Onchidium stuxbergi (Westerlund, 1883) Westerlund, 1883

Dayrat, Benoit, Goulding, Tricia C., Apte, Deepak, Bhave, Vishal, Joseph Comendador,, Qua, ng, Ngo Xuan, Tan, Siong Kiat & Tan, Shau Hwai, 2016, Integrative taxonomy of the genus Onchidium Buchannan, 1800 (Mollusca, Gastropoda, Pulmonata, Onchidiidae), ZooKeys 636, pp. 1-40 : 17-27

Onchidium stuxbergi (Westerlund, 1883) comb. n. Figs 9, 10, 11, 12, 13

Vaginulus stuxbergi Westerlund, 1883: 165; Westerlund 1885, p.191-192, pl. 2, fig. 2 a–c.

Oncidium nigrum Plate, 1893: 188-190, pl. 8, fig. 31a, pl. 10, fig. 53, pl. 11, fig. 75; Hoffmann 1928: 78; Labbé 1934: 223-224, figs. 58-61. New synonym.

Elophilus ajuthiae Labbé, 1935: 312-317, figs 1-3. New synonym.

Type locality

( Vaginulus stuxbergi ). "Borneo in silva, ad flum Kalias" means that the slugs were found in forests by a river now called the Klias River. The latter runs into the Bru nei Bay, which is a small bay bordered by Brunei Darussalam in the South, by Sabah (Malaysia) in the north, and by the small island of the Labuan Territory (Malaysia) in the west. Several of the labels of the type material indicate Labuan as the locality. So, it is possible that the type material is a mix of specimens collected at Labuan Island itself and on the shore of Borneo, facing Labuan. Here is what the different labels read for the first jar: " Vaginulus stuxbergi Westerlund, 1885. Borneo, Labuan. On the beach, mangroves. Leg. Vega Exp 1878-1880, sta. 633. SMNH-Type-7523-syntype(s);" " Onchidium . Mangrover Sump, Labuan vid Borneo [i.e., meaning Labuan opposite (seeing) Borneo], Vega Exp. n° 633, 18/11 1879;" and " Oncis stuxbergi Wstld, 1883. Hab. Labuan b. Borneo (Mangrove - Sumpf). Leg Vega-Expedition (N°633) 18-xi-1879, Jena, Jan 1927, Hoffmann determ." Here is what the different labels read for the second jar: " Vaginulus stuxbergi Westerlund, 1885. Borneo, Labuan. On the beach, mangroves. Leg. Vega Exp 1878-1880, sta. 633. SMNH-Type-1334-syntype(s);" " Vaginulus Stuxbergi ;" " Vaginulus Stuxbergi Borneo Vega Exp, det. Westerlund;" and " Oncis stuxbergi Westerlund, 1883 [Typ fur Vaginula stuxbergi Wsterld]. Hab. Borneo. Leg. Vega-Expedition, Westerlund det. Jena, Jan 1927, Hoffmann determ." Our specimens here were collected from Brunei Darussalam, which borders the Brunei Bay and faces the island of Labuan, i.e., from a locality that is extremely close to the type locality.

Type locality

( Onchidium nigrum ). “Borneo” is the only geographic information provided by Plate (1893) in the original description as well as on the label of the type, which reads " Oncidium nigrum Plate. 22749. Borneo. Gera S." The mention of "Gera S." does not refer to Sungai Geras, a river near the city of Bintulu, on the west coast of Borneo, in Sarawak, Malaysia, but most likely to the fact that the specimen was collected by Gerard, as indicated in Plate’s (1893: 188) original description ("1 Exemplar von Borneo, durch Gerard gesammelt"). Thus, the type locality of Onchidium nigrum could be anywhere in Borneo.

Type locality

( Elophilus ajuthiae ). The "Province d’Ajuthia (Siam)" is the province of Ayutthaya, in Thailand, approximately 80 kilometers north of Bangkok, which used to be the capital of the Kingdom of Siam. Because that province is inland, Labbé assumed that the slugs had been collected in fresh water. However, the Chao Phraya River of the basin than runs through the province of Ayutthaya actually is under the influence of the tide year round. The salt front (brackish water) goes up to 75 and 175 kilometers from the river mouth in the wet and dry seasons, respectively, and it was even more so the case in the past when the river side was still not developed (syntypes collected by M. Bocourt in 1862 according to the label of the type material). In the province of Ayutthaya, the river is approximately at its kilometer 140 ( Singkran 2015: 28). The label of the type specimens of Elophilus ajuthiae says that it lived in the "eaux dormantes de la province d’Ajuthia.” The French expression "eaux dormantes" means “swamps.” Given what is known of the basin of Chao Praya River, those swamps were brackish water under the influence of daily tides.

Type material

( Vaginulus stuxbergi ). One lectotype hereby designated (43/25 mm; entire and never dissected; SMNH 1334). All eleven other syntypes become paralectotypes with no name-bearing status. Originally, the type material included a total of twelve specimens split in two different jars: five specimens (all paralectotypes) with catalog number SMNH 7523 (from 35/30 to 15/12 mm, all entire except one specimen opened by a previous investigator, with a vial including the male copulatory system); and seven specimens with catalog number SMNH 1334 (the lectotype 43/25 mm, entire, and six paralectotypes from 20/18 to 15/15 mm, all entire except for one specimen dissected by a previous investigator, with a vial including a male copulatory system). All paralectotypes are Platevindex and the lectotype clearly is an Onchidium . The only one Onchidium specimen was selected as the lectotype because it is the only specimen that Westerlund (1885) illustrated for his new species Vaginulus stuxbergi . In fact, Westerlund’s figures are a perfect image of the lectotype and, most importantly, it is exactly how Onchidium slugs (in the strict sense) look like when they are preserved without relaxation or when they are alive but disturbed.

Type material

( Onchidium nigrum ). Holotype, by monotypy (ZMB 22749). One specimen 40/30 mm, completely dissected (by Plate) and empty. There is a vial with destroyed pieces of the digestive system (mostly the digestive gland and the intestine).

Type material

( Elophilus ajuthiae ). Three syntypes (MNHN 22965) 20/17, 20/15 and 20/14 mm. All three specimens, dissected by Labbé, are now empty. Only a few destroyed pieces of the digestive system remain in a vial.

Additional material examined.

Malaysia, Peninsular Malaysia, Kuala Sepatang, 04° 50.434N, 100° 38.176E, 18 July 2011, 1 specimen (42/24 [DNA 971] mm), leg. B. Dayrat & T. Goulding, [station 27, old forest with tall, old Rhizophora trees, high in the tidal zone (ferns), following boardwalk in educational preserve, reached a creek lower in the tidal zone, with mud] (USMMC 00006); Brunei Darussalam, Mentiri, Jalan Batu Marang, 04° 59.131N, 115° 01.820E, 29 July 2011, 3 specimens (33/18 [#1], 20/16, and 16/13 [DNA 1048] mm), leg. T. Goulding & S. Calloway, [station 36, old mangrove with tall Rhizophora trees with high roots and Thalassina mounds] (BDMNH); Philippines, Bohol, Inabanga, 10°04.255'N, 124°04.416'E, 13 July 2014, 3 specimens (from 30/20 [DNA 3251] to 25/17 mm), leg. J. Comendador, B. Dayrat & T. Goulding, [station 187, mostly Nypa palms with Thalassina mounds] (PNM 041199); Inabanga, 10°04.432'N, 124°04.691'E, 13 July 2014, 1 specimen (27/17 mm), leg. J. Comendador, B. Dayrat & T. Goulding, [station 188, old forest, untouched for about 30 years, mostly Avicennia , many old logs] (PNM 041200); Mabini, 09°51.532'N, 124°31.685'E, 17 July 2014, 1 specimen (35/25 mm), leg. J. Comendador, B. Dayrat & T. Goulding, [station 194, narrow forest on the edge of fish ponds, tall Rhizophora and Avicennia trees, many old logs, muds of different types] (PNM 041201); Mabini, 09°51.402'N, 124°30.982'E, 18 July 2014, 4 specimens (from 35/28 [#1] and 35/22 [#2, DNA 3363] to 12/9 mm), leg. J. Comendador, B. Dayrat & T. Goulding, [station 195, narrow forest with tall trees on the edge of fish ponds, cemented ditches between the mangrove patches and the ponds] (PNM 041202); Vietnam, Can Gio, 10°24.171'N, 106°53.960'E, 10 July 2015, 1 specimen (30/20 [DNA 5602] mm), leg. T. & J. Goulding, [station 221, hard mud by a small road and then a steep bank to the soft, deep mud, Avicennia and some Rhizophora trees spread out] (ITBZC IM 00001); Can Gio, 10°26.703'N, 106°53.694'E, 12 July 2015, 7 specimens (from 55/35 to 25/16 mm; 44/30 [DNA 5605], 35/25 [#1] mm), leg. T. & J. Goulding, [station 223, margin of mangrove by creek, shrimp ponds behind the mangrove, fairly high intertidal] (ITBZC IM 00002); Can Gio, 10°27.620'N, 106°53.316'E, 17 July 2015, 3 specimens (from 28/18 to 12/8 mm), leg. T. & J. Goulding, [station 231, open mangrove with large Avicennia trees, soft mud, some old logs] (ITBZC IM 00003); Can Gio, 10°24.157'N, 106°53.950'E, 19 July 2015, 1 specimen (20/12 mm), leg. T. & J. Goulding, [station 233, hard mud by a small road and then a steep bank to the soft, deep mud, open forest of Avicennia and Rhizophora , rocks and gravel on side of mangrove] (ITBZC IM 00004); China, Macau, 3 specimens (42/28 to 32/28 mm), leg. Heynemann, (SMF 333591/3).

Distribution

(Fig. 2). Malaysia, Sabah (type locality of Vaginulus stuxbergi ); Brunei Darussalam (present study, new record); Malaysia, Peninsular Malaysia (present study, new record); Philippines, Bohol (present study, new record); Vietnam (present study, new record); Thailand (type locality of Elophilus ajuthiae ); China (present study; Sun et al. 2014, one individual misidentified as " Onchidium struma " nomen nudum). A specimen of Onchidium stuxbergi was found in Singapore (in the mangrove by the Mandai River) but was unfortunately lost. The presence of Onchidium stuxbergi was also documented (as Onchidium nigram , which is a spelling mistake) in a guide to the mangroves of Singapore ( Ng and Sivasothi 2002: 115). The type locality of Onchidium nigrum simply was cited as “Borneo,” which could be anywhere on the island in Indonesia (Kalimantan) or Malaysia (Sabah or Sarawak). Our record in Macau is the northernmost (22°10'N) confirmed locality on the coast of southern China.

Habitat

(Fig. 9). The habitat of Onchidium stuxbergi is very similar to that of Onchidium typhae : directly on mud (not soft, i.e. mud that is not very watery), on muddy trunks, old logs, lobster mounds, and even under Nypa leaves (Philippines). However, Onchidium stuxbergi was not observed on mud as soft as the mud on which Onchidium typhae was found in West Bengal.

Abundance.

Onchidium stuxbergi is a rare species. Only one individual was found in Malaysia (where 18 mangrove sites were explored), three individuals at one site in Brunei Darussalam (7 sites), nine individuals at four sites in Bohol (17 sites), and 12 individuals at four sites in Vietnam (19 sites). Even though it will need to be confirmed in the future, it seems that Onchidium stuxbergi tends to be slightly more common (although still rare, overall) in more northern latitudes (Vietnam and Philippines).

Color and morphology of live animals

(Fig. 10). Live animals are not always covered with mud and the color of their dorsum can normally be seen. The dorsum is brown, with no particular pattern. Exceptionally, it can be almost black. The hyponotum varies between grayish and yellowish, and sometimes even greenish. It occasionally bears conspicuous black dots. The foot is bright orange, which is different from the two other species described here. In a tiny specimen (12 mm in length, St. 195/17), the foot was pale yellow. The long ocular tentacles are cream to brown distally, and darker proximally. The head is brown to black, with many evenly distributed white markings. The morphology of live specimens is similar to that of Onchidium typhae . The only difference is that the central papilla with a few dorsal eyes is prominent in Onchidium stuxbergi . Crawling individuals normally measure between 30 and 40 mm in length. Preserved specimens no longer display the distinct color traits seen in live animals. The color of preserved animals is meaningless and uninformative. The background color of the notum is brown. Some individuals, including old ones (SMF 333491) bear a few irregular, darker markings. The hyponotum and the foot of preserved animals are homogenously white.

Internal anatomy

(Figs 11-13). Examples of radular formulae are: 70 × 75-1-75 in USMMC 00006 (42 mm long), 50 × 68-1-68 in BDMNH #1 (33 mm long), and 70 × 80-1-80 in PNM 041202 #1 (35 mm long). The intestine is long, narrow, and of the so-called "type III," characterized by the fact that a transverse line can intersect the intestine eight times.

The oviduct is narrow, short, and straight. The hollow spine of the penial accessory gland is slender and slightly curved. It measures between 0.5 (USMMC 00006) and 1.4 mm (PNM 041202 #2) in length, and between 20 (USMMC 00006) and 35 (PNM 041202 #2) µm in diameter. The diameter of the opening at its tip is nearly 10 µm. The hollow spine does not open directly into the vestibule. Instead, the end of the tube of the accessory gland is a disc which is more or less flat (approximately 0.3 mm in diameter) and does not seem to bear distinct conical papillae. The hollow spine must thus go through that disc in order to be outside and shared with the partner.

The penial sheath is long (to the posterior third of the visceral cavity) and coiled in a few spirals. In less mature individuals, the coils may not be as marked. The retractor muscle is short and inserts into the posterior third of the visceral cavity. There is an additional retractor muscle attaching the anterior portion of the penial sheath to the left wall of the visceral cavity, near the buccal mass. In some individuals, that left additional retractor muscle is very thick and strong. The deferent duct is highly convoluted with many loops, but less so in immature specimens. The penis is elongated, round, narrow, and hollow. Its diameter is less than 200 µm. Its distal part is covered with hooks. When the penis is retracted inside the penial sheath, the hooks are inside the tube-like penis. During copulation, the penis is exerted like a glove and the hooks are then on the outside. Hooks are very densely packed inside the penis, with multiple, irregular rows of an average of 15 hooks around the circumference of the penis. Hooks are conical, slender, sharply pointed, and measure from 40 µm up to 300 µm in length. The longer they are the more slender they are.

Distinctive diagnostic features.

Externally, Onchidium stuxbergi differs from other Onchidium species by the color of the foot, which is bright orange (see below the dichotomous identification key, before the final conclusion). Internally, Onchidium stuxbergi is the only Onchidium species known so far (and the only onchidiid species, for that matter) with a strong, additional retractor muscle attaching the anterior penial sheath to the left, anterior wall of the visceral mass, near the buccal mass.

Remarks.

The status of Onchidium stuxbergi has been problematic from the start because Westerlund unknowingly based his original description on specimens that are part of two distinct species (see Type materials, above): eleven former syntypes (now paralectotypes) are Platevindex and another former syntype (now the lectotype) is an Onchidium (in the strict sense, as defined here). Two years after the original description, Westerlund (1885) again published the description of Vaginulus stuxbergi , as a new species again. Although that contribution is not the original description, its figure 2 helps confirm that Vaginulus stuxbergi is an onchidiid and, most importantly, illustrates the one former syntype (here designated as the lectotype) that is part of Onchidium . Therefore, even though the brief and vague description may be confusing (because it is based on two different species), the illustration makes the identification absolutely clear, hence our decision to designate the illustrated specimen as the lectotype. Note that Westerlund did not describe any internal characters.

However, as a direct consequence of Westerlund’s ambiguous original description and type material, many authors have proposed synonymies between Onchidium stuxbergi and some species names that clearly belong to Platevindex . Those cases are briefly discussed here, but they will be discussed in more detail in our revision of the genus Platevindex . Labbé (1934: 235) and Hoffmann (1928: 88) both regarded Onchidium stuxbergi as a Platevindex (as Oncis stuxbergi ) and suggested that Onchidella condoriana Rochebrune, 1882 and Oncis inspectabilis Plate, 1893, could be synonyms of Onchidium stuxbergi . Onchidium condoriana and Oncis inspectabilis clearly belong to Platevindex (types were examined) and are not synonyms of Onchidium stuxbergi . Hoffmann (1928: 88) also regarded Onchidium coriaceum Semper, 1885, as a synonym of Onchidium stuxbergi but it actually is a valid name of the genus Platevindex . Finally, Hoffmann (1928: 88) suggested that Onchidium ponsonbyi Collinge, 1901, could possibly be a synonym of Onchidium stuxbergi but it is very unlikely because Onchidium ponsonbyi is a terrestrial species known from 850 to 1,050 meters high at Mt Penrissen, Borneo. Onchidium ponsonbyi likely belongs to the genus Semperoncis Starobogatov, 1976.

There is no doubt that Onchidium nigrum , which is only known from the holotype, belongs to the genus Onchidium as re-defined here: the mantle of the preserved holotype bears the typical papillae of Onchidium . Also, Plate described both a rectal gland and an accessory gland, which are found in all three known Onchidium species. Plate did not mention the presence of an additional, left, retractor muscle for the penial sheath. He only mentioned that the insertion of the retractor muscle is of "type II" (i.e., near the pericardium). According to Plate, the penial hooks are from 14 to 87 µm long and the spine of the penial accessory gland is 1.2 mm long. The penial hooks observed here are from 40 to nearly 300 µm in length. It is possible that Plate, who observed only one specimen, could not fully evaluate the variation of penial hooks. Also, penial hooks are extremely challenging to extract and observe without SEM. However, Plate’s description of the penial accessory gland spine is fully compatible with our observations (from 0.5 to 1.2 mm long). Finally, Plate described the intestine loops of Onchidium nigrum as of a unique and exceptional pattern, which he referred to as "type III." The latter, as illustrated by Plate (1893: plate 8, fig. 31a) is slightly more coiled than what was observed for the present study, but they are basically identical patterns. That intestinal pattern is absent from Onchidium typhae but it has also been observed in some species from other genera. Given that Plate did not know the color of the live animal, it will never be known whether it matched the diagnostic color of the foot that was observed for our specimens (bright orange). According to our data, Onchidium stuxbergi is distributed from Malaysia to Vietnam and the Philippines and therefore mostly encompasses Borneo. As a result, the synonymy of Onchidium nigrum and Onchidium stuxbergi is warranted, even though it cannot be completely excluded that Onchidium nigrum could refer to an Onchidium species remotely endemic to the south east of Borneo.

The three syntypes described as Elophilus ajuthiae by Labbé (1935) were earlier identified by him as Onchidium nigrum ( Labbé, 1934). His first identification was supported by a pattern of intestinal loops ( Plate’s "type III") only known from Onchidium nigrum . Labbé changed his mind after the observation of what he thought were tiny dorsal gills ( “microbranchies”) in those three syntypes from Thailand. Indeed, according to Labbé’s (1934) onchidiid classification, dorsal gills are only found in the Dendrobranchiatae (which includes genera such as Peronia , Paraperonia , and Scaphis ) while all other onchidiids (such as Onchidium , Onchidella , Platevindex and Onchidina ), the Abranchiatae, lack dorsal gills. The three specimens from Thailand (with gills) could not belong to Onchidium (no gills) and, as a result, Labbé created a new species name and a new genus name for those specimens with an intestine of "type III" and dorsal gills. Labbé’s new genus Elophilus , preoccupied, was replaced by Starobogatov (1976) by Labella . Those three syntypes from Thailand were re-examined for the present study; unfortunately, they are mostly empty. A few destroyed pieces of the intestine system remain but they are completely useless. However, the mantle clearly does not bear any “microbranchies” (i.e., microgills). It is very likely that Labbé’s first intuition was correct and that he was just looking at large Onchidium papillae retracted within the mantle. Those three specimens from Thailand are part of Onchidium nigrum , which means Onchidium stuxbergi . Unfortunately, Labbé did not describe the male copulatory complex in detail and so the sizes and shapes of the penial accessory spine and of the penial hooks are unknown. However, our specimens from Vietnam suggest that there is only one species of Onchidium distributed in the region of the South China Sea, Onchidium stuxbergi . Naturally, it cannot be excluded that the Gulf of Thailand actually hosts a distinct species; however, there is nothing in Labbé’s description supporting that hypothesis. As a result, the synonymy of Labella ajuthiae with Onchidium stuxbergi is warranted. Also, Labbé was confused about the type locality of Onchidium nigrum because he claimed that “Plate’s unique specimen came from Borneo (Guam)" ( Labbé 1934: 223, our translation) and that “Plate’s unique specimen came from the Marianna Islands" ( Labbé 1935: 312, our translation). Borneo is with no doubt the type locality of Onchidium nigrum . Finally, Labbé’s claim that Labella ajuthiae lived in fresh water was unfounded. Even though the specimens were collected far inland, it was still in brackish water and under the influence of the tides (see above, Type localities).

The name Onchidium struma , introduced by Qiu (1991) and used occasionally in the Chinese literature to refer to some onchidiids from the coast of China (e.g., Shen et al. 2006; Sun et al. 2014), is a nomen nudum (to our knowledge, Onchidium struma has not been formally described as a new species). A survey of the diversity of onchidiids from China based on molecular data was recently published ( Sun et al. 2014). The sequences of the specimens identified as Onchidium " struma " by Sun et al. were all included in our analyses here, and that name appears in two distinct species units (Fig. 1), which demonstrates that the name Onchidium " struma " used by Sun et al. referred to two distinct species. One of their species, identified here as Onchidium reevesii , is mostly subtropical and is distributed from 22°30' to 34°36' latitude north along the coast of China. So far, it seems to be endemic to China. In the data set of Sun et al., Onchidium reevesii is represented by eight individuals (under the name Onchidium " struma "). Note that in that same contribution, Sun et al. (2014) apply the name Paraoncidium reevesii to a different species but that is also a misidentification. The species they refer to as Paraoncidium reevesii obviously cannot be Onchidium reevesii , but it does not belong to Paraoncidium either. The other species referred to as Onchidium " struma ", identified here as Onchidium stuxbergi , is tropical and, in China, is only found in the southernmost coastline. In the data set of Sun et al., only one individual from Hainan Island (19°56'N) can be safely referred to as Onchidium stuxbergi . Another individual in their data set (from Hong Kong, at 22°28'N) is problematic because its CO1 and 16S sequences give contradictory results, and so it is likely that one of those sequences is a mistake. The three specimens examined for the present study from Macau (SMF 333591/3) are the northernmost confirmed locality of Onchidium stuxbergi in China at 22°10'N.