Vignadula mangle Tan & Tan & Sanpanich & Duangdee & Ambarwati, 2022

Tan, Koh Siang, Tan, Samuel Hui Ming, Sanpanich, Kitithorn, Duangdee, Teerapong & Ambarwati, Reni, 2022, Xenostrobus or Vignadula (Bivalvia: Mytilidae)? A taxonomic re-evaluation of small black mussels inhabiting the upper intertidal zone of the estuaries of Southeast Asia, Zoological Journal of the Linnean Society 196, pp. 316-345 : 325-333

VIGNADULA MANGLE ( OCKELMANN, 1983) COMB. NOV.

( FIGS 1–3 , 5 , 6 , 9–11 , 12B , 13 )

Xenostrobus mangle Ockelmann, 1983: 104–106 , figs 32–35, 38, 40, 43, 44;

type locality: Jeram , Kuala Selangor, west coast of Malaysia. Holotype NHMD 77043 (BIV-001811) : missing; NHMD 916051 : 57 paratypes (27 intact individuals and 30 valves).

Xenostrobus balani – Ockelmann, 1983: 107–111, figs 36, 37, 39, 41, 43–45; type locality: mangrove of Ao Nam Bor, Phuket, Thailand. Holotype missing. NHMD 915909: five paratypes; PMBC 3104: three paratypes ( Aungtonya et al., 1999).

Xenostrobus aratus (Dunker, 1856) – Nguyen, 2001: 417 (not aratus [sic]).

Xenostrobus atrata (Lischke, 1871) – Lee & Morton, 1985: 54–55, pl. 2A (not atrata ); – Wang, 1997 (in part): 208, fig. 87 (not atrata ).

Xenostrobus atratus – Bernard et al., 1993: 33–34 (in part); – Kurozumi, 2000: 429–430 and pl. 429, fig. 6 (not atratus ?); – Kurozumi, 2017: 1173 and pl. 472, fig. 9 (not atratus ?).

Xenostrobus sp. – Ng & Sivasothi, 1999: 119, fig.

Diagnosis: Dark purple to black mytiliform shells with a trapezoid outline and mostly subterminal umbones; anterior half of ventral region often yellow to orange crossed by dark purple commarginal lines; animal with inner and outer plicate glands; labial palps long, each palp with ≤ 60 folds; mantle edge at posterior region of animal bears ≤14 simple and/or branched guard papillae. Nomenclature: Ockelmann (1983) designated and illustrated the holotype of X. mangle but did not isolate the specimen from amongst some 27 intact individuals and 30 valves designated paratypes at NHMD. However, none of the paratype shells in NHMD matches the overall appearance and dimensions of the holotype provided by Ockelmann. In the case of X. balani , the holotype specimen is missing from the vial marked holotype in the NHMD collection (T. Schiøtte, pers. comms.). The ten separated valves (referable to five individuals) in the paratype lot at NHMD all do not match exactly the holotype dimensions given by Ockelmann (1983). Nevertheless, these irregularities do not affect the conclusions reached in this study.

Material examined: Taiwan: Husia, Kinmen (off Xiamen, China) (ZRC.MOL.24925); Vietnam: Sung Sot Cave, Ile de la Surprise off Cat Ba Island, Gulf of Tonkin

(ZRC.MOL.24926); Thailand: Andaman Sea–Pak Bara, Satun (ZRC.MOL.24927, 24928; PMBC 25331); Ko Talibong, Trang (ZRC.MOL.24929; PMBC 25332); Pak Meng, Trang (ZRC.MOL.24067); Phuket, Ao Nam Bor (NHMD 915909; PMBC 3104, 3732); Gulf of Thailand- Pranburi (ZRC.MOL.24930; PMBC 25333); Hua Hin (ZRC.MOL.24004, 24005); Bangsaen (ZRC.MOL.24931, 24932; PMBC 25334); Pattaya (ZRC.MOL.24006); Laem Hin, Trat (ZRC.MOL.24933, 24934; PMBC 25335, 25336); Malaysia: Johor west coast-Sungei Punggur (ZRC.MOL.24935); Sungei Lurus (ZRC.MOL.24936); Pontian Kechil (ZRC.MOL.24937); Minyak Beku (ZRC. MOL.24938); Selangor-Jeram (NHMD 77043); Perak- Teluk Batik, Lumut (ZRC.MOL.24039, 24939); Teluk Senangin (ZRC.MOL.24940); Kedah-Tanjung Jaga (ZRC.MOL.24941); Penang-Gertak Sanggul (ZRC. MOL.24942); Batu Ferringhi (ZRC.MOL.24943); Sarawak-Santubong (ZRC.MOL.24944); Kampung Bako, north of Kuching (ZRC.MOL.24022, 24945); Singapore: Raffles Marina (ZRC.MOL.24946); Tengeh Buoy (ZRC.MOL.24947); Sarimbun (ZRC.MOL.24064); Sungei Buloh (ZRC.MOL.24948, 24949); Kranji (ZRC. MOL.24950); Sembawang Park (ZRC.MOL.24951); Tanjong Irau, Sembawang (ZRC.MOL.24952, 24953); Punggol Point (ZRC.MOL.24053); Changi Village (ZRC.MOL.24056, 24057, 24058); Paku Buoy (ZRC. MOL.24954); Siglap Canal (ZRC.MOL.24955);

Indonesia: Surabaya , East Java ( ZRC.MOL.24956). See also the Supporting Information ( Table S1 ) .

Shell: Variable in outline and shape, length ≤ 15 mm, thin, overall outline trapezoid; the blunt, inflated anterior region is defined by distinct umbonal and anteroventral extensions, while the posterior region is broad and flat. Surface mostly entirely purplish black over the posterior two-thirds of length of valves, while the anteroventral region is translucent yellow to orange crossed by fine dark purple, closely set commarginal lines where the periostracum is intact. Umbones in most cases subterminal to the distinct, rounded anteroventral margin. Shell interior iridescent, margins devoid of teeth. Ligament narrow, smooth without teeth; resilial pits absent.

Shell microstructure: Shell generally comprises two 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: Up to 14 simple or branched (bifid, trifid or quadrifid) guard papillae ( Figs 10E–G , 12B ) along one edge of inner mantle forming inhalant siphon. Papillae translucent pink to white, with small white subcutaneous pigment grains. Smaller individuals tend to have fewer guard papillae, which are invariably simple (not branched). In individuals with SLs <2–3 mm, the inner mantle edge forming the inhalant aperture is generally entire, with no papillae. Foot yellowish white, vermiform.

Two adjacent bundles of the posterior adductor muscle can be distinguished. One is generally larger than the other, although their relative size varies between individuals. Posterior byssal retractor muscles are split into two bundles near base of foot, but the anterior bundle is further divided into two or three subbundles towards the shell (as observed on animal, but the muscle scars on the valves do not reflect this separation). Pericardium lies anterior to the posterior byssal retractor muscle complex (Category 2 of Morton, 2015a).

Plicate organ (organ of Sabatier; Fig. 10D ) present on the roofs of the suprabranchial chamber, along length of bases of outer and inner demibranchs on the left and right sides of the animal. 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 nine and 55 for SLs 2–13 mm ( Fig. 11 ).

Ecology: Found singly or in small groups of two to four individuals inside empty barnacle (often Balanus amphitrite Darwin, 1854 ) shells together with Acari, collembolans and the estuarine bryozoan Sundanella sibogae (Harmer, 1915) in the upper- to mid-intertidal zone of estuaries and mangroves. They can also be found in large numbers clustered together over rocks or concrete surfaces in the intertidal zone in estuaries. However, they can also occur subtidally, as observed by Madin et al. (2009) on recruitment nylon net panels placed vertically in floating net cages. Leh et al. (2012) showed that V. mangle , together with other bivalves [ Anadara granosa (Linnaeus, 1758) and Placuna sp. ] composed ≤ 50% of the diet of the eel catfish Plotosus canius Hamilton, 1822 along the west coast of peninsular Malaysia.

Geographical distribution ( Fig. 13 ): Kinmen (off Xiamen, China) southwards to the Gulf of Thailand, Malaysia, Singapore, Strait of Malacca and the Andaman Sea; Borneo and Java. Curiously, there were no records of V. mangle or V. atrata from the main island of Taiwan itself in the National Museum of Natural Science in Taichung, Taiwan.

Taxonomic remarks: Ockelmann (1983) described two Xenostrobus species, X. mangle and X. balani , from Jeram in the Malacca Strait and Phuket Island adjoining the Andaman Sea, respectively. They were distinguished chiefly based on the sizes of their shells and labial palps, which are both relatively larger in X. mangle compared with X. balani as diagnosed by Ockelmann ( Table 1 ). Also, size asymmetry of the two bundles making up the posterior adductor muscle was noticed to be more marked in X. balani than in X. mangle . ‘Special byssal secretions’ ( Fig. 10C ) on the shell surface of X. balani (see paratypes: Fig. 9E, G, H ) that comprise commarginal thickenings or serrations are weak or absent in X. mangle , according to Ockelmann (1983). In addition, the byssal gland of X. balani is branching and penetrates deeply into the visceral mass, in contrast to the condensed, nonbranching gland in X. mangle (for a summary of characters, see Table 1 ).

In this study, we collected, examined and sequenced material from across a wide geographical range encompassing locations in the East China Sea, South China Sea, Malacca Strait, the Andaman Sea and Java Sea. Our observations suggest the existence of a continuum of morphological character states, instead of disjunct states that Ockelmann used to distinguish X. mangle and X. balani . This is particularly the case for the relative sizes of two muscle bundles of the posterior adductor muscle, the presence or absence of byssal secretions on the valves, the condition of the byssal gland and the number of folds on the labial palps. Nevertheless, the existence of two phenotypes [i.e. individuals having labial palps that are either short or long (corresponding to a smaller or larger number of folds) as shown by Ockelmann (1983)] appears to be real (see Fig. 11 ) amongst the otherwise genetically indistinguishable individuals we observed in this study. Interestingly, the two phenotypes seem to be location specific; for instance, all individuals collected in this study and sequenced from Singapore, Gulf of Thailand, Andaman Sea and Taiwan had small labial palps, whereas those from localities in Johor, Sarawak (Borneo) and Surabaya in Java had large labial palps. However, they all shared the apomorphy of having a plicate gland along the bases of the outer and inner demibranchs on both left and right sides of the animal.

Ockelmann (1983) also observed that ‘almost all’ X. balani were found singly in empty barnacle shells, whereas X.mangle were‘…living together with Balanus amphitrite …’ and ‘…were found byssally attached to their congeners’ valves’, occurring ‘…gregariously as members of the epifauna on mangrove roots and on stony substrata’. Our observations showed that X. mangle can indeed be found inside barnacle shells, not only singly, but gregariously in groups of up to several individuals depending on the size of the empty barnacle shells. These tended to be smaller individuals of X. mangle possessing largely intact ‘special byssal secretions’ ( Fig. 10C ), possibly because of the sheltered nature of the microhabitat, which is also used by other organisms, such as mites, collembolans and the high shore bryozoan Sundanella sibogae . Conversely, Ockelmann (1983) stated that many individuals of X. mangle living outside barnacle shells had live barnacles attached to their shells, which he attributed to the absence of ‘special byssal secretions’. We also observed some individuals fouled by barnacles, but these were not restricted to those living outside dead barnacle shells. Hence, based on these field observations, considered together with their similar morphology and genetic composition, we conclude that X. balani and X. mangle are one and the same species.

Figures

Figure 1. Concatenated four-gene Bayesian inference/maximum likelihood tree based on COI, ITS1, 28S D1R and H3 sequences of Xenostrobus mussels from East Asia and Australasia. Both 313 bp and full-length COI barcodes were used in the analysis. In several cases (e.g. SG X18 and SG X18-2; XTSW1 and XTSW1-2), two sequences from the same individual were incorporated into the analysis. For readability, some nodes on the branch tips have been omitted and the lengths of some branches truncated (marked by a double slash). Sequence codes with asterisks were obtained from specimens shown in Figures 4–7. Line and text colours correspond to the colour scheme used to differentiate the geographical distributions of the species represented in the clades A, B, C and D. Sequence code abbreviations: E Th, Gulf of Thailand; FJP, Fukuoka, Japan; GB, GenBank; HK, Hong Kong; Indon, Indonesia; J MY, Johor, Malaysia; JP, Japan; K EMY, Kuching, East Malaysia; K JP, Kagoshima, Japan; K TW, Kinmen, Taiwan; N JP, Nagasaki, Japan; P MY, Perak, Malaysia; SG, Singapore; S Indon, Surabaya, Indonesia; SW Th, south-west Thailand; Th, Thailand; Z Tan, Zanzibar, Tanzania.

Figure 2. Concatenated three-gene Bayesian inference/maximum likelihood tree based on COI, 28S D1R and H3 sequences of mussels from East Asia and Australasia, including additional sequences from GenBank. For readability, some nodes on the branch tips have been omitted and the lengths of some branches truncated (marked by a double slash). Sequence codes with asterisks were obtained from specimens shown in Figures 4–7. Line and text colours correspond to the colour scheme used to differentiate the geographical distributions of the species represented in the clades A, B, C and D.For an explanation of sequence code abbreviations, see legend to Figure 1.

Figure 3. Concatenated two-gene Bayesian inference/maximum likelihood tree based on COI and H3 sequences of mussels. For readability, some nodes on the branch tips have been omitted and the lengths of some branches truncated (marked by a double slash). Sequence codes with asterisks were obtained from specimens shown in Figures 4–7. Line and text colours correspond to the colour scheme used to differentiate the geographical distributions of the species represented in the clades A, B, C and D. For an explanation of sequence code abbreviations, see legend to Figure 1.

Figure 5. Vignadula mangle (Ockelmann, 1983) comb. nov. Selected sequenced specimens from Indonesia (A, B), Malaysia (C, D, I–L) and Singapore (E–H). A, B, X4I, Surabaya, E. Java; LV, SL = 9.9 mm (ZRC.MOL.24045). C, D, XA MYKS 1219A, Kampong Bako, Sarawak (Borneo); LV, SL = 12.8 mm (ZRC.MOL.24021). E, F, X8, Punggol Point; LV, SL = 10.1 mm (ZRC. MOL.24054). G, H, MX2, Sembawang Park; LV, SL = 14.8 mm (ZRC.MOL.24046). I, J, XAJ 12, Sungei Punggur, Johor (Malacca Strait); LV, SL= 6.8 mm (ZRC.MOL.24015). K, L, XMJ 3, Sungei Punggur, Johor (Malacca Strait); LV, SL = 10.9 mm (ZRC.MOL.24019). See also Clade D in Figures 1–3.

Figure 6. Vignadula mangle (Ockelmann, 1983) comb. nov. Selected sequenced specimens from Malaysia (A, B), Thailand (C–H) and Taiwan (I–L). A, B, XA MYPK 0419C, Lumut, Perak (Malacca Strait); RV, SL = 6.2 mm (ZRC.MOL.24041). C, D, XBT3, Pak Meng, Trang Province, Thailand (Andaman Sea); LV, SL = 11.9 mm (ZRC.MOL.24067). E, F, XTE6, Hua Hin, Prachuap Khiri Khan Province (Gulf of Thailand); LV, SL = 10.1 mm (ZRC.MOL.24004). G, H, XM TBB 0719C, Bangsaen Beach, Chonburi Province (Gulf ofThailand); LV,SL= 9.7 mm (ZRC.MOL.24007).I, J, XTW1, Kinmen,Taiwan; LV,SL = 9.2 mm (ZRC.MOL.24033). K, L, XTW6, Kinmen, Taiwan; LV, SL = 9.5 mm (ZRC.MOL.24038). See also Clade D in Figures 1–3.

Figure 9. Type material of Vignadula mangle comb. nov. and Vignadula balani. A–D, Xenostrobus mangle Ockelmann, 1983 paratypes NHMD 916051 Jeram, Selangor, peninsular Malaysia. A, B, SL = 8.2 mm. C, D, SL = 11.0 mm. E–H, Xenostrobus balani Ockelmann, 1983 paratypes NHMD 915909 Phuket, Thailand. E, F, SL = 6.4 mm. G, LV, SL = 7.2 mm. H, LV, SL = 8.5 mm.

Figure 10. Vignadula mangle (Ockelmann, 1983) comb. nov. A, numerous individuals amongst barnacles (Amphibalanus amphitrite (Darwin, 1854)) on an intertidal rocky shore at Teluk Senangin, Perak, Malaysia. Scale bar: 1 cm. B, intact living individual seen from its left, with extended inhalant (inner mantle lobes bearing simple guard papillae, gp) and exhalant (es) siphons. Scale bar: 1 mm. C, surface of posterior half of left valve (SL = 8.0 mm; specimen collected from Sembawang, Singapore) with ‘special byssus secretions’ (sbs), or commarginal series of thickenings or blunt serrations, sensu Ockelmann (1983). Scale bar: 500 µm. D, outer plicate gland, after removal of ctenidium. Scale bar: 1 mm. E–G, detail of the posterior region of living individuals, showing simple and branched (yellow asterisks) guard papillae. Both types of papillae can occur in the same individual, as shown in E and F. Blue and green arrows depict the direction of inhalant and exhalant currents. Scale bars: 1 mm. D, Hat Yao jetty, Trang, Gulf of Thailand. E, Teluk Batik, Lumut, Perak. B, F, G, Bangsaen Beach, Chonburi, Gulf of Thailand.

Figure 11. Relationship between shell size and number of folds observed on the labial palps of four Vignadula species from East and Southeast Asia. Data from Ockelmann (1983) for Vignadula atrata, Vignadula balani and Vignadula mangle are indicated separately. Apart from data points based on the study by Ockelmann (1983) and three individuals of Vignadula kuraburiensis sp. nov., all are referable to genetically sequenced individuals based on this study.

Figure 12. Guard papillae of Vignadula (A–D) and Xenostrobus (E–G) species on the posterior region of the inner mantle margin attached to LV. All animals shown preserved in ethanol. A, Vignadula atrata, SL = 11.4 mm, Toishi-ko, Nagasaki, Japan. B, Vignadula mangle comb. nov., SL = 13.5 mm, Kampong Bako, Kuching, Sarawak. C, Vignadula kuraburiensis sp. nov., SL = 13.2 mm, Kuraburi, Phangnga, Thailand. D, V. kuraburiensis sp. nov., SL = 8.8 mm, Ban Baen, Ranong, Thailand. E, Xenostrobus inconstans, SL = 22.7 mm, Dunnalley Bay, Tasmania, Australia (TMAG E21829). F, Xenostrobus securis, SL = 24.8 mm, Swan River, Perth, WA,Australia. G, Xenostrobus pulex, SL = 13.1 mm, Cottesloe, Perth, WA, Australia. Scale bars: 1 mm in A–C, E–G; 500 µm in D.

Figure 13. Geographical distribution of Vignadula and Xenostrobus species in East Asia, Australia and New Zealand. Locations are based on examined museum material or material collected in the field for this study by the authors, and supplemented by information contained in papers by Colgan (2017: Xenostrobus securis); Colgan et al. (2020: Xenostrobus neozelanicus and X. securis); Horikoshi & Okamoto (2007: X. securis); Iwasaki (2013: X. securis); Iwasaki & Yamamoto (2014: X. securis); Kimura (1996: Vignadula atrata); Kimura et al. (1999: X. securis [?], as Xenostrobus sp.); Lee & Morton (1985: Vignadula mangle comb. nov., as Xenostrobus atratus); Lamarck (1819: X. securis [?], as Mytilus securis; MNHN-IM-2000-34894, three syntypes); Lutaenko et al. (2019: V. atrata); Morton (1999: Xenostrobus pulex); Morton (2004: Xenostrobus inconstans); Morton & Leung (2015: X.securis); Park et al. (2017: V. atrata);Wang et al. (2011: V.atrata); Wilson (1967: Xenostrobus spp.).