Morphological and molecular evidence for Gothus teemo gen. et sp. nov., a new xanthid crab (Crustacea, Brachyura, Xanthoidea) from coral reefs in the South China Sea, with a review of the taxonomy of Actaeodes consobrinus (A. Milne-Edwards, 1867) Author Yuan, Zi-Ming https://orcid.org/0009-0008-0009-2693 Department of Marine Organism Taxonomy and Phylogeny, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China & Laoshan Laboratory, Qingdao 266237, China & Shandong Province Key Laboratory of Experimental Marine Biology, Institute of Oceanology, Chinese Academy of Sciences, Qingdao, China Author Jiang, Wei 0000-0003-4767-8196 Department of Marine Organism Taxonomy and Phylogeny, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China & Laoshan Laboratory, Qingdao 266237, China Author Sha, Zhong-Li 0000-0002-2192-3758 Department of Marine Organism Taxonomy and Phylogeny, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China & Laoshan Laboratory, Qingdao 266237, China text Zoosystematics and Evolution 2024 2024-07-09 100 3 965 987 journal article 299644 10.3897/zse.100.117859 f17d3dc6-151e-4b2c-bfe6-1e3978daa701 15CA9ED4-C24C-4AD7-81AC-F699FA953FE8 Gothus gen. nov. Figs 1 , 2 , 3 , 4 , 5 Type species. Gothus teemo sp. nov. , by present designation. Diagnosis. Small species, CW under 10 mm . Carapace broader than long, dorsal surface bearing round granules, regions clearly defined; front wide, not protruding, divided into two slightly triangular lobes by a V-shaped notch; frontal lobes and dorsal inner orbital angle separated by shallow depression; eyestalks densely granulated; area beneath outer orbital angle slightly concave, not forming a subhepatic cavity; anterolateral margin with four teeth, first tooth flattened, sometimes completely reduced to appear as three teeth; posterolateral margin almost straight; subhepatic region densely granulated. Epistome central region with low median projection on posterior margin. Maxilliped 3 granulated, anterior edge of merus indented, external terminal angle expanded. Antennule folding transversely; basal segment of antenna subrectangular; contacting ventral external frontal margin and ventral internal orbital angle; antennal flagellum filling orbital hiatus. Chelipeds symmetrical, merus short; carpi robust, surface granulated, aggregated into nodules; outer and dorsal surfaces of palm densely granulated; fingers elongated, with triangular teeth; tips sharp, crossing at extremities when closed; dorsal surface of movable finger with three granulated ridges. Fingers brownish-black, coloration of immovable finger extending onto inner and outer surfaces of palm in male. Ambulatory legs with meri flattened, granulated along anterior and posterior edges; dactyli elongated, margins with granules and setae, terminal end chitinous, sharp, slightly curved backward, dactylo-propodal lock present but underdeveloped. Male thoracic sternum with sternites 1 and 2 completely fused, suture between sternites 2 and 3 straight, complete, sternites 3 and 4 mostly fused, suture between them visible only at margins, sternites 3 short, sternite 4 with central longitudinal groove, tubercle of sterno-pleonal lock located on posterior of sternite 5. Male pleon narrow, pleonites 3 to 5 completely fused, lateral margins of pleonite 6 slightly concave. Telson long, broad, truncated oval, base margin wider than terminal margin of pleonite 6. G 1 slender, curving slightly outward, distal lobe spoon-shaped, long setae on inner subdistal part, small spines on outer part. G 2 not exceeding 1 / 6 length of G 1 , distal lobe elongated. Etymology. The genus is named after the game of Go, alluding to the intermingled black and white patterns on the carapace, beneath which lie circular granules resembling the pieces of the game. “ - thus ” is a common suffix for species names within the Xanthidae family. Gender masculine. Comparative material. Rizalthus anconis Mendoza & PKL Ng, 2008 (Fig. 6 A ). China • 1 female ; CW 4.2 mm , CL 2.7 mm ; Meiji Reef , Nansha Islands ; 9 ° 52 ' 38.19 " N , 115 ° 31 ' 17.08 " E ; 8 m ; 7 May 2022 ; Ziming Yuan , Yuli Sun , Shaobo Ma coll.; NS-MJ- 2022-1457 . Hypocolpus haanii Rathbun, 1909 (Fig. 6 B ). China • 1 male ; CW 45.3 mm , CL 34.2 mm ; Lingao Cape , Hainan Island ; 15–30 m ; 20 Aug. 2018 ; Yunhao Pan coll.; MBM 286755 . Euxanthus exsculptus (Herbst, 1790) (Fig. 6 C ). China • 1 male ; Wenchang , Hainan Island ; 20 Dec. 2018 ; Yunhao Pan coll.; Xan 016 • 1 male ; Yongxing Island , Xisha Islands ; 15–17 May 1957 ; MBM 163793 • 2 males ; Wood Island , Xisha Islands ; 1957; MBM 163791 • 1 male , 2 females ; Wood Island , Xisha Islands ; 15–17 May 1957 ; MBM 163788 • 3 males , 3 females ; East Island , Xisha Islands ; 28–31 May 1980 ; MBM 163785 • 1 female ; Yongxing Island , Xisha Islands ; 11–13 Jun. 1980 ; MBM 163784 • 1 female ; East Island , Xisha Islands ; 12 Jun. 1975 ; Xianqiu Ren coll.; MBM 163792 . CW 15–52.8 mm , CL 9.7–33.2 mm . Euxanthus huonii (Hombron & Jacquinot, 1846) (Fig. 6 D ). China • 1 male ; Dengqing Island , Xisha Islands ; 11–17 Apr. 1958 ; MBM 163762 • 1 female ; Tree Island , Xisha Islands ; 1 May 1958 ; MBM 163761 • 1 female ; Yongxing Island , Xisha Islands ; 7 May 1980 ; MBM 163780 • 2 females ; Drummond Island , Xisha Islands ; 1980; MBM 163781 • 1 male ; Meiji Reef , Nansha Islands ; 9 ° 53 ' 30.84 " N , 115 ° 34 ' 22.05 " E ; 10 m ; 10 May 2022 ; Ziming Yuan , Yuli Sun , Shaobo Ma coll.; NS-MJ- 2022-1734 . CW 19.3–37.3 mm , CL 13.1–26.4 mm . Psaumis cavipes (Dana, 1852) (Fig. 6 E ). China • 1 female ovigerous; Xisha Islands , Jinqing Island ; 10 Jul. 2019 ; azp 01 • 1 male , 3 females ; Sanya Station Front, Hainan ; 30 Apr. 2021 ; Zhang Xu coll.; aop 01 • 2 males ; Xisha Islands , Yongle blue hole; 10 Jul. 2019 ; aop 02 • 1 female ovigerous; Xisha Islands , Yongle blue hole; 10 Jul. 2019 ; aop 03 • 1 male ; Phoenix Island , Sanya , Hainan ; Zhang Xu coll.; 2022010 • 1 juvenile ; Xisha Islands , Yuzhuo Reef ; 9 Jul. 2019 ; aop 04 • 2 males ; Hainan , subtidal 9–10 m ; 21 Nov. 2016 ; Xan 020-2. CW 5.1–17.4 mm , CL 3.3–10.6 mm Remarks. Gothus gen. nov. exhibits the closest resemblance to the subfamily Euxanthinae , particularly to Eux 1, as defined and morphologically summarized in the molecular systematic study by Lai et al. (2011) ., mainly considering its anterolateral margin of the carapace, which does not clearly meet the orbit but instead continues down to the subhepatic region, presenting an ambiguous starting point (Fig. 1 B ). Other characteristics justifying its inclusion are chelipeds almost completely symmetrical, which can be coapted against the carapace (Fig. 1 A ); male pleon long, with the telson reaching to the level above the coxo-sternal condyles of pereiopod 1, and base of somite 3 only slightly wider than tip of somite 5 (Figs 2 F , 3 F ) (see also Serène, 1984 ; Lai et al. 2011 ). However, its ambulatory legs do not form a similar perfect coapted structure, especially since the corresponding posterolateral margin is nearly non-concave (Fig. 1 A, C ). Other features notably distinguishing it from any member of the subfamily Euxanthinae are its extremely narrow male pleon with a long and broad, overall truncated oval telson (Figs 2 F , 3 F ), the base of which is wider than the width of the end of the sixth pleonite, the terminal end wide and rounded, with the lateral edges barely converging inward but rather forming two opposing arcs, unlike the typically triangular telson common in the Euxanthinae . Figure 1. Gothus teemo sp. nov. A – C. Holotype, male, CW 3.7 mm, CL 2.6 mm, MBM 287027; D – F. Paratype, CW 3.2 mm, CL 2.2 mm, MBM 287026; A, D. Dorsal view; B, E. Frontal view; C, F. Carapace. Scale bar: 1 mm. Figure 2. Gothus teemo sp. nov. A, C, E. Holotype, male, CW 3.7 mm, CL 2.6 mm, MBM 287027; B, D, F, G. Paratype, female, CW 3.2 mm, CL 2.2 mm, MBM 287026; A. Male chelipeds; B. Female chelipeds; C. Male thoracic sternites; D. Female thoracic sternites; E. Male pleon; F. Female pleon; G. Female vulva. Scale bar: 1 mm ( A – F ); 0.2 mm ( G ). Figure 3. Gothus teemo sp. nov. A – C, F, G – L. Holotype, male, CW 3.7 mm, CL 2.6 mm, MBM 287027; D, E. Paratype, female, CW 3.2 mm, CL 2.2 mm, MBM 287026; A. Carapace; B. Maxilliped 3; C. Cheliped; D. Pereopod 5; E. Female pleon; F. Male pleon; G. right G 1 , ventral view; H. Same, dorsal view; I. Right G 1 , distal part, ventral view; J. Same, dorsal view; K. Right G 2 , ventral view; L. Same, dorsal view. Scale bar: 1 mm ( A, C, D ); 0.5 mm ( B, E, F, G, H ); 0.1 mm ( I – L ). Gothus gen. nov. shares the closest similarities with the genus Rizalthus Mendoza & PKL Ng, 2008 , due to both possessing a granule-covered carapace surface, similar carapace outlines and front, developed cheliped carpus, and analogous G 1 structures. However, Gothus can be easily distinguished from Rizalthus by several key characteristics: its anterolateral margin with four teeth, first tooth flattened, sometimes completely reduced to appear as three teeth (Figs 1 A, C , 3 A ) (vs. no clearly defined teeth in Rizalthus ; Fig. 6 A ; cf. Mendoza and Ng 2008 : fig. 1 A); absence of etched depressions on body (Fig. 1 A ) (vs. distinct etched depressions on thoracic sternum in Rizalthus ; cf. Mendoza and Ng 2008 : fig. 1 C); central part of epistome raised (Fig. 1 B ) (vs. central part of epistome not protruding in Rizalthus ; cf. Mendoza and Ng 2008 : fig. 1 B); robust cheliped carpus, sometimes slightly expanded (Fig. 1 A ) (vs. strongly expanded and protruding in Rizalthus ; Fig. 5 A ; cf. Mendoza and Ng 2008 : fig. 1 A); male pleon with a long, broad, truncated oval telson (Figs 2 F , 3 F ) (vs. a smaller, triangular telson in Rizalthus ; cf. Mendoza and Ng 2008 : fig. 2 C); G 1 distal lobe curved inwards (Fig. 3 G – J ) (vs. nearly straight, not curved inwards in Rizalthus ; cf. Mendoza and Ng 2008 : fig. 2 F – H) and G 2 with a longer, straighter distal lobe (Fig. 3 K, L ) (vs. shorter and curved in Rizalthus ; cf. Mendoza and Ng 2008 : fig. 2 I). Due to its similar carapace outline, particularly the less concave posterolateral margins, Gothus also resembles Visayax Mendoza & Ng, 2008 . However, it can be easily differentiated by the following characteristics: Gothus lacks erosive depressions across body (Fig. 1 A ) (vs. chelipeds, ambulatory legs, carapace, and thoracic sternum with erosive depressions in Visayax ; cf. Mendoza and Ng 2008 : figs 3–6); carapace regions more flattened (Fig. 1 A, C ) (vs. carapace regions more pronounced in Visayax ; cf. Mendoza and Ng 2008 : figs 3 A, C, 5 A, C); posterior three teeth on anterolateral margin of carapace well-developed (Fig. 1 A, C , 3 A ) (vs. absence of developed teeth on anterolateral margin in Visayax ; cf. Mendoza and Ng 2008 : figs 3 A, 5 A); male abdominal telson larger, truncated oval (Figs 2 F , 3 F ) (vs. smaller, semi-circular in Visayax ; cf. Mendoza and Ng 2008 : figs 4 E, 6 D); G 1 more slender (Fig. 3 G – J ) (vs. G 1 more robust in Visayax ; cf. Mendoza and Ng 2008 : figs 4 F, G, 6 F, G). Figure 4. Gothus teemo sp. nov. A, B. Holotype, male, CW 3.7 mm, CL 2.6 mm, MBM 287027; C – E. Paratypes, 2 juvenile, CW 1.8–2.2 mm, CL 1.3–1.5 mm, MBM 287023; A – D. Live coloration; E. Habitat and substrate conditions. Scale bar: 1 mm. Figure 5. Gothus teemo sp. nov. , paratype, female, CW 3.2 mm, CL 2.2 mm, MBM 287026, artistic illustration, displaying live coloration. Drawn by Fei Gao. Figure 6. Euxanthinae and Actaeodes species in comparative material. A. Rizalthus anconis Mendoza & PKL Ng, 2008 , female; CW 4.2 mm, CL 2.7 mm, NS-MJ- 2022-1457 ; B. Hypocolpus haanii Rathbun, 1909 , 1 male, CW 45.3 mm, CL 34.2 mm, MBM 286755; C. Euxanthus exsculptus (Herbst, 1790) , 1 male, CW 52.6, CL 33.2 mm, MBM 163793; D. Euxanthus huonii (Hombron & Jacquinot, 1846) , CW 34.0 mm, CL 24.0 mm, NS-MJ- 2022-1734 ; E. Psaumis cavipes (Dana, 1852) , 1 male, CW 13.8 mm, CL 8.6 mm, aop 01; F. Actaeodes mutatus Guinot, 1976 , 1 female, CW 20.8 mm, CL 12.5 mm, BF 02; G. Actaeodes hirsutissimus ( Rüppell, 1830 ) , 1 male, CW 31.9 mm, CL 21.0 mm, MBM 164298; H. Actaeodes tomentosus (H. Milne Edwards, 1834), 1 male, CW 35.8 mm, CL 22.5 mm, Xan 041. Scale bar: 1 mm ( A ); 5 mm ( E, F ); 10 mm ( B – D, G, F ). The new genus exhibits a general morphological similarity to typical Euxanthinae members such as Euxanthus Dana, 1851 , and Hypocolpus Rathbun, 1897 . In addition to the existing comparative specimens, Guinot-Dumortier (1960) provided excellent descriptions and photographs of species from the above two genera. Subsequently published species also have relatively clear morphological descriptions and images available for comparison (cf. Guinot 1971 b ; Galil and Vannini 1990 ; Crosnier 1996 ). The new genus can be easily distinguished from Euxanthu s by the following features: entire body covered with granules and short pubescence (Fig. 1 A ) (vs. relatively smooth in Euxanthus ; Fig. 6 C, D ; cf. Guinot-Dumortier 1960 : pl. VIII, figs 42, 44, 46, pl. IX, figs 48–52); carapace anterolateral margin with four teeth, first tooth flattened, sometimes completely reduced to appear as three teeth (Figs 1 A, C , 3 A ) (vs. 4–6 teeth on anterolateral margin in Euxanthus ; Fig. 6 C, D ; cf. Guinot-Dumortier 1960 : pl. VIII, figs 42, 44, 46, pl. IX, figs 48–52); front not prominent, divided by a V-shaped notch (Figs 1 C , 3 A ) (vs. more protruding, divided by a narrow fissure in Euxanthus ; Fig. 6 C, D ; cf. Guinot-Dumortier 1960 : pl. VIII, figs 42, 44, 46, pl. IX, figs 48–52); male abdominal telson large and truncated oval (Figs 2 F , 3 F ) (vs. small and triangular in Euxanthus ; cf. Guinot-Dumortier 1960 : pl. VIII, fig. 47); G 1 with a prominent, spoon-shaped distal lobe, and long setae on inner subdistal part (Fig. 3 G – J ) (vs. G 1 with a short, non-protruding distal lobe, inwardly curved and encircling, with short setae on inner subdistal part in Euxanthus ; cf. Guinot-Dumortier 1960 : pl. VI, figs 36–39). Similarly, the new genus is easily distinguishable from Hypocolpus by the absence of a developed subhepatic cavity (Fig. 1 B ) (vs. a developed subhepatic cavity in Hypocolpus ; cf. Guinot-Dumortier 1960 : pl. II, figs 40–41); posterior three teeth on anterolateral margin well-developed (Figs 1 C , 3 A ) (vs. underdeveloped teeth in Hypocolpus ; Fig. 6 B ; cf. Guinot-Dumortier 1960 : pl. VII, figs 40–41); front not prominent, divided by a V-shaped notch (Figs 1 C , 3 A ) (vs. more protruding, divided by a narrow fissure in Hypocolpus ; Fig. 6 B ; cf. Guinot-Dumortier 1960 : pl. VII, figs 40–41); male abdominal telson large and truncated oval (Figs 2 F , 3 F ) (vs. small and triangular in Hypocolpus ; cf. Guinot-Dumortier 1960 : pl. IX, fig. 53, pl. X, fig. 55); G 1 with a prominent, spoon-shaped distal lobe (Fig. 3 G – J ) (vs. G 1 with a short, non-protruding distal lobe, inwardly curved and encircling in Hypocolpus ; cf. Guinot-Dumortier 1960 : pl. VI, figs 32–35). The new genus slightly resembles Psaumis Kossmann, 1877 , and Paractaeopsis Serène, 1984 , but can be readily distinguished. Gothus can be easily distinguished from Psaumis by lack of erosive depressions across body (Fig. 1 A ) (vs. chelipeds, ambulatory legs, carapace with strong erosive depressions in Psaumis ; Fig. 6 E ; cf. Serène 1984 : pl. XVIII, fig. E); front divided by a V-shaped notch (Figs 1 C , 3 A ) (front divided by a narrow fissure in Psaumis ; Fig. 6 E ; cf. Serène 1984 : pl. XVIII, fig. E); anterolateral margin with four teeth, first tooth flattened, sometimes completely reduced to appear as three teeth (Figs 1 A, C , 3 A ) (anterolateral margin with very flat teeth, except for fourth tooth at junction of anterior and posterior lateral margins, which is more prominent in Psaumis ; Fig. 6 E ; cf. Serène 1984 : pl. XVIII, fig. E). It can be distinguished from Paractaeopsis by anterolateral margin with four teeth, first tooth flattened, sometimes completely reduced to appear as three teeth (Figs 1 A, C , 3 A ) (anterolateral margin with four well development teeth in Paractaeopsis ; cf. Takeda and Miyake 1968 : fig. 1 a); carapace broad, approximately 1.5 times as wide as long, with a relatively flat dorsal surface (Figs 1 A, C , 3 A ) (carapace narrower, with a width not exceeding 1.4 times the length, and dorsal surface convex both anteroposteriorly and laterally in Paractaeopsis ; cf. Serène 1984 : pl. XVII, fig. E); carapace 2 M region divided into two lobes (Figs 1 A, C , 3 A ) (carapace 2 M region divided into four lobes in Paractaeopsis ; cf. Takeda and Miyake 1968 : fig. 1 a); ambulatory legs comparatively slender (Figs 1 A , 3 D ) (ambulatory legs very short and stout in Paractaeopsis ; cf. Takeda and Miyake 1968 : fig. 1 c). Given the above comparisons, the current species cannot be placed within any known genera, necessitating the establishment of a new genus. The main morphological characteristics comparing Gothus gen. nov. with closely related genera are listed in Table 2 . Table 2. Comparison of the characters of Gothus gen. nov. and six related genera included in the subfamily Euxanthinae .

Character	Gothus gen. nov.	Rizalthus	Visayax	Euxanthus	Hypocolpus	Psaumis	Paractaeopsis
carapace dorsal surface	with round granules, with clustered long setae or scattered short setae	with large granules, surrounded by short setae basally	with distinct or faint reticulate pattern of fused granules	relatively smooth, without well-developed granules.	with granules and setae	densely covered with granules	with pearly granules, scattered long tubular setae
carapace anterolateral margin	with four anterolateral teeth, first tooth flattened, posterior three teeth well-developed	without clearly defined anterolateral teeth	absence of developed anterolateral teeth	with four to six anterolateral teeth	anterolateral teeth underdeveloped	with very flat anterolateral teeth, only fourth tooth prominent	with four developed anterolateral teeth
carapace posterolateral margin	non-concave	concave	concave	concave	concave	concave	non-concave
front	not protruding, divided by a V-shaped notch	not protruding, divided by a V-shaped notch	not protruding, divided by a V-shaped notch	protruding, divided by a narrow fissure	protruding, divided by a narrow fissure	not protruding, divided by a narrow fissure	protruding, divided by a V-shaped notch
epistome	central part protruding	central part not protruding	central part not protruding or slightly protruding	central part protruding	central part protruding	central part protruding	unknown
subhepatic cavity	absent	absent	absent	absent	present	absent	absent
etched depressions	absent	present on thoracic sternum	present on chelipeds, ambulatory legs, carapace, and thoracic sternum	absent	present on thoracic sternum	present on chelipeds, ambulatory legs and carapace	absent
male telson	broad, truncated oval	smaller, triangular	smaller, semi-circular	smaller, triangular	smaller, triangular	smaller, semi-circular	unknown
cheliped carpus	robust, sometimes slightly expanded	strongly expanded and protruding	robust	robust	robust	slightly robust	robust
ambulatory leg	comparatively slender	comparatively slender	comparatively slender	comparatively slender	comparatively slender	comparatively slender	very short and stout
male first gonopod	slender, distal lobe prominent, spoon-shaped, curved inwards, with long setae on inner subdistal part	slender, distal lobe prominent, spoon-shaped, nearly straight, with long setae on inner subdistal part	robust, distal lobe prominent, nearly straight, with long setae on inner subdistal part	slender, distal lobe non-protruding, inwardly curved and encircling, with short setae on inner subdistal part	slender, distal lobe non-protruding, inwardly curved and encircling, with long setae on inner subdistal part	slender, distal lobe prominent, spoon-shaped, nearly straight, with long setae on inner subdistal part	robust, distal lobe prominent, nearly straight, with long setae on inner subdistal part