Paromoionchis tumidus Dayrat & Goulding & Khalil & Apte & Bourke & Comendador & Tan 2019, comb. nov.

Paromoionchis tumidus (Semper, 1880) comb. nov.

Figs 7 –25

Onchidium tumidum Semper, 1880: 262 –263, pl. 20, figs 3–4, pl. 23, fig. 4.

Onchidium samarense Semper, 1880: 268 –269, pl. 20, figs 9, 13, pl. 23, fig. 7. Syn. nov.

Onchidium mertoni Simroth, 1918: 294 –296, pl. XX, figs 43–47. Syn. nov.

Onchidium hongkongense Britton, 1984: 188 –190, figs 6–7. Syn. nov.

Onchidium samarense – Semper 1882: 268–269, pl. 21, fig. 5.

Material examined

Type material SINGAPORE • lectotype (here designated; 28/ 22 mm); ZMB 39019a • 15 paralectotypes; ZMB 39019b • 2 paralectotypes; NHMD 300305 • 1 paralectotype; SMF 333603 /1.

AUSTRALIA • 2 paralectotypes; Queensland, Mackay; ZMB 39020.

Other type material

PHILIPPINES • lectotype of Onchidium samarense (here designated; 22/ 17 mm); Samar Island, Palapa harbor; ZMB 39025a • 2 paralectotypes of O. samarense (24/20 and 20/ 15 mm); same locality as lectotype; ZMB 39025b.

INDONESIA • lectotype of Onchidium mertoni (here designated; 15/ 9 mm); Aru Islands, Kobroor, Sungai; 5 Jan. 1908; ZMB 121591 a • 4 paralectotypes of O. mertoni (14/8, 14/10, 15/14 and 14/ 10 mm); same data as for lectotype; ZMB 121591 b.

CHINA • holotype of Onchidium hongkongense (17/ 13 mm); Hong Kong; NHM 1982290 • 15 paratypes; same locality as holotype; NHM 1982291 to 1982292.

Notes on type material

Onchidium tumidum. Lectotype, 28/ 22 mm, designated here (ZMB 39019a). All other syntypes become paralectotypes (the 15 paralectotypes from the same lot are now ZMB 39019b). According to the original description, the type material included 42 specimens from Singapore and an unknown number of specimens from Port Mackay, Queensland, Australia. A total of 21 syntypes were located in museum collections: 19 specimens from Singapore (16 specimens, ZMB 39019; 2 specimens, NHMD 300305; 1 specimen, SMF 333603 /1) and 2 specimens from Mackay (ZMB 39020). There also are two possible syntypes from Australia (ZMH 27480/2). Two similar species of Paromoionchis gen. nov. are found at Port Mackay, P. tumidus and P. daemelii, which anatomically can only be distinguished based on the insertion of the retractor muscle of the penis. In the lectotype designated here from Singapore, the retractor muscle inserts near the heart, exactly as in the species described here. However, in one of the two paralectotypes of P. tumidus from Mackay (ZMB 39020), the retractor muscle is vestigial, as in P. daemelii (in the other paralectotype from Mackay, the male apparatus was destroyed prior to the present investigation and could not be examined). Hence, it was necessary to designate a lectotype from Singapore in order to clarify the application of P. tumidus. Note that the type material was fixed in formalin more than 130 years ago and no DNA sequencing could be attempted.

Onchidium samarense. Lectotype, 22/ 17 mm, designated here (ZMB 39025a). The two other syntypes become paralectotypes (ZMB 39025b). According to the original description, the type material included only two specimens from the same locality in Samar, Philippines. However, the jar with the type material currently contains three similar-looking specimens (syntypes), all of which were dissected prior to the present study. It is not excluded that the original description was based on only two of those three specimens but it is also possible that Semper himself identified all three specimens as O. samerense [sic] (with a minor typo in the original description). The lectotype still contains all its internal organs, including the male copulatory parts. One paralectotype (24/ 20 mm) is mostly destroyed, with no internal organs left except the digestive gland (a few destroyed pieces of organs are in a vial). The other paralectotype (20/ 15 mm) still contains internal organs, but the male parts are missing. Our observations and comments are mostly based on the only specimen with male parts; hence its designation as a lectotype. Note that the type material was fixed in formalin more than 130 years ago and no DNA sequencing could be attempted. Note also that if, in the future, Onchidium samarense were to be regarded as a valid species name in Paromoionchis gen. nov., the specific name samarense (neuter) would need to become samarensis (masculine).

Onchidium mertoni. Lectotype, 15/ 9 mm, designated here (ZMB 121591 a). The four other syntypes become paralectotypes (ZMB 121591 b). Simroth mentioned in the original description that all five specimens were very hard. Indeed, it seems that they dried out at some point and they are very poorly preserved. The lectotype designated here is the only specimen that is complete. It was partially dissected for the present study (the penial hooks, identical to those of O. tumidum, are illustrated here). Two paralectotypes (14/8 and 14/ 10 mm) were dissected prior to the present study and are completely empty. Two other paralectotypes (15/14 and 14/ 10 mm) are in very poor condition (the body is extremely hard and the digestive system is partly outside the body through the foot). A lectotype is designated here to clarify the application of the name O. mertoni because several species of Paromoionchis gen. nov. are potentially sympatric in the Aru Islands and so it cannot be excluded that the five original syntypes belong to different species. Note that the type material was fixed in formalin more than 100 years ago and no DNA sequencing could be attempted.

Onchidium hongkongense. Holotype, 17/ 13 mm, by original designation (NHM 1982290) and 15 paratypes (NHM 1982291, NHM 1982292). The holotype is largely destroyed due to prior dissection, likely by Britton. Large parts of the notum and of the reproductive organs are missing. Even though it is mostly destroyed, the digestive system is confirmed to be of type II. A few paratypes were checked for the present study and their anatomy matches that of the holotype. Note that the type material was fixed in formalin more than 40 years ago and no DNA sequencing could be attempted. Note also that the specific name hongkongensis (masculine or feminine gender) originally used by Britton is corrected to hongkongense (neuter) for gender agreement with Onchidium. Should Onchidium hongkongense ever become a valid species name in Paromoionchis gen. nov., hongkongense would then need to be changed back to hongkongensis.

Other material

AUSTRALIA – New South Wales • 1 spec. (20/15 [1522] mm); Sydney, Pittwater, Careel Bay; 33°37.323´S, 151°19.878´E; 24 Nov. 2011; station 40; supratidal zone on margin of salt marsh, mangrove patch on side of creek; AM C.468918.005 • 1 spec. (35/20 [1529] mm); Sydney, Hawkesbury River, Cheero Point; 33°30.687´S, 151°11.669´E; 25 Nov. 2011; station 42; open mangrove with old logs; AM C.468924.001 • 1 spec. (33/20 [1528] mm); same data as for preceding; AM C.468923.002 • 1 spec. (32/20 [1530] mm); same data as for preceding; AM C.468925.001. – Northern Territory • 1 spec. (45/32 [1634] mm); Darwin, near Channel Island Road; 12°34.979´S, 130°55.992´E; 16 Aug. 2012; station 65; sequence of Sonneratia, Rhizophora and Ceriops; NTM P.57620 • 1 spec. (40/25 [1686] mm); Darwin, end of Channel Island Road; 12°33.557´S, 130°52.894´E; 17 Aug. 2012; station 66; sequence of Sonneratia, Rhizophora and Ceriops; NTM P.57621 • 1 spec. (42/38 [1638] mm); same data as for preceding; NTM P.57623 • 2 spec. (30/17 [1705] and 17/12 [1645] mm); Darwin, close to Tiger Brenan Road (small service road); 12°28.782´S, 130°54.750´E; 19 Aug. 2012; station 69; high tidal Ceriops; NTM P.57622 • 1 spec. (36/22 [1651] mm); Darwin, Elizabeth Road; 12°32.893´S, 130°57.642´E; 20 Aug. 2012; station 70; Ceriops and old logs in Rhizophora forest; NTM P.57624. – Queensland • 1 spec. (45/30 [2562] mm); Cairns, Yorkey’s Knob; 16°48.558´S, 145°42.768´E; 17 Jun. 2013; station 101; hard, red mud with grasses; MTQ • 1 spec. (45/30 [2602] mm); Townsville, Magnetic Island; 19°09.938´S, 146°49.029´E; 24 Jun. 2013; station 109; water on the mud; MTQ • 1 spec. (15/10 [2627] mm); Bowen; 20°00.658´S, 148°15.878´E; 1 Jul. 2013; station 115; back of mangrove across from beach, dense Rhizophora, Avicennia trees with soft mud around; MTQ • 1 spec. (55/30 [2637] mm); Bowen; 20°00.913´S, 148°15.745´E; 1 Jul. 2013; station 116; mangrove away from ocean, small area of open Avicennia mangrove, surrounded by Rhizophora; MTQ • 1 spec. (20/10 [2652] mm); Bowen, Doughty Creek; 20°01.264´S, 148°14.345´E; 2 Jul. 2013; station 117; narrow Avicennia and Rhizophora mangrove, by creek, some muddy areas and some very sandy; MTQ • 1 spec. (35/20 [2657] mm); Bowen; 20°01.478´S, 148°14.224´E; 3 Jul. 2013; station 119; Rhizophora and Avicennia mangrove; MTQ • 1 spec. (30/20 [2701] mm); Mackay; 20°08.511´S, 149°12.076´E; 8 Jul. 2013; station 125; large, dense and sandy mangrove and, by side of river, small strip of mud with Avicennia and Rhizophora; MTQ • 1 spec. (30/25 [1531] mm); Thirsty Sound, Plum Tree, beach in front of Endeavour Park; 22°08.144´S, 150°01.856´E; 14 Sep. 2002; I. Loch, D.L. Beechey and A.C. Miller leg.; sheltered, muddy cobble shore; AM C.575588.

BRUNEI DARUSSALAM • 3 spec. (55/30 [1036], 35/20 [1035] and 20/15 [1062] mm); Pulau Pyatan, Teluk Brunei; 04°55.246´N, 115°02.764´E; 27 Jul. 2011; station 32; open Avicennia and Rhizophora mangrove, with hard mud; BDMNH.

INDIA • 1 spec. (26/17 [1119] mm); Andaman Islands, Middle Andaman, Shantipur, Kadamtala; 12°19.843´N, 092°46.377´E; 12 Jan. 2011; station 58; open area with hard mud and many old logs, next to a mangrove with medium trees; BNHS 88.

INDONESIA – Sumatra • 1 spec. (24/15 [1732] mm); Kualapenet; 05°16.275´S, 105°51.287´E; 17 Oct. 2012; station 77; narrow band of mangrove between ocean and fish ponds; UMIZ 0 0 121 • 2 spec. (35/20 [1754] and 26/16 [1755] mm); Bakauheni; 05°50.560´S, 105°46.200´E; 21 Oct. 2012; station 81; small mangrove, not far from road and next to large harbor, very impacted mangrove; UMIZ 0 0 122 • 1 spec. (38/30 [1794] mm); same data as for preceding; UMIZ 0 0 138 • 1 spec. (20/12 [1798] mm); S of Bandar Lampung; 05°32.66´S, 105°15.113´E; 28 Oct. 2012; station 83; high intertidal, fairly dense roots with some Avicennia and Nypa, edge of mangrove by road; UMIZ 0 0 123. – Sulawesi • 2 spec. (25/15 [2200] and 20/12 [2201] mm); Tamperong; 01°41.513´N, 125°00.797´E; 12 Mar. 2013; station 87; muddy mangrove with small Rhizophora in dense patches; UMIZ 0 0 124 • 1 spec. (27/15 [2240] mm); Sondaken; 01°21.777´N, 124°32.594´E; 13 Mar. 2013; station 89; sand, small rocks, pieces of wood outside narrow coastal mangrove of mostly Rhizophora; UMIZ 0 0 125 • 1 spec. (30/20 [2345] mm); Makassar, Tallo mangrove; 05°06.117´S, 119°26.777´E; 21 Mar. 2013; station 92; small mangrove used as outhouse by village, very impacted with trash; UMIZ 0 0 126 • 1 spec. (20/13 [2355] mm); Barru; 04°25.437´S, 119°35.953´E; 22 Mar. 2013; station 93; forest of mostly Avicennia and Rhizophora, with hard and sandy mud; UMIZ 0 0 127. – Ambon • 1 spec. (25/15 [3541] mm); Lateri; 03°38.261´S, 128°14.716´E; 12 Feb. 2014; station 128; mudflat next to small creek in low intertidal of mangrove preserve; UMIZ 0 0 128 • 3 spec. (45/30 [2832], 22/12 [2839] and 35/22 [2840] mm); Lateri; 03°38.237´S, 128°14.783´E; 14 Feb. 2014; station 131; muddy mangrove with Rhizophora; UMIZ 0 0 129. – Seram • 2 spec. (20/15 [2874] and 30/15 [2875] mm); Kawa; 02°58.240´S, 128°07.066´E; 18 Feb. 2014; station 135; mud next to a seawall adjacent to a mangrove; UMIZ 0 0 130. – Lombok • 2 spec. (30/20 [2950] and 20/12 [2952] mm); Tanjung Batu village; 08°45.748´S, 116°02.892´E; 24 Mar. 2014; station 145; Avicennia forest; UMIZ 0 0 131 • 1 spec. (20/14 [2961] mm); Seriwe Bay; 08°51.960´S, 116°32.838´E; 25 Mar. 2014; station 146; Avicennia mangrove with hard mud and rocks; UMIZ 0 0 132 • 1 spec. (18/10 [2960] mm); same data as for preceding; UMIZ 0 0 139. – Bali • 1 spec. (25/14 [3051] mm); Denpasar; 08°47.435´S, 115°13.197´E; 1 Apr. 2014; station 153; large mangrove by road, very soft mud; UMIZ 0 0 133 • 1 spec. (30/20 [3070] mm); Denpasar; 08°46.126´S, 115°10.803´E; 2 Apr. 2014; station 154; large mangrove by road, with shallow mud; UMIZ 0 0 134. – Halmahera • 1 spec. (25/15 [5082] mm); Akelamo; 01°01.329´N, 127°39.091´E; 10 Mar. 2015; station 207; sandymuddy beach at margin of mangrove near village; UMIZ 0 0 135 • 2 spec. (50/35 [5102] and 55/35 [5103] mm); Buli; 00°55.446´N, 128°20.612´E; 16 Mar. 2015; station 212; logged area in front of old Rhizophora forest, by the road; UMIZ 0 0 136 • 1 spec. (55/30 [5042] mm); Buli; 00°55.367´N, 128°20.647´E; 17 Mar. 2015; station 213; tall and old Rhizophora forests, high intertidal; UMIZ 0 0 137.

JAPAN • 1 spec. (22/10 [3761] mm); Ehime Prefecture, Misho Bay; 32°57.634´N, 132°33.205´E; 4 Aug. 2014; station o28; mudflats; NSMT Mo 78984.

MALAYSIA • 1 spec. (40/25 [963] mm); Peninsular Malaysia, Nibong Tebal, Pulau Burung; 05°12.488´N, 100°25.564´E; 11 Jul. 2011; station 17; soft mud, open mangrove of Rhizophora, with a few Sonneratia; USMMC 0 0 0 57 • 1 spec. (18/12 [928] mm); E Peninsular Malaysia, Balok; 03°53.219´N, 103°21.978´E; 14 Jul. 2011; station 19; mostly Rhizophora, with some Avicennia, hard mud with shallow pools, patches of soft mud; USMMC 0 0 0 58.

PAPUA NEW GUINEA • 1 spec. (18/14 [5432] mm); Madang, Meiro River, near airport; 05°12.2´S, 145°47.4´E; 5 Nov. 2012; MNHN expedition Papua Niugini leg.; station PM01; Nypa palm swamp; MNHN IM-2013-10478 • 1 spec. (17/12 [5433] mm); same data as for preceding; MNHN IM-2013- 10479.

PHILIPPINES – Luzon • 1 spec. (26/18 [3610] mm); Lian, Batangas; 13°58.130´N, 120°37.471´E; 5 Jul. 2014; station 179; narrow and impacted mangrove of Avicennia near village, very sandy, little to no mud; PNM 0 41261 • 2 spec. (30/20 [3171] and 25/15 [3192] mm); Nasugbu, Batangas; 14°10.714´N, 120°36.817´E; 6 Jul. 2014; station 182; near village, well-preserved and dense forest of Avicennia and Rhizophora; PNM 0 41255 • 1 spec. (22/14 [3172] mm); same data as for preceding; PNM 0 41262 • 2 spec. (30/20 [3200] and 35/22 [3205] mm); Calantagan, Batangas; 13°55.319´N, 120°37.260´E; 7 Jul. 2014; station 183; rocks next to Avicennia and Rhizophora forest; PNM 0 41256 • 1 spec. (30/18 [3202] mm); same data as for preceding; PNM 0 41263 • 1 spec. (35/22 [3222] mm); Calantagan, Batangas; 13°53.278´N, 120°37.124´E; 8 Jul. 2014; station 184; narrow forest on the shore, Avicennia and young Rhizophora; PNM 0 41257 • 1 spec. (33/15 [3229] mm); same data as for preceding; PNM 0 41264 • 1 spec. (27/18 [3237] mm); Calantagan, Batangas; 13°51.264´N, 120°37.383´E; 8 Jul. 2014; station 185; next to village, impacted, narrow Avicennia mangrove by the shore; PNM 0 41265. – Bohol • 1 spec. (40/25 [3344] mm); Mabini; 09°51.532´N, 124°31.685´E; 17 Jul. 2014; station 194; narrow mangrove on edge of fish ponds, tall Rhizophora and Avicennia trees, many old logs; PNM 0 41258 • 1 spec. (26/15 [3371] mm); Mabini; 09°51.586´N, 124°34.155´E; 18 Jul. 2014; station 196; Avicennia and Sonneratia open forest with sand, algae and coral rubble; PNM 0 41259 • 1 spec. (30/18 [3416] mm); Maribojoc; 09°44.280´N, 123°49.389´E; 20 Jul. 2014; station 202; uplifted coral rubble with sand and algae, near Sonneratia trees; PNM 0 41260.

VIETNAM • 2 spec. (45/30 [5619] and 40/30 [5682] mm); Nha Trang; 12°12.778´N, 109°09.572´E; 27 Jul. 2015; station 237; small strip of mud with a few Rhizophora trees next to a small river, by fish ponds and houses; ITBZC IM 0 0 0 19 • 1 spec. (25/15 [5642] mm); Nha Trang; 12°24.168´N, 109°10.058´E; 29 Jul. 2015; station 239; mostly Avicennia and some small Rhizophora, with shallow mud; ITBZC IM 0 0 0 20.

Color and morphology of live animals (Figs 7–10)

Live animals of units #1 and #2 are often abundantly covered with mud, in which case their dorsal color can hardly be seen. The background of the dorsal notum is brown, light to dark. That background can be homogenous or clearly mottled with darker or lighter areas and, occasionally, also with red areas. In addition, in some animals the tip of the dorsal papillae (with and without dorsal eyes) can be bright yellow. The foot varies from gray (light or dark) to yellow or orange. The hyponotum is almost always yellow, from pale yellow to bright yellow and even orange. This variable yellow component can cover the entire hyponotum or just an outer ring (the inner ring being light to dark gray). The color of the foot and of the hyponotum of an individual can change rapidly, especially when disturbed. The ocular tentacles are brown (variable from light to dark) and may or may not be speckled with tiny white dots, exactly like the head. The ocular tentacles are short (just a few millimeters long).

No live pictures were available for unit #3, so the following description is based on preserved specimens (Fig. 10). It is possible that bright colors (yellow, orange) were lost during preservation on both the ventral and dorsal sides. The background of the dorsal notum is brown, mottled with darker or lighter areas. The foot is light gray. The hyponotum is gray-brown, with a reddish hue on the margin (which could possibly be orange in live animals). The color of the ocular tentacles (retracted, likely short) cannot be determined.

Generally speaking, the dorsal notum of any given live animal can rapidly change from almost perfectly smooth to covered by many papillae. However, when animals are not disturbed, the dorsum is usually covered by papillae of various sizes. In some animals, larger papillae may be arranged in two longitudinal and lateral ridges (on either side of the median line), but those ridges can appear and disappear rapidly. Some papillae bear from two to four black dorsal eyes at their tip (most papillae bear three eyes). The number of papillae with dorsal eyes is variable (between 10 and 15, on average) and they mostly are on the central part of the notum. Their tip is usually yellow, but not always. A central, much larger papilla, which bears four dorsal eyes (sometimes three), is entirely retractable within the notum. In addition to all these large papillae, the notum is covered by smaller, rounded papillae, which can make it look granular.

External morphology (Fig. 11 A–B)

Preserved specimens no longer display the color of live animals. The body is not flattened (although, exceptionally, animals on mud with a thin layer of water can look flattened). The notum is oval. Dorsal gills are absent. The large, central, retractable papilla at the center of the notum can only be seen in live animals. In preserved specimens, it is retracted inside the notum. The hyponotum is horizontal. The width of the hyponotum relative to the total width of the ventral surface (pedal sole and hyponotum) varies among individuals, from approximately one third to half. In the anterior region, the left and right ocular tentacles are superior to the mouth. Eyes are located at the tip of the two ocular tentacles. Inferior to the ocular tentacles, superior to the mouth, the head bears a pair of oral lobes. The latter are smooth, with no transversal protuberance. The male opening (of the copulatory complex) is below and to the left of the right ocular tentacle (i.e., between the two ocular tentacles). The anus is posterior, median, close to the edge of the pedal sole. On the right side (to the left in ventral view), a peripodial groove is present at the junction between the foot and the hyponotum, running longitudinally from the buccal area to the posterior end, a few millimeters from the anus and the pneumostome. The pneumostome is median. Its position on the hyponotum relative to the notum margin and the edge of the pedal sole varies among individuals but averages in the middle. The position of the female pore (at the posterior end of the peripodial groove) does not vary much among individuals.

Visceral cavity and pallial complex

The anterior pedal gland is oval and flattened, lying free on the floor of the visceral cavity below the buccal mass. The heart, enclosed in the pericardium, is on the right side of the visceral cavity, slightly posterior to the middle. From the anterior ventricle an anterior vessel exits that supports several anterior organs such as the buccal mass, the nervous system and the copulatory complex. The auricle is posterior. The kidney is more or less symmetrical, the right and left parts being equally developed. The kidney is intricately attached to the respiratory complex. The lung is in two more or less symmetrical parts, left and right.

Digestive system (Figs 12–15)

There are no jaws. The left and right salivary glands, heavily branched, join the buccal mass dorsally, on either side of the esophagus. The radula is in between two large postero-lateral muscular masses. Radulae measure up to 5.2 mm in length (unit #1), 4.1 mm (unit #2) and 2.8 mm (unit #3). Each radular row contains a rachidian tooth and two half rows of lateral teeth of similar size and shape. Examples of radular formulae are presented in Table 4. The rachidian teeth are unicuspid: the median cusp is always present; there are no conspicuous cusps on the lateral sides of the base of the rachidian tooth. The length of the rachidian teeth (approximately 25 µm) tend to be approximately half the size of the lateral teeth (approximately 50 µm). The lateral aspect of the base of the rachidian teeth is straight, occasionally slightly convex. The half rows of lateral teeth form an angle of 45° with the rachidian axis. With the exception of the few innermost and few outermost lateral teeth, the size and shape of the lateral teeth do not vary along the half row, nor do they vary among half rows. The lateral teeth seem to be unicuspid with a flattened and curved hook (approximately 50 µm long) with a rounded tip, but there is also a pointed spine on the outer lateral expansion of the base (basal lateral spine). In most cases, the basal lateral spine cannot be observed because it is hidden below the hook of the next, outer lateral tooth. It can only be observed when the teeth are not too close (such as in the innermost and outermost regions) or when teeth are placed in an unusual position. The inner and outer lateral aspects of the hook of the lateral teeth are straight (i.e., not wavy and not with any protuberance).

The esophagus is narrow and straight, with thin internal folds. The esophagus enters the stomach anteriorly. Only a portion of the posterior aspect of the stomach can be seen in dorsal view because it is partly covered by the lobes of the digestive gland. The dorsal lobe is mainly on the right. The left, lateral lobe is mainly ventral. The posterior lobe covers the posterior aspect of the stomach. The stomach is a U-shaped sac divided into four chambers. The first chamber, which receives the esophagus, is delimited by thin tissue and receives the ducts of the dorsal and lateral lobes of the digestive gland. The second, posterior chamber, delimited by thick muscular tissue, receives the duct of the posterior lobe of the digestive gland. The third, funnel-shaped chamber is delimited by thin tissue with high ridges internally. The fourth chamber is continuous and externally similar to the third, but it bears only low, thin ridges internally. The intestine is long and narrow and the intestinal loops are of type II. There is no rectal gland.

Nervous system (Fig. 11C)

The circum-esophageal nerve ring is post-pharyngeal and pre-esophageal. The paired cerebral ganglia are close and the cerebral commissure is short (but its length does vary among individuals). Paired pleural and pedal ganglia are also all distinct. The visceral commissure is short but distinctly present and the visceral ganglion is more or less median. Cerebro-pleural and pleuro-pedal connectives are short and pleural and cerebral ganglia touch each other on either side. Nerves from the cerebral ganglia innervate the buccal area and the ocular tentacles and, on the right side, the penial complex. Nerves from the pedal ganglia innervate the foot. Nerves from the pleural ganglia innervate the lateral and dorsal regions of the mantle. Nerves from the visceral ganglia innervate the visceral organs.

Reproductive system (Figs 16 –23)

Sexual maturity is correlated with animal length. Mature individuals have large female organs (with a large female gland mass) and fully-developed male copulatory parts. Immature individuals (<15 mm long) may have inconspicuous (or no) female organs and rudimentary anterior male parts.

The female organs are located at the posterior end of the visceral cavity, mixed with some male parts (Figs 16A, 17 A–B). The hermaphroditic gland is a single mass, joining the spermoviduct through the hermaphroditic duct (which conveys the eggs and the autosperm). There is a narrow and bent receptaculum seminalis (caecum) along the hermaphroditic duct. The female gland mass contains various glands (mucus and albumen) which can hardly be separated by dissection and of which the exact connections remain uncertain. The hermaphroditic duct becomes the spermoviduct (which conveys eggs, exosperm and autosperm). Proximally, the spermoviduct is not divided (at least externally) and is embedded within the female gland mass. Distally, the spermoviduct branches into the deferent duct (which conveys the autosperm up to the anterior region, running through the body wall) and the oviduct. The free oviduct conveys the eggs up to the female opening and the exosperm from the female opening up to the fertilization chamber. The large, ovate-spherical spermatheca connects to the oviduct through a narrow and short duct. The oviduct is narrow and straight. There is no vaginal gland.

The male anterior organs consist of the penial complex (penis, penial sheath, vestibule, deferent duct, retractor muscle) and the accessory penial gland (Figs 16 B–C, 17C–D). The penial complex and the accessory penial gland share the same vestibule and the same anterior male opening. The penial gland is a long, tube-like flagellum with a proximal dead end. The length of the flagellum of the penial gland varies among individuals but it is always heavily coiled. Near its distal end (just before the hollow spine), the flagellum is enlarged into a thick muscular sac. Distally, the flagellum ends in a hard, hollow spine protected by a sheath which opens into the vestibule. The hollow spine is narrow, elongated and slightly curved (Figs 18–20). Its base is conical. Its diameter is between 60 and 100 μm. The diameter of the opening at its tip measures between 30 and 60 μm. Its length ranges from 1 mm ([1634] NTM P.57620) to 2 mm ([5619] ITBZC IM 0 0 0 19, [5102] UMIZ 00136) for unit #1, from 1.2 mm ([1638] NTM P.57623) to 1.8 mm ([3237] PNM 0 41265, [3172] PNM 041262) for unit #2 and from 0.8 mm ([5433] MNHN IM-2013-10479) to 1 mm ([5432] MNHN IM-2013-10478) for unit #3, and its shape does vary between individuals (Figs 18–20). There is no disc separating the spine of the penial gland and the vestibule.

The penial sheath is narrow and elongated (Figs 16 B–C, 17C–D). The penial sheath protects the penis for its entire length. The beginning of the retractor muscle marks the separation between the penial sheath (and the penis inside) and the deferent duct. The retractor muscle is shorter than the penial sheath and inserts on the wall of the body cavity, near the heart. The deferent duct is also highly convoluted, with many loops. Inside the penial sheath, the penis is a narrow, elongated, soft, hollow tube of approximately 200 μm in diameter. Inside the tube-like penis, six longitudinal ridges bear sparse, tiny, conical (but not pointed) hooks which are less than 20 μm long in unit #1, less than 22 μm long in unit #2 and less than 28 μm in unit #3 (Figs 21–23). When the penis is retracted inside the penial sheath, the hooks are inside the tube-like penis; during copulation, the penis is evaginated like a glove and the hooks are outside.

Distinctive diagnostic features

Externally, Paromoionchis tumidus (Semper, 1880) cannot be distinguished from other species of Paromoionchis gen. nov. Internally, the presence of penial hooks distinguishes it from other species of the genus (Table 3).

Distribution (Fig. 6)

All records here are new, except for the type localities.

Unit #1. Australia: New South Wales, Northern Territory, Queensland. Brunei Darussalam. Hong Kong (type locality of Onchidium hongkongense). India: Andaman Islands. Indonesia: Ambon, Aru Islands, Bali, Halmahera, Lombok, Seram, Sulawesi, Sumatra. Japan. Malaysia: Peninsular Malaysia. Singapore (type locality of Onchidium tumidum). Philippines: Bohol, Luzon, Samar (type locality of Onchidium samarense). Vietnam.The southernmost locality is in Sydney, New South Wales, Australia (33°37.323´S) and the northernmost locality is in Misho Bay, Ehime Prefecture, Japan (32°57.634´N).

Unit #2. Australia: Northern Territory. Indonesia: Sumatra, Lombok. Philippines: Luzon.

Unit #3. Papua New Guinea: Madang.

Habitat (Figs 24–25)

Paromoionchis tumidus unit #1 is predominantly found on mud, hard or soft, inside or near mangroves, or on mudflats (Fig. 24). It is also found on old, muddy logs, inside or near mangroves. It occasionally is found on muddy sand, or even rocks and coral rubble, usually in the proximity of some mangrove trees. It is not found on rocky shores. Paromoionchis tumidus unit #2 is found in mangroves, mostly on mud and occasionally on sand (Fig. 25). Paromoionchis tumidus unit #3 is found in Nypa palm swamps and seems rare (only two specimens are known).

Paromoionchis tumidus is very common across its entire distribution range. It is by far the most abundant species of Paromoionchis gen. nov. and arguably the most abundant onchidiid species in the Indo-West Pacific. Most individuals of P. tumidus are part of unit #1, because P. tumidus unit #2 is rare across its entire distribution (it is only known from a total of nine specimens collected at nine stations) and P. tumidus unit #3 is restricted to two individuals from Papua New Guinea.

Remarks

The publication dates of the various sections of the volume on Landmollusken by Carl Semper in the Reisen im Archipel der Philippinen series were clarified by Johnson (1969). The species name Onchidium tumidum was published by Semper with a complete description (text and figures) in 1880.

The anatomy of the species described here is fully compatible with Semper’s original description of Onchidium tumidum as well as our own observation of the lectotype (and paralectotypes) from Singapore (Table 3). The most important characters are the lack of a rectal gland, a digestive system of type II, an accessory penial gland, a retractor muscle of the penis inserting near the heart, a male opening between the two eye tentacles (not just below the right eye tentacle), and a penis with hooks (Figs 12H, 21D). According to Semper (1880: 263, our translation), the male opening is “almost exactly midway between the two [eye tentacles],” but it actually is closer to the right tentacle. Also, Semper (1880: 263, our translation) described a penis with an “anterior tooth-bearing portion [which] is reduced, namely at most 2 mm long.” It is confirmed here with SEM (Fig. 21D) that the penis of the lectotype bears tiny hooks (<20 μm) and is fully compatible with the species described here.

The lectotype of O. mertoni is anatomically identical to the species described here, P. tumidus. Simroth did not describe the internal anatomy of O. mertoni, but a description of its lectotype is provided here. Simroth mentioned a male aperture below the right ocular tentacle, but it clearly is to the left of the right tentacle (Fig. 11A); the intestinal loops are of type II; the male apparatus includes an accessory penial gland; the penial retractor muscle inserts to the body wall near the heart; the penis bears tiny hooks which are identical to the hooks of the species described here (Fig. 21C).

The type material of O. hongkongense is anatomically identical to the species described here and it is characterized by the exact same combination of characters: no rectal gland, intestinal loops of type II (Fig. 12D), an accessory penial gland, a retractor muscle inserting near the heart, a male opening between the two eye tentacles and a penis with tiny hooks (which Britton illustrated and measured as <20 μm).

Strictly speaking, Onchidium samarense probably should be regarded as a nomen dubium because 1) the jar with the type material includes three specimens while Semper only mentioned two specimens in the original description, 2) the male organs of two of the syntypes are gone (possibly dissected by Semper) and cannot be checked, and 3) important features (e.g., the insertion of the retractor muscle) are not mentioned in the original description. However, our observations are compatible with the original description and it is possible that all three specimens were actually identified as O. samarense by Semper himself. According to Semper (1882: 269, our translation), the penis of O. samarense is “very similar to that of O. tumidum.” Indeed, the male apparatus of the lectotype is very similar to that of the species described here. In particular, the retractor muscle of the penis (not described by Semper) is thin but reaches the heart, following some nerves, as in the species described here. Semper described no ʻcartilaginous teeth.’ However, that can be easily explained by the fact that the hooks inside the penis of O. tumidum are soft and tiny (<20 μm) and, unlike the large and solid hooks found in some other onchidiids, can hardly be seen under a light microscope (in fact, because there are not many hooks, they are hard to find even with SEM). Our collections currently do not include any specimen from Samar, the type locality of O. samarense, but we did collect many species of onchidiids in Luzon, just next to Samar, as well as in Bohol, a bit further south in the Philippines. Given the anatomy of O. samarense (no rectal gland, digestive system of type II, male opening clearly on the left of the right eye tentacle, accessory penial gland and retractor muscle inserting near the heart), it is most likely that O. samarense applies to the same species as O. tumidum. Because its nomenclatural status still remains problematic (it could be regarded as a nomen dubium) and because its written description was published in 1882 and not in 1880, O. samarense is regarded as a junior synonym of O. tumidum.

Plate (1893) identified five specimens from Ponape (now Pohnpei, Micronesia) and one specimen from Singapore as Onchidium tumidum. Given that their intestinal loops were of type I, the specimens from Ponape were misidentified. It is impossible to determine whether the specimen from Singapore (with intestinal loops of type II) actually belongs to P. tumidus. Bretnall (1919) listed previous records but did not examine any new material. Bretnall also suggested that Onchidium punctatum Quoy & Gaimard, 1832 could possibly refer to the same species as O. tumidum. However, O. punctatum clearly belongs to the genus Peronia. In fact, it was transferred to Scaphis Labbé, 1934 by Labbé (1934a: 203) as Scaphis punctata and Scaphis is a junior synonym of Peronia. Hoffmann (1928) mentioned O. tumidum, O. samarense and O. mertoni but did not examine any new material. Hoffmann also suggested that the record of Onchidium tabularis (Tapparone-Canefri, 1883), listed by Boettger (1923) from the Aru and Kei Islands, was a misidentification for O. mertoni (Boettger provided a record but did not describe any specimens). However, Hoffmann’s claim cannot be checked because the application of Onchidella tabularis Tapparone-Canefri, 1883 (as Oncidiella) is very unclear (the type material could not be located and the original description is not informative).

Three onchidiid sequences from mainland China were obtained from GenBank, the only ones that are not new in our data set (Table 1). These sequences were misidentified as Paraoncidium reevesii (J.E. Gray, 1850). Paraoncidium Labbé, 1934 is not a valid name: it is a junior synonym of Onchidina Semper, 1882. Also, Onchidium reevesi (J.E. Gray, 1850) is actually one of the three valid species of Onchidium (Dayrat et al. 2016).