (NSMT-Cr S 1253), Japan. A, dorsal habitus (right side partially brushed); B, carapace (right side denuded); C, left third maxilliped; D, frontal view of cephalothorax; E, epistome; F, anterior thoracic sternum and part of pleon; G, left chela; H, dorsal view of right cheliped.

Fig. 9. Takedaplax sextuberculata (Takeda & Kurata, 1984), new combination. A–C, holotype female (8.2 × 6.2 mm) (NSMT-Cr S 8949) (after Takeda & Kurata, 1984: figs. 14–16); D–J, male (4.0 × 3.2 mm) (NSMT-Cr S 1253), Japan. A, carapace; B, dorsal view of left cheliped; C, right third maxilliped; D, epistome; E, right P4; F, right P5; G, pleon (missing somite 6 and telson) showing exposed sternite 8 lateral to somite 2; H, right G1 (ventral view); I, right G1 (dorsal view); J, right G2. Scales: D, G = 0.5 mm; E, F = 1.0 mm; H–J = 0.2 mm.

Fig. 1. Terminology of carapace structures (based on Myopilumnus sculpta (Stimpson, 1858)). A, dorsal view of carapace; B, right lateral view of cephalothorax. Abbreviations: brs = branchiostegite ridge; brt = branchiostegite; car = cardiac region; car-bra = cardio-branchial areole; cha = channel; epi = epibranchial areole; epi-bra = epibranchial region; hep = hepatic areole; int = intestinal region; int-bra = intestino-branchial areole; meo = Milne Edwards opening; mes-bra = mesobranchial region; met-bra = metabranchial region; met-gas = metagastric region; porb = postorbital areole; pos = posterior carapace margin; pot = tubercles on posterolateral margin; pro = protogastric areole; t1–t4 = anterolateral teeth 1–4.