Lophoplax bicristata Tesch, 1918

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 : 338-341

Lophoplax bicristata Tesch, 1918

( Figs. 2 , 3 )

Material examined. Lectotype (here designated): RMNH-ZMA D103002 , male (5.8 × 4.3 mm), 2.3 miles from north point of Nuhu Jaan , northwest of Kei Islands, Indonesia, 5°36.5′S 132°55.2′E, 90 m, Siboga Expedition station 260, coll. M. Weber, 16–18 December 1899 . Paralectotype (here designated): RMNH-ZMA D103001 , male (4.5 × 3.7 mm), Borneo Bank , Strait of Makassar, Indonesia, 59 m, Siboga Expedition station 77, coll. coll. M. Weber, 10 June 1899 .

Diagnosis. Carapace frontal margin almost straight, granulate, not distinctly produced anteriorly, lateral lobe very low but just visible ( Fig. 2A, B ); epigastric areole low but visible; protogastric areolet distinct, smooth ( Fig. 2A, B ); metabranchial region with prominent raised granules ( Fig. 2A ); median lobe on posterior margin of epistome obtusely triangular ( Fig. 3B ). Distal margin of ischium of third maxilliped smooth ( Fig. 3A ). Cheliped carpus dorsal surface with subquadrate areole with crenulate ridged margins ( Fig. 2A–C ); proximal basal part of male chela almost smooth or with very low tubercles ( Fig. 2D ). P2–4 merus extensor margin distinctly crenulated, with low lobes. P5 merus unarmed ( Fig. 3C, D ). Male thoracic sternum anteriorly relatively wide ( Fig. 2E ). G1 gently sinuous, distal strongly recurved ( Fig. 3F–I ). G2 about ¼ length of G1.

Description. Carapace subquadrate; frontal and anterolateral margins with scattered tufts of short and long setae, not brush-like, not concealing margins; regions well demarcated with few areoles distinct: protogastric areole transversely ovate, distinct; epigastric region low, visible when denuded, not areolate; hepatic region with 1 tubercle, no areole visible; cardiac region raised, covered with granules, setose, not areolate, lateral parts with raised patches of granules, but no visible cardio-branchial areole; gastro-cardiac region depressed, with grooves visible but not deep ( Fig. 1A, B ); intestinal region convex, forming rounded areole, adjacent surfaces gently raised, with small granules, setose, separated from cristate posterior carapace margin by wide space, no obvious channel ( Fig. 1A, B ); meso- and metabranchial regions each with 1 gently raised area covered with granules; metabranchial region with tubercle just posterior to raised patch of granules ( Fig. 1A ). Subhepatic and suborbital regions slightly rugose to almost smooth; pterygostomial region with small granules. Frontal margin weakly bilobed, almost straight, with shallow median concavity, margins granulate; lateral lobe, small, low, just visible in dorsal view ( Fig. 1A ). Supraorbital margin short, sinuous, with distinct submedian lobe, without fissure or cleft ( Fig. 1A ). Orbit dorsal, supraorbital margin, eyes clearly visible in dorsal view; eyes freely movable, ocular peduncle relatively long, filling orbit ( Fig. 1A ). Anterolateral margin with 4 small, sharp teeth (including external orbital tooth), external orbital tooth broader, next 3 teeth acutely triangular, tip slightly enlarged ( Fig. 2A, B ). Posterolateral margin uneven, with 2 tubercles, each with smaller basal granules; margins gently converging towards posterior carapace margin ( Fig. 1A ). Antennules rectangular, relatively large; antennules folding laterally. Basal antennal article subquadrate; flagellum entering orbital hiatus. Epistome with median lobe of posterior margin obtusely triangular, no obvious fissure in frontal view; lateral parts sinuous with small cleft ( Fig. 3B ). Endostome distinct, sloping gradually posteriorly into buccal cavern, endostomial ridges low, short.

Third maxilliped with ischium relatively short, subquadrate, length 1.2× width, with shallow median oblique sulcus; merus subquadrate, anterolateral angle gently auriculiform. Exopod relatively stout, tip reaching to just before distal edge of merus ( Fig. 3A ).

Male thoracic sternum surface relatively smooth, slightly rugose; sternites 1 and 2 completely fused, wide, lateral margins gently concave; sternites 3 and 4 fused with only very shallow median groove and lateral notches visible; suture between sternites 2 and 3 almost straight ( Fig. 2E ). Sternite 8 clearly visible as subrectangular plate adjacent to pleonal somite 2 when pleon closed ( Fig. 3E ). Tubercle of male pleonal locking mechanism on anterior third of thoracic somite 5.

Adult male chelipeds almost symmetrical, stout ( Fig. 1A, B ). Basis-ischium fused but suture visible; extensor margins uneven but unarmed ( Fig. 1C ). Merus short, surface slightly rugose; flexor and extensor margins with tubercles and granules; inner distal angle dentiform with subdistal angle tuberculate ( Fig. 1C ). Carpus surface granulate; dorsal surface with subquadrate areole with crenulate ridged margins; inner distal angle with low sharp tooth ( Fig. 1A–C ). Chela with dorsal surface covered with granules; inner surface almost smooth; median and ventral surfaces almost smooth or with very low tubercles; margins of chela with scattered long and short setae, not dense or brush-like; fingers relatively slender, slightly shorter than palm, pigmented brown along distal half; occlusal margins with low teeth, forming small gape when closed ( Fig. 1C, D ).

Ambulatory legs long, P4 longest, about 1.8× carapace width; P4 merus length about 5× height, distinctly longer than half carapace width; outer surface slightly rugose to almost smooth; coxa rounded; merus elongate, P2–4 extensor margin not cristate with low, broad, well-spaced setose prominences, P5 merus extensor margin smooth, fully setose; surface of P2–5 gently convex, without median swelling or ridge; P2–5 carpus relatively smooth, with very low submarginal ridge, lowest 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. 2A , 3C, D ).

Male pleon triangular, all somites and telson free; somite 1 broadly rectangular, reaching base of P5 coxae; somite 2 transversely much narrower than somite 1, trapezoidal; somite 3 broadly subtrapezoidal, widest, somites 4–6 trapezoidal, gradually decreasing in width with somite 6 almost rectangular; telson slightly wider than long, rounded, slightly narrowly distally, lateral margins convex, longer than somite 6 ( Fig. 3E ).

G1 relatively stout, gently sinuous, distally tapering to narrowly rounded tip, strongly hooked inwards; subdistal surface with 2 long stout setae ( Fig. 3F–I ). G2 sigmoid, about ¼ length of G1 ( Fig. 3J ).

Females not known. Remarks. Lophoplax bicristata is only known from two males collected from relatively near each other ( Kei Islands and Borneo Bank , Indonesia). Both are mature and were covered with silt and mud, suggesting they live on soft substrates. They were collected from depths of 59–90 m.

Figures

Fig. 2. Lophoplax bicristata Tesch, 1918. A, B, D–F, lectotype male (5.8 × 4.3 mm) (ZMA D103002), Kei Islands; C, paralectotype male (4.5 × 3.7 mm) (ZMA D103001), Strait of Makassar. A, dorsal habitus (right side partially brushed); B, dorsal habitus (right side denuded); C, dorsal habitus (not denuded); D, dorsal view of left cheliped; E, left chela; F, anterior thoracic sternum and pleon.

Fig. 3. Lophoplax bicristata Tesch, 1918, lectotype male (5.8 × 4.3 mm) (ZMA D103002), Kei Islands. A, right third maxilliped; B, epistome; C, left P4; D, left P5; E, pleon, showing exposed sternite 8 lateral to somite 2; F, left G1 (ventral view); G, distal part of left G1 (ventral view); H, distal part of left G1 (dorsomedial view); I, distal part of left G1 (dorsal view); J, left G2. Scales: A, B = 0.5 mm; C–E = 1.0 mm; F, J = 0.25 mm; G–I = 0.1 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.