Platevindex tigrinus ( Stoliczka, 1869 ) Goulding & Bourke & Comendador & Khalil & Quang & Tan & Tan & Dayrat, 2021

Goulding, Tricia C., Bourke, Adam J., Comendador, Joseph, Khalil, Munawar, Quang, Ngo Xuan, Tan, Shau Hwai, Tan, Siong Kiat & Dayrat, Benoît, 2021, Systematic revision of Platevindex Baker, 1938 (Gastropoda: Euthyneura: Onchidiidae), European Journal of Taxonomy 737 (1), pp. 1-133 : 60-68

Platevindex tigrinus ( Stoliczka, 1869) comb. nov.

Figs 26–30

Onchidium tigrinum Stoliczka, 1869: 105–107 , pl. 15, fig. 2.

Platevindex mortoni – Sun et al. 2014: 63. — Zhang et al. 2017: fig. 1 [non Platevindex mortoni Britton, 1984 ].

Material examined

Neotype (here designated)

INDIA • neotype (28/18 [1066] mm); Sundarbans , Bali Island; 22°04.387′ N, 88°41.828′ E; 6 Jan. 2011; station 50; soft mud by the sea with a few mangrove trees and dead logs; BNHS 18-1066 .

Notes on type material

The designation of a neotype is needed in order to clarify the application of the name Onchidium tigrinum (ICZN Article 75.3.1). The characters that differentiate P. tigrinus from other species of Platevindex are mentioned below in a section on ‘Distinctive diagnostic features’ (ICZN Article 75.3.2). The neotype is preserved separately and will be easily recognized (ICZN Article 75.3.3). The syntypes could not be located after an exhaustive search of museum collections ( Dayrat 2009) (ICZN Article 75.3.4). What is known of the former syntypes, from the original description and illustrations, is consistent with the neotype (ICZN Article 75.3.5). The type locality is the banks of the Mutlah River at Port Canning [Matla River, Ganges Delta, India] and the neotype was collected as nearly as practicable from the original type locality in West Bengal (ICZN Article 75.3.6), and it is already the property of a scientific institution, the Bombay Natural History Society, in Mumbai, India (ICZN Article 75.3.7). The neotype was collected on soft mud by the sea with a few mangrove trees and dead logs. The designation of a neotype is exceptionally needed in order to make sure that the application of the name Onchidium tigrinum remains unambiguous (see Remarks below).

Other material

BRUNEI • 2 specs (29/18 [1033] and 15/14 [1032] mm); Sungai ; 04°53.756′ N, 114°59.496′ E; 26 Jul. 2011; station 31; very large, tall Rhizophora mangrove with soft mud; BDMNH • 1 spec. (23/16 [1030] mm); Pulau Pyatan , Teluk Brunei; 04°55.246′ N, 115°02.764′ E; 27 Jul. 2011; station 32; open mangrove with a few sparse old trees and large old logs, by the river; BDMNH .

INDIA – West Bengal • 1 spec. (13/10 [1065] mm); same collection data as for neotype; BNHS 18-1065 • 1 spec. (22/15 [#1] mm); same collection data as for neotype; BNHS 18 . – Andaman Islands • 1 spec. (40/30 [1094] mm); Shamkund , near Rangat; 12°29.448′ N, 92°50.620′ E; 11 Jan. 2011; station 57; by a large river, deep mangrove with tall trees, small creeks and old muddy logs, next to a road and a small cemented bridge across creek; BNHS 2-1094 • 2 specs (35/22 [#1] and 24/15 [1114] mm); Shantipur, Kadamtala ; 02°19.844′ N, 92°46.377′ E; 12 Jan. 2011; station 58; open area with hard mud and many old logs, next to a mangrove; BNHS 12-1114 • 1 spec. (25/22 [1123] mm); Shoal Bay, by Bamboo Flat; 11°47.531′ N, 92°42.576′ E; 13 Jan. 2011; station 59; open mangrove with hard mud, old logs, next to a road and a small cemented bridge across creek; BNHS 1-1123 . – Maharashtra • 1 spec. (25/16 [1140] mm); Khandra mangrove; 19°03.876′ N, 72°49.296′ E; 18 Dec. 2011; station 43; very small Avicennia trees in mud; BNHS 35-1140 • 1 spec. (36/26 [1160] mm); Vatad; 17°15.791′ N, 73°17.623′ E; 23 Dec. 2011; station 46; Avicennia mangroves by field with deep and very watery mud; BNHS 104-1160 • 1 spec. (29/16 [1178] mm); Areware mangrove; 17°04.404′ N, 73°17.747′ E; 24 Dec. 2011; station 47; mangrove of Avicennia and a few small Rhizophora with soft mud and areas with pooled water; BNHS 33-1178 .

MALAYSIA – Peninsular Malaysia • 3 specs (23/17 [953], 21/18 [955] and 14/12 [956] mm); Matang, Crocodile River off Kuala Septang; 04°49.521′ N, 100°37.630′ E; 10 Jul. 2011; station 16; mangrove with tall Rhizophora ; USMMC 00022 • 2 specs (27/18 [939] and 26/14 [5522] mm); Merbok; 05°39.035′ N, 100°25.782′ E; 18 Jul. 2011; station 21; deep Rhizophora forest with old, tall trees, hard mud, many small creeks and dead logs; USMMC 00023 • 1 spec. (14/13 [936] mm); Langkawi Island , Tanjung Rhu; 06°25.771′ N, 99°49.436′ E; 13 Jul. 2011; station 23; young Rhizophora , Sonneratia and Avicennia in a dense forest with small creeks; USMMC 00024 .

SINGAPORE • 1 spec. (21/17 [992] mm); Lim Chu Kang ; 01°26.785′ N, 103°42.531′ E; 2 Apr. 2010; station 7; open mangrove forest with tall trees and soft mud by river, ending on sun-exposed mudflat; ZRC.MOL.10472 • 1 spec. (18/17 [993] mm); Semakao Island ; 01°12.083′ N, 103°45.585′ E; 3 Apr. 2010; station 8; landfill island with newly planted mangrove, very dense; ZRC.MOL.10473 • 1 spec. (19/13 [994] mm); Lim Chu Kang ; 01°26.785′ N, 103°42.531′ E; 5 Apr. 2010; station 9; open mangrove forest with tall trees and soft mud by river, ending on sun-exposed mudflat; ZRC.MOL.10474 .

Description

Color and morphology of live animals ( Fig. 26 )

The dorsal surface is bumpy, not smooth, and typically brown with dark brown or yellow-brown markings. The hyponotum color varies from bluish grey to light grey. The edge of the hyponotum may be slightly lighter, but there is no prominent light-colored line around the entire edge. The foot color varies from dark blue-grey to light yellow. The number of papillae with dorsal eyes is variable (generally between 25 and 50). Dorsal eyes are distributed across the notum but are absent from its margin (i.e., eyes are never <2 mm from the notum edge).

Digestive system ( Figs 27A , 28 )

Radulae measure up to 5 mm in length. Examples of radular formulae are presented in Table 5 . The intestinal loops are of type II, with transitional loops oriented between 6 and 9 o’clock ( Fig. 27A ).

Reproductive system ( Figs 27 B–C, 29)

The oviduct is much wider than the deferent duct (approximately up to three times). Its distal section (distal to the spermatheca) is up to three times as long as its proximal section ( Fig. 27B ). The deferent duct is closely attached to the oviduct and almost straight (with only loose turns). The distal, flexible, hook-bearing section of the penis is 1 to 1.5 mm long. The penial hooks are approximately 60 to 100 µm long, and can be seen inside the semi-transparent penis. In mature specimens, the retractor muscle inserts on the right side of the visceral cavity near the heart. The retractor muscle is from one quarter as long as the penial sheath to equal to it in length ( Fig 27C ). The deferent duct is not highly convoluted, with only a few loops (and is even less convoluted in immature specimens).

Distinctive diagnostic features ( Table 4 )

Externally, the yellow foot of Platevindex tigrinus clearly distinguishes it from P. martensi (orange foot) and P. aptei sp. nov. (black foot). Platevindex tigrinus is very similar externally to P. amboinae , except that the notum of P. tigrinus bears many dorsal eyes which are absent in P. amboinae . Platevindex tigrinus can be distinguished from P. latus by its granular dorsal surface and the lack of large, thorny dorsal papillae. Its granular dorsal surface also helps to distinguish it from P. luteus and P. applanatus , which bear prominent dorsal papillae when disturbed, although it is still possible to confuse these three species. Platevindex tigrinus cannot be distinguished externally from P. coriaceus , which has been found in many of the same sites, except in India (where P. coriaceus has not been found so far).

Internally, the intestinal loops clearly distinguish P. tigrinus (type II) from P. applanatus (type I). Some individuals of P. luteus are characterized by intestinal loops of type II and cannot be distinguished from P. tigrinus based only on the digestive system. The reproductive system can also be used to differentiate P. tigrinus from P. coriaceus , P. luteus and P. burnupi . In P. tigrinus , the retractor muscle for the penis inserts near the heart on the right side of the visceral cavity (i.e., in the posterior half or third of the cavity) while it inserts at the posterior end of the cavity in P. coriaceus . In P. tigrinus the deferent duct is closely attached to the oviduct (in the posterior reproductive system), while it is only loosely attached to it in P. luteus and P. burnupi . This trait also differs slightly between P. tigrinus and P. coriaceus : the deferent duct, which is closely attached to the oviduct in both species, is almost straight in P. tigrinus and highly-convoluted, with tight, U-shaped turns in P. coriaceus .

Distribution ( Fig. 10B )

Brunei. India: Andaman Islands, Maharashtra and West Bengal (type locality and fresh specimens newly collected). Peninsular Malaysia. Singapore. Southern China (specimens misidentified as Platevindex mortoni in Sun et al. 2014 and Zhang et al. 2017). All records are new, except for the type locality.

Habitat ( Fig. 30 )

Platevindex tigrinus is found in mangroves, on tree trunks and roots and dead logs, but is not found directly on mud. Those logs and trees may be in silty mud saturated with water or in the mid-intertidal. It does not live on rocky shores. Platevindex tigrinus is common across most of its geographic range. It is one of the most abundant onchidiid species in the mangroves of India.

Remarks

Stoliczka’s illustration of the external appearance of O. tigrinum clearly shows a Platevindex slug ( Stoliczka 1869: fig. 2). In his description of O. tigrinum, Stoliczka emphasized the flattened shape of the slug, the narrow foot and the hardened notum, which are all diagnostic of Platevindex . Other aspects of the original description are also in agreement with Platevindex : a granular dorsal surface, a foot from one third to one fourth the total width of the body, dorsal eyes on papillae of the notum and a male opening above the right oral lobe. Therefore, O. tigrinum is transferred to Platevindex .

Stoliczka’s description of the color of the ventral surface is as follows: “Young specimens have the mantle below uniform light bluish with very numerous and white dots; large ones have occasionally a number of dark green or rusty, more or less confluent spots along the lateral margins, and the general colour is pale. The foot (...) is of a uniform dark bluish grey colour.” ( Stoliczka 1869: 106). The color of Stoliczka’s young specimens perfectly matches the species described here and the color of the large ones is also within the observed color variation. The species treated here is the only one that we found at the type locality of O. tigrinum with a blue-grey ventral surface, strongly suggesting that the name O. tigrinum applies to the species described here. Also, Stoliczka mentioned that, at the type locality, O. tigrinum was common and found crawling on old wood, which is in complete agreement with our observations in the field, including at the type locality.

There is, however, a problem with Stoliczka’s original description of O. tigrinum . Indeed, Stoliczka (1869: 107) stated that the “internal vas deferens is about 5 inches, and its supplementary albuminous string [Stoliczka’s term for the accessory penial gland] about 8 inches long.” An accessory penial gland is not present in any species of Platevindex and this strongly suggests that a species from another genus was part of the type material, although it cannot be excluded that he made a mistake. Thus, the designation of a neotype is needed because it allows us to ensure that, in the future, O. tigrinum will always apply to the species described here. The designation of a neotype is far more preferable to a lectotype designation based on an illustration or description. Indeed, even though the species described here is the only Platevindex we found at the type locality of O. tigrinum , it cannot be excluded that other species, especially P. luteus , will be found there in the future. Because P. luteus and P. tigrinus are hardly distinguishable externally, it is possible that Stoliczka also examined some specimens of P. luteus . For all the reasons above, the neotype designated here efficiently clarifies the application of O. tigrinum .

In his remarks on Onchidium coriaceum, Semper (1882: 273) addressed the similarity between the species he described and Stoliczka’s description of Onchidium tigrinum . Semper pointed out that the only difference between the two descriptions is the presence of an accessory penial gland in O. tigrinum , but doubted that one was actually present in Stoliczka’s specimens of O. tigrinum . Semper considered O. tigrinum to be a synonym of O. coriaceum (although with a question mark), even though he was not able to properly compare the anatomy of the two species, and even though, strictly speaking, O. tigrinum had priority over O. coriaceum . Platevindex tigrinus is shown here to be genetically and anatomically distinct from P. coriaceus .

Hoffmann (1928: 77) noted Semper’s comments on the similarity between Onchidium tigrinum and Oncis stuxbergi , but did not provide additional comments on the species or add new localities. Hoffmann’s indication that the species was from “Port Canning (Penang, Malayische Halbinsel)” [Penang, Peninsular Malaysia] appears to be a mistake for Stoliczka’s locality of Port Canning in West Bengal, India. Labbé (1934: 223) repeated Hoffmann’s erroneous record but did not provide new information about the species.

The type material of Platevindex mortoni , a species originally described by Britton (1984) from Hong Kong, includes a holotype by original designation (NHMUK 1982288) and three paratypes (NHMUK 1982289/1,2 and NHMUK 1982289/3). The holotype (18/ 16 mm) is missing both the male and female reproductive parts, which were not described in detail by Britton. Also, its digestive system is largely destroyed (the intestinal loops cannot be observed) even though the radula remains. The two largest paratypes (22/18 and 15/ 15 mm) are empty of all internal organs. All organs remain in the smallest (9/ 9 mm) paratype but it is not fully mature (the posterior, female reproductive parts are only partly developed). The intestinal loops of the smallest paratype are clearly of type II. Britton did not describe the color of the ventral surface (foot and hyponotum) of live animals. He only mentioned that the preserved holotype and two preserved paratypes (NHMUK 1982289/1,2) were brown with a darker mantle rim, while the third paratype (NHMUK 1982289/3) was grey with darker mottling. However, the dorsal color is useless for identifying species of Platevindex and a brown dorsal surface with a darker mantle rim (color of the holotype) could apply to any species. Britton’s description of the type of intestinal loops is also problematic. He described intestinal loops of type I in the holotype, of type II in what he called the “grey paratype ” (NHMUK 1982289/3), which is the largest paratype, and intermediate between types I and II in the smallest paratype. Intestinal loops of type II are clearly present in the smallest paratype (the only type specimen with remaining intestinal loops) which Britton called the “smaller brown paratype.” There are three possible explanations for that. First, Britton correctly described the intestinal loops of the holotype of P. mortoni (the only specimen with a name-bearing function) and the name P. mortoni cannot apply to P. tigrinus or P. coriaceus , two species with intestinal loops of type II and which are known to be present in Hong Kong (see below). However, the name P. mortoni could apply to P. luteus , a species with intestinal loops of type I, but which is not known from Hong Kong (although it is known from the Philippines and Vietnam, among other places). Second, it is also possible (and quite likely) that Britton simply made a mistake in identifying the type of the intestinal loops in the holotype of P. mortoni because he also misidentified the intestinal type in another onchidiid that he called Paraoncidium reevesii (J.E. Gray, 1850) (see Goulding et al. 2018b). If Britton misidentified the intestinal loops in the holotype of P. mortoni , the description would be compatible with P. tigrinus . Third, it is also possible that the original description of P. mortoni was based on specimens from two different species (one with a type I and one with a type II), even though, strictly speaking, only the holotype is the name-bearer. Because Britton’s description of P. mortoni is confusing and since critical characters cannot be checked in the type material, P. mortoni is regarded as a nomen dubium. It does apply to a species of Platevindex , but which one cannot be confidently determined.

DNA sequences of onchidiids from China published by Sun et al. (2014) show that there are several species of Platevindex in China and at least two in Hong Kong. For unclear reasons, Sun et al. applied the name P. mortoni to all Chinese onchidiids with a bluish-grey hyponotum and foot. By comparing their DNA sequences with ours, we have determined that the sequenced specimens they called P. mortoni belong to P. tigrinus and P. coriaceus . The specimens called P. mortoni from southern China, east of Hainan (including Hong Kong), are here referred to P. tigrinus , while a specimen collected from southern China west of Hainan belongs to P. coriaceus .

Zhang et al. (2017) recently re-described a species they identified as Platevindex mortoni based on specimens from Hong Kong. This description did not address the inconsistency in the original description regarding the intestinal loops and appears to be based on multiple species of Platevindex . Despite the fact that COI sequences from Sun et al. (2014) showed that several species of this genus are present in China, including two in Hong Kong, Zhang et al. did not discuss how the species they re-described was different from other sympatric species of Platevindex . Photographs of live specimens by Zhang et al. clearly show that there are two different kinds of ventral coloration. The individual with a blue-grey foot and hyponotum ( Zhang et al. 2017: fig. 1c) was either P. tigrinus or P. coriaceus (both could potentially be present in Hong Kong), and the individual with a bright orange foot ( Zhang et al. 2017: fig. 1E) is identified here as P. martensi .

Platevindex tigrinus is distributed from western India to southern China ( Hong Kong). It is one of few onchidiid species which is broadly distributed from the Indian Ocean to the South China Sea. It was not found in Tamil Nadu (southeastern India) during our recent (2016) survey in the mangroves of Pichavaram, but the diversity of mangrove molluscs was very low. Further exploration of southeastern India could reveal whether Platevindex tigrinus is present in low abundance, whether it varies in abundance seasonally, or is simply absent.

Figures

Fig. 26. Platevindex tigrinus (Stoliczka, 1869), live animals. A. Dorsal view, 35 mm long [#1], India, Andaman Islands, Shantipur, Kadamtala (BNHS 12).B. Dorsal view, 22 mm long [#1], India, Sundarbans, Bali Island (BNHS 18). C. Dorsal view, 25 mm long [1140], India, Maharashtra, Khandra mangrove (BNHS 35-1140). D. Dorsal view, 19 mm long [994], Singapore, Lim Chu Kang (ZRC.MOL.10474). E. Dorsal view, 29 mm long [1178], India, Maharashtra (BNHS 33-1178). F. Dorsal view, neotype, 28 mm long [1066], India, West Bengal, Sundarbans, Bali Island (BNHS 18-1066). G. Ventral view, 27 mm long [939], Malaysia, Merbok (USMMC 00023). H. Ventral view, same as B. I. Ventral view, same as A.

Fig. 27. Platevindex tigrinus (Stoliczka, 1869), digestive and reproductive systems, Peninsular Malaysia [5522] (USMMC 00023). A. Digestive system, dorsal view, type II with a transitional loop oriented at 9 o’clock. B. Posterior reproductive system. C. Anterior, male copulatory apparatus. Abbreviations: dd = deferent duct; ddg = dorsal digestive gland; fgm = female gland mass; hd = hermaphroditic duct; hg = hermaphroditic gland; i = intestine; ov = oviduct; pdg = posterior digestive gland; ps = penial sheath; rg = rectal gland; rm = retractor muscle; rs = receptaculum seminis; sp = spermatheca; st = stomach; v = vestibule. Scale bars: A = 3 mm; B–C = 2 mm.

Fig. 28. Platevindex tigrinus (Stoliczka, 1869), radula, Peninsular Malaysia, Merbok [5522] (USMMC 00023). A. Rachidian and innermost lateral teeth. B. Lateral teeth with rachidian teeth. C. Lateral teeth. D. Outermost lateral teeth. Scale bars: A = 20 μm; B = 30 μm; C = 200 μm; D = 50 μm.

Fig. 29. Platevindex tigrinus (Stoliczka, 1869), penial hooks, Brunei [1030] (BDMNH). A. Penial hooks. B. Close up of penial hook. Scale bars: A = 50 μm; B = 20 μm.

Fig. 30. Habitats of Platevindex tigrinus (Stoliczka, 1869). A. Brunei, Sungai, very large, tall Rhizophora with soft mud, access by boat from river (station 31). B. Peninsular Malaysia, Merbok, old, tall Rhizophora trees with hard mud (station 21). C. India, Maharashtra, small Avicennia trees and some Rhizophora with soft mud, trees in clumps with open area in between (station 47). D. Singapore, Lim Chu Kang, Avicennia mangrove (station 7).

Fig. 10. Geographic distributions of species of Platevindex. Colored areas correspond to hypothetical ranges proposed based on all known records, and the colors are the same as those used for each species in the phylogenetic trees (Figs 1–5). Distinct colors are used for each subspecies and mitochondrial unit in P. coriaceus (Semper, 1880) as well as for each subspecies of P. burnupi (Collinge, 1902). Black dots show sites sampled for this study and red dots (China and Japan) show localities of previously published genetic sequences included in the phylogenetic analyses. A. Platevindex luteus (Semper, 1880), P. applanatus (Simroth, 1920), P. burnupi. B. Platevindex tigrinus (Stoliczka, 1869), P. coriaceus coriaceus (units #1, #2, and #3) and P. coriaceus darwinensis subsp. nov. C. Platevindex martensi (Plate, 1893), P. aptei Goulding & Dayrat sp. nov., P. amboinae (Plate, 1893) and P. latus (Plate, 1893).