Xenostrobus or Vignadula (Bivalvia: Mytilidae)? A taxonomic re-evaluation of small black mussels inhabiting the upper intertidal zone of the estuaries of Southeast Asia Author Tan, Koh Siang Author Tan, Samuel Hui Ming Author Sanpanich, Kitithorn Author Duangdee, Teerapong Author Ambarwati, Reni text Zoological Journal of the Linnean Society 2022 2022-05-14 196 316 345 https://www.mendeley.com/catalogue/f5a3130a-cb03-3121-b8eb-32409c478f89/ journal article 133184 10.1093/zoolinnean/zlac031 fe110920-333b-4cdc-98f2-a4cc24124ea8 0024-4082 7035193 968C576E-4747-4586-9D48-C24C92741905 VIGNADULA ATRATA (LISCHKE, 1871) ( FIGS 1–3 , 8 , 11 , 12A , 13 ) Mytilus atratus Lischke, 1871a: 44 and Lischke, 1871b: 146 , pl. 10, figs 4, 4a, 5, 5b; type locality: Nagasaki , Japan . Holotype : ZIN 2/111 (see Lutaenko & Chaban, 2016 ). Modiola aterrima – Dall, 1871: 154 and pl. 14, fig. 13; type locality: Bay of Yeddo ( Tokyo Bay ). Holotype : USNM 185505 . Volsella atrata – Kuroda, 1932: 134 , sp. 407. Adula atrata – Habe & Kosuge, 1967: 127 , pl. 47 fig. 10. Xenostrobus atratus – Wilson, 1967: 284 ; – Ockelmann, 1983: 109–110 , fig. 44 (graph); – Bernard et al. , 1993: 33–34 (in part); – Kimura, 1996: 97–100, figs 1–6; – Lutaenko et al. , 2019: 185 and pl. 9, figs I, J. Hormomya atrata – Habe, 1968: 167–168 , pl. 50, fig. 19. Vignadula atrata – Kuroda et al. , 1971: 549 [in Japanese], 348 [in English], pl. 74, figs 5, 6; – Wang et al. , 2011: 87–88 , figs. 3-71. Xenostrobus atrata – Wang, 1997: 207–208 , fig. 87; – Wang, 2004 : p. 229 , pl. 121E. Diagnosis: Dark purplish blue to black mytiliform shells with terminal umbones; anterior half of ventral region often yellow to orange crossed by dark purple commarginal lines; animal without plicate gland; labial palps short, each palp bearing with ≤ 16 folds; mantle edge at posterior region of animal bears up to ten simple and/or branched guard papillae. Material examined: Japan : Fukuoka-Amakusa (NSMT 49028); Kagoshima - Shoujigawa Figure 4. Vignadula atrata (Lischke, 1871) . Sequenced specimens from Kyushu, Japan: A–H, Nagasaki; I–L, Kagoshima. A, B, XM NJ 0519; LV, SL = 11.2 mm (ZRC.MOL.24028). C, D, XNJ 0519A; LV, SL = 12.4 mm (ZRC.MOL.24029). E, F, XNJ 0519B; LV, SL = 10.6 mm (ZRC.MOL.24030). G, H, XNJ 0519C; LV, SL = 9.6 mm (ZRC.MOL.24031). I, J, XKJ 0519B; LV, SL = 8.0 mm (ZRC.MOL.24025). K, L, XKJ 0519C, SL = 6.6 mm (ZRC.MOL.24026). See also Clade C in Figures 1–3. (ZRC.MOL.24027, 24916, 24917; PMBC 25330); Red Cross Hospital, Hirakawa (ZRC.MOL.24918, 24919); Ibusuki (ZRC.MOL.24920); Mie –Tsu (NSMT 58821); Miyagi-Fukuura Island (NSMT 44150); Nagasaki-Obama (ZRC.MOL.24921); Toishi-ko (ZRC. MOL.24922); Isomichimachi (ZRC.MOL.24923, 24924); Okayama-Kasaoka Bay (NSMT 49029). See also the Supporting Information ( Table S1 ). Shell: Variable in outline and shape, length ≤ 15 mm , mostly entirely dark purplish blue to black over the posterior two-thirds of length of valves, while the anterior region is translucent yellow to orange crossed by fine dark purple commarginal lines where the periostracum is intact. Umbones terminal or in line with the edge of the anteroventral margin. Periostracal surface with fine, close-set commarginal lines free of byssal secretions. Ligament robust, long, stretching posteriorly from umbones to nearly half the length of the shell. Resilial pits are absent. At the region immediately below the umbones, there is a short, narrow shelf, but there are no hinge teeth anterior or posterior to the umbones. Likewise, there are no teeth posterior to the ligament. Interior of shell can generally be divided into a dark purplish brown dorsoposterior region and a lighter-coloured anteroventral region. Anterior region with prominent, oval anterior adductor muscle scar. Posterior adductor muscle scar continuous with byssal retractor muscle scars. Shell microstructure: Shell generally comprises of two recognizable layers below the periostracum: a thin prismatic layer, under which a thick sheet nacreous layer is present. A thin, fibrous prismatic myostracum may be present within the sheet nacreous layer. Anatomy: Inner mantle lobes forming the inhalant aperture are translucent orange-brown, with a high density of yellow subcutaneous pigment grains. The guard papillae are six to ten in number on each side. Each papilla is translucent white with small, white subcutaneous pigment grains. They may be simple or mixed with bi- or trifid papillae ( Figs 8 , 12A ). The papillae all taper to a point when fully extended. They are highly contractile and if disturbed can be withdrawn completely to appear as folds or creases along the mantle edge. Juveniles < 2 mm SL do not possess guard papillae ( Fig. 8A ). Plicate organ is absent. Foot is yellowish white. Two bundles of the posterior adductor muscle can (barely) be distinguished, and one is generally larger than the other, although their relative size varies between individuals. The posterior byssal retractor muscles are divided into two main bundles near the base of the foot. Of these, the anterior bundle is further divided into two or three bundles proximal to the shell. The pericardial complex is situated anterior to the posterior byssal retractor muscles (Category 2 of Morton, 2015a ). Labial palps vary in length according to shell size (see Theisen, 1982 ; Ockelmann, 1983 ), as does the number of folds (sorting ridges), which can range between five and 16 on each palp for SLs 4–12.5 mm (see Fig. 11 ). Geographical distribution ( Fig. 13 ): Korea ( Jeju Island; see Lutaenko et al. , 2019 ), Japan and northeast China . Taxonomic remarks: Mytilus atratus was first described from Nagasaki Bay, Japan by C. E. Lischke in 1871 (see Lutaenko & Chaban, 2016 ). Subsequent Japanese workers were undecided on its generic status, assigning the species variously to Adula H.Adams &A.Adams, 1857 , Hormomya Mörch, 1853 and Volsella Scopoli, 1777 (see Kuroda, 1932 ; Habe & Kosuge, 1967 ; Habe, 1968 ). A century later, the species was assigned to Vignadula Kuroda & Habe in Kuroda et al. (1971) , although Wilson (1967) recognized the possible connection between V. atratus and the Australian species of Xenostrobus . This was later followed up by Ockelmann (1983) and Lee & Morton (1985) , who considered Vignadula a junior synonym of Xenostrobus . At about the same time, Limnoperna fortunei kikuchii Habe (1981) was described, which afterwards proved to be X. securis introduced from Australia and/or New Zealand ( Kimura et al. , 1999 ). Kimura (1996) also described X. atratus in detail, and subsequently, Kimura et al. (1999) compared it with other congeners (see also Table 1 ). However, owing to its external resemblance to the tropical V. mangle (see below), the two species have sometimes been confused with each other (e.g. Wang, 1997 ; Liu et al. , 2011 ), particularly along the Chinese coast, where both species may occur and overlap in distribution.