Gustavus holthuisae Ng & Anker, 2024, sp. nov.
Creators: name: Ng, Peter K. L.
name: Anker, Arthur
Publisher: name: Zenodo
Year: 2024
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Decapoda
Aphanodactylidae
Gustavus
Gustavus holthuisae

description: Gustavus holthuisae sp. nov.
(Figs. 1–6)
Type material. Holotype: male (5.3 × 3.4 mm) (FLMNH UF 72876), Saudi Arabia, Red Sea coast, Sta. RS-41, Thuwal, King Abdullah University of Science & Technology (KAUST), near King Abdullah Monument, 22.340608°N, 39.087519°E, sand flat with fine sand near mangrove roots, less than 0.5 m at low tide, suction (yabby) pump, in terebellid tube, coll. A. Anker, 21 January 2023 [AA-22-520/M]. Paratypes: 1 ovigerous female (7.3 × 4.0 mm) (ZRC 2023.0580), same collection data as for the holotype [AA-22-520/F]; 1 ovigerous female (6.3 × 3.6 mm) (FLMNH UF 72877), same collection data as for the holotype [AA-22-509; terebellid host, AA-22-508, deposited in RSRC / KAUST]; 1 ovigerous female (5.9 × 3.4 mm) (FLMNH UF 72878), same collection data as for the holotype [AA-22-510].
Comparative material. Gustavus mecognathus Ahyong & Ng, 2009. Holotype: male (8.8 × 6.1 mm) (ZRC 2010.0251); paratype: ovigerous female (13.9 × 8.0 mm) (ZRC 2010.0252), Mariana Islands, Guam, Southwest Cocos Barrier, near small pass, with large terebellid polychaete worm, coll. G. Paulay, 20 March 2000.
Diagnosis. Carapace with metagastric region distinctly transversely depressed in both sexes (Figs. 2A, C, 3A, C, 4A, B); male frontal width proportionately narrow, median lobes acutely triangular (Fig. 4C, 5A); female front wider, median lobes low, separated by shallow concavity (Fig. 4D, 5E). Mxp3 ischium with inner margin gently concave (Fig. 6A, B). Male and female P4 and P5 basis-ischium with 2 or 3 spines on flexor margin (Fig. 6E, F, M, N); male and female P4 merus with 1 spine on flexor margin (Fig. 6E, M); male and female P5 merus with 2 spines on flexor margin (Figs. 6F, N). Male pleon with somites 4–6 immobile, with sutures visible between somites (Fig. 6G). G1 overall relatively stout, apex short, rounded, with sharp subdistal projection (Figs. 4E, F, 5D, 6H, I).
Description. Male. Carapace subpentagonal, broader than long, width to length ratio 1.56; dorsal surface almost smooth, regions poorly demarcated, with numerous, small, shallow pits (Figs. 2A, 4A); surface and margins finely pubescent; short, dense setae present on front, partially obscuring margin; metagastric region transversely depressed, covered with low, soft, easily detachable setae (Figs. 2A, C, 4A); anterolateral margin entire, gently convex laterally, subcristate, continuing onto posterolateral margin but not supraorbital margin (Figs. 2A, 4A). Front deflexed, medially emarginate in dorsal view; median lobes triangular, separated by distinct cleft, triangular in anterior view; lateral tooth distinct in dorsal view (Figs. 4C, 5A). Orbital margin entire, slightly narrowed laterally; infraorbital margin terminating mesially as rounded angle; supraorbital margin terminating mesially as small triangular, ventrally directed tooth (Figs. 4C, 5A). Epistome very short, partially sunken medially; posterior margin gently concave, entire (Fig. 5A). Eyes mobile, filling orbit; cornea simple, pigmented (Fig. 4C, 5A). Antennular fossae large; antennule folding obliquely, filling fossa (Fig. 5A). Antenna short, fully entering orbit, with basal antennal article stout, quadrate, not fused to carapace, not reaching distolateral angle of carapace (Fig. 5A).
Mxp3 covering most of buccal cavern when closed; ischium distinctly longer than merus, slender, subtriangular, distomesial angle produced into rounded lobe extending mesially beyond level of mesial meral margin, inner margin gently concave; merus smaller than ischium, slightly wider than long; palp articulating on distal margin of merus; basal part of exopod relatively slender, reaching almost to just above merus; exopod flagellum short, not reaching distal margin of merus (Fig. 6A).
Chelipeds equal, covered with short, fine setae partially obscuring margins and surface (Figs. 2A–C, 4A); merus triangular in cross-section, with plumose setae proximally; carpus smooth, inner angle rounded, with sparser plumose setae; chela surfaces relatively short, surface smooth, except for scattered, low, rounded granules and punctae on lower portion of palm; palm longer than dactylus, ventral margin near pollex concave; pollex with convex occlusal margin, with blunt teeth proximally, distal half with small teeth fused basally, forming blade-like structure; dactylus with occlusal margin armed with 3 low, blunt teeth proximally, distal part unarmed, tip strongly curved (Fig. 5B).
P2–P5 relatively short, P3 longest, P5 shortest; P5 dorsal to other pereopods (Figs. 2A, 4A); propodus and carpus covered with short, fine tomentum partially obscuring surface and margins; merus with plumose setae along flexor and extensor margins; P2 and P3 merus longer than carpus and propodus combined; P4 and P5 merus subequal to carpus and propodus combined; dactylus short, stout, unarmed laterally, with corneous apex (Fig. 6C–F). P2 and P3 merus with margins almost entire (Fig. 6C, D). P4 and P5 ischium each armed with 2 stout spines on flexor margin; P4 merus with 1 larger spine on flexor margin; P5 merus with 2 spines on flexor margin (Fig. 6E, F). P2–P5 propodus with 2 or 3 small, fixed spines on distal flexor margin opposing dactylus (Fig. 6C–F).
Thoracic sternum relatively wide; sternites 1–3 completely fused, sternites 1 and 2 demarcated only by row of setae; sternites 3 and 4 partially fused, separated by shallow median groove lined with setae; sternopleonal cavity wide, deep, its distal part subcristate, tip reaching to imaginary line joining subproximal edge of cheliped coxae; proximal third of sternite 5 with rounded tubercle representing pleonal locking mechanism (Fig. 5C, D). Thoracic sternite 8 visible when pleon closed.
Pleon broadly triangular; somites 4–6 with sutures visible but functionally fused, immobile; somites 1–3 subequal in width; somites 4 and 5 trapezoidal; somite 6 subrectangular; telson wide, semicircular (Fig. 6G).
G1 relatively stout, with tip directed mesially (Fig. 5D); distal quarter distinctly bent at about 45° to longitudinal axis; distally tapering gradually to short, rounded apex, with numerous stiff setae; with short, sharp subdistal projection on lower margin; groove for G2 present on mesioventral surface (Figs. 4E, F, 6H, I). G2 short, about onethird length of G1; distally spatulate, apex bluntly angular (Figs. 4G, 6J).
Female. Carapace transversely ovate, much broader than long, width to length ratio 1.74–1.83; dorsal surface almost smooth, with numerous small shallow pits, regions poorly demarcated (Figs. 3A–C, 4B); surface finely pubescent on surface and margins; front with short, dense setae partially obscuring margin; lower lateral margins with longer, dense, plumose setae obscuring surface; metagastric region transversely depressed, covered with low, soft setae, latter easily brushed off (Figs. 3A, C, 4B). Front deflexed, medially gently emarginate in dorsal view, separated by shallow concavity, triangular in anterior view; lateral tooth low but distinct, rounded in dorsal view (Figs. 4D, 5E). Orbital margin entire, gently concave, slightly narrowed laterally; infraorbital margin terminating mesially as rounded angle; supraorbital margin terminating mesially as low, slender, ventrally directed tooth (Figs. 4D, 5E). Anterolateral margin entire, convex laterally, subcristate, continuing onto posterolateral margin but not supraorbital margin, with very shallow, short, transverse groove along proximal third (Figs. 3A, 4B). Epistome very short, partly sunken medially; posterior margin gently concave, entire (Fig. 5E). Eyes mobile, filling orbit; cornea simple, pigmented (Figs. 4D, 5E). Antennular fossae large; antennule folding obliquely, filling fossa (Fig. 5E). Antenna short, completely entering orbit, with basal antennal article stout, quadrate, not fused to carapace, not reaching distolateral angle of carapace (Fig. 5E).
Mxp3 covering most of buccal cavern when closed; ischium distinctly longer than merus, slender, subtriangular, distomesial angle produced into rounded lobe extending mesially beyond level of mesial meral margin, mesial margin gently concave; merus smaller than ischium, slightly wider than long, squircular; palp articulating on distal margin of merus; basal part of exopod slender, tip reaching almost to just above merus; exopod flagellum short, not reaching distal margin of merus (Fig. 6B).
Chelipeds equal, densely covered with fine, short, setae partially obscuring margins and surface (Figs. 4B, 5F); merus triangular in cross-section, with plumose setae proximally; carpus smooth, inner angle rounded, with sparse plumose setae; chela somewhat elongate, surfaces smooth except for scattered, low, rounded granules on lower portion of palm; palm distinctly longer than dactylus, with ventral margin near pollex concave; pollex with occlusal margin convex, with blunt teeth proximally, distal two-thirds with small teeth fused basally, forming blade-like structure; dactylus with occlusal margin armed with 2 or 3 low, blunt teeth proximally, distal part unarmed, tip curved (Fig. 5F).
P2–P5 relatively short, P3 longest, P5 shortest, dorsal to other pereopods (Figs. 3A–C, 4B); propodus and carpus covered with short, fine tomentum partially obscuring surface and margins; merus with plumose setae along flexor and extensor margins; P2 and P3 merus longer than carpus and propodus combined; P4 and P5 merus subequal to carpus and propodus combined; dactylus short, stout, unarmed laterally, with corneous apex (Fig. 6K–N). P2 and P3 merus with flexor margin almost smooth except for scattered, very low, sharp granules (Fig. 6K, L). P4 and P5 basis-ischium with 3 sharp, stout, fixed spines on flexor margin (Fig. 6M, N); P4 merus with 1 large, fixed spine on proximal third of flexor margin (Fig. 6M); P5 with 2 large spines and 1 sharp, basal tubercle on proximal third of flexor margin (Fig. 6N). P2–P5 propodus with 2 or 3 small, sharp, distoflexor spines opposing dactylus (Fig. 6K–N).
Thoracic sternum similar in form to that of male in non-sexual characters, but distinctly broader (Fig. 5H).
Pleon wide, ovate, all somites and telson free; somites 3 and 4 widest; telson wide, with strongly convex margins (Figs. 3B, 5G). Vulvae positioned wide on sternite 6; vulvar cover tube-like, operculum soft, covering most of opening (Fig. 5H).
Variation. There is a pronounced sexual dimorphism in Gustavus holthuisae sp. nov., with the carapace distinctly different in shape and proportions (cf. Figs. 2A, 3A, 4A, B). The largest paratype female (7.3 × 4.0 mm, ZRC 2023.0580) is noticeably proportionally wider than the other two females, its carapace width to length ratio being 1.83 versus 1.74 and 1.75. The female chelae are proportionally longer than those of the male, with the fingers more elongate and less curved (cf. Fig. 5B, F).
Etymology. The authors take a great pleasure in naming the species after our friend, the marine biologist Bernadette Holthuis (University of Washington Friday Harbor Laboratories), who is also a relative of one of the greatest carcinologists of the 20 th century, Lipke B. Holthuis (1921–2008).
Biology. All specimens of Gustavus holthuisae sp. nov. were collected near mangrove roots (dominant species: Avicennia marina (Forskål)), in water less than 0.5 m deep; they were found in tubes of unidentified terebellid worms (Fig. 1), which were extracted from fine sand/mud with the aid of a metallic suction pump. Although a complete specimen of the terebellid host was collected and deposited in KAUST, its identification to genus level was impossible based on examination of photographs alone (P. Hutchings, pers. comm.).
Remarks. Gustavus holthuisae sp. nov. can easily be separated from G. mecognathus by the following features: the metagastric region of the carapace is transversely distinctly depressed in both sexes (Figs. 2A, C, 3A, C, 4A, B) (versus the entire dorsal carapace surface being gently convex, including the metagastric region, in G. mecognathus; cf. Ng & Ahyong 2009: figs. 4A, 5A, 7A); the male frontal margin is proportionally narrower, with the median lobes acutely triangular (Fig. 5C) (versus broader and with the median lobes lower and more obtuse in G. mecognathus; Ng & Ahyong 2009: fig. 4A); the female frontal margin has the median lobes separated by a shallow concavity (Fig. 5D) (versus separated by a distinct V-shaped notch in G. mecognathus; cf. Ng & Ahyong 2009: fig. 5A); the Mxp3 ischium has a gently concave inner margin (Fig. 6A, B) (versus with a distinctly concave margin in G. mecognathus; cf. Ng & Ahyong 2009: figs. 4C, 5C); the basis-ischium of the male and female P4 and P5 has two or three spines on the flexor margin (Fig. 6E, F, M, N) (versus a cluster of three or four small and large spines in G. mecognathus; cf. Ng & Ahyong 2009: figs. 4G, H, 5G, H); the merus of the male and female P5 has two spines on the flexor margin (Fig. 6F, N) (versus only one spine in G. mecognathus; cf. Ng & Ahyong 2009: figs. 4H, 5H); the merus of the female P4 has only one spine on the flexor margin (Fig. 6M) (versus five spines in G. mecognathus; cf. Ng & Ahyong 2009: fig. 5G); the male pleon is proportionally more elongate, with all sutures distinct and clearly visible, despite the immobility of the somites 4–6 (Fig. 6G) (versus with lateral sutures between somites 4–6 shallow to undiscernible in G. mecognathus; cf. Ng & Ahyong 2009: fig. 4J); and, finally, the G1 is overall stouter, with the distal part short and rounded, and with a subdistal projection (Figs. 5E, F, 6H, I) (versus the G1 being relatively more slender, with the distal part more elongate and without a subdistal projection in G. mecognathus; cf. Ng & Ahyong 2009: fig. 4K).
description: Published as part of Ng, Peter K. L. & Anker, Arthur, 2024, A second species in the polychaete-associated crab genus Gustavus Ahyong & Ng, 2009 (Decapoda: Brachyura: Aphanodactylidae), pp. 369-380 in Zootaxa 5497 (3) on pages 370-378, DOI: 10.11646/zootaxa.5497.3.3, http://zenodo.org/record/13618519