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Research Article
Halechiniscidae (Heterotardigrada, Arthrotardigrada) of Oura Bay, Okinawajima, Ryukyu Islands, with descriptions of three new species
expand article infoShinta Fujimoto
‡ Kyoto University, Kyoto, Japan
Open Access

Abstract

Marine tardigrades of the family Halechiniscidae (Heterotardigrada: Arthrotardigrada) are reported from Oura Bay, Okinawajima, one of the Ryukyu Islands, Japan, including Dipodarctus sp., Florarctus wunai sp. n., Halechiniscus churakaagii sp. n., Halechiniscus yanakaagii sp. n. and Styraconyx sp. The attributes distinguishing Florarctus wunai sp. n. from its congeners is a combination of two characters, the smooth dorsal cuticle and two small projections of the caudal alae caestus. Halechiniscus churakaagii sp. n. is differentiated from its congeners by the combination of two characters, the robust cephalic cirrophores and the scapular processes with flat oval tips, while Halechiniscus yanakaagii sp. n. can be identified by the laterally protruded arched double processes with acute tips situated dorsally at the level of leg I. A list of marine tardigrades reported from the Ryukyu Islands is provided.

Keywords

Meiobenthos, northwestern Pacific, subtidal, subtropic, Tardigrada, taxonomy

Introduction

Halechiniscidae (Heterotardigrada: Arthrotardigrada) is a group of unarmoured marine tardigrades possessing cephalic appendages, including the median cirrus, and legs with four digits terminating in distal claws. More than half of the described marine species are assigned to this family, which comprises 29 genera in seven subfamilies.

In January 2014, the first Umisawa-kai (Field Workshop for Young Marine Biologists) was held to survey the invertebrate fauna of Oura Bay, Okinawajima, one of the Ryukyu Islands, Japan. During this survey, the following five species of Halechiniscidae were encountered: Dipodarctus sp., Florarctus wunai sp. n., Halechiniscus churakaagii sp. n., Halechiniscus yanakaagii sp. n. and Styraconyx sp.

Materials and methods

Specimens were found in five sediment samples (each sample was approximately 1L in volume) collected from Oura Bay, Okinawajima, one of the Ryukyu Islands, Japan by SCUBA diving. The geographical coordinates, water depth, sediment type and date of collection are listed for each sediment sample in Table 1.

Sediment samples collected from Oura Bay, Okinawajima, Ryukyu Islands.

Sediment sample number Dive site Geographical coordinates of dive site Water depth Sediment type Date Species collected (Number of specimens)
1 Off Sedake 26°32'39.06"N, 128°2'52.8"E 6 m Coarse sand 24th Jan. 2014 Halechiniscus churakaagii sp. n. (1)
2 Off Camp Schwab 26°31'51.78"N, 128°3'10.74"E 9 m Coarse sand 25th Jan. 2014 Florarctus wunai sp. n. (2)
3 Off Camp Schwab 26°31'51.78"N, 128°3'10.74"E 6 m Coarse sand 25th Jan. 2014 F. wunai sp. n. (2)
4 Off Futami 26°32'42.47"N, 128°2'26.34"E 6 m Muddy sand 27th Jan. 2014 Dipodarctus sp. (2)
H. churakaagii sp. n. (1)
Styraconyx sp. (2)
5 Off Thima 26°32'0.81"N, 128°3'49.61"E 6 m Coarse sand 28th Jan. 2014 F. wunai sp. n. (1)
H. yanakaagii sp. n. (1)

The samples were freshwater-shocked (Kristensen 1983), sieved through a 32-μm-mesh net and fixed in 3% formaldehyde. To extract specimens from the remaining sediment the fixed samples were treated using a modified density separation method from Burgess (2001). The sample was rinsed with distilled water to remove formaldehyde. Subsequently, the sample was thoroughly mixed with distilled water-diluted LUDOX® HS-40 colloidal silica (density slightly above 1.15 g cm-3) before allowing the sediment to settle (for at least 15 minutes). The supernatant was sieved through a 32-μm-mesh net to collect the specimens, and the procedure repeated three times per sample. The specimens were sorted under a stereomicroscope before being mounted in glycerol and observed under a phase-contrast microscope (Olympus BX53). The terminology for the genus Florarctus follows Hansen (2011).

Systematics

Order ARTHROTARDIGRADA Marcus, 1927
Family Halechiniscidae Thulin, 1928

Subfamily Dipodarctinae Pollock, 1995
Genus Dipodarctus Pollock, 1995

Dipodarctus sp.

Fig. 1

Material examined

Two four-clawed juveniles found in sediment sample 4 (Table 1).

Remarks

The species resembles Dipodarctus borrori Pollock, 1995 and D. susannae Jørgensen, Boesgaard, Møbjerg & Kristensen, 2014 by having digits of unequal length on legs I–III and the lack of lateral processes between legs III and IV. It is distinguished from the two species by the lateral cirrus lack of scapus, which is present in the both D. borrori and D. susannae. It is also distinguished from D. borrori by the shorter digit 1 of legs I–III and from D. susannae by its shorter papillate leg IV sense organ. These observations are based on comparing juveniles with descriptions of adults so while this species is probably an undescribed species observation of an adult specimen is required for confirmation.

Figure 1.

Phase contrast micrograph of Dipodarctus sp. Scale bar = 20 μm.

Subfamily Florarctinae Renaud-Mornant, 1982
Genus Florarctus Delamare, Deboutteville & Renaud-Mornant, 1965

Florarctus wunai sp. n.

Figs 2, 3, Table 2

Diagnosis

Florarctus with smooth dorsal cuticle; six alae with continuous caestus; frontal ala with no caestus; antero-lateral alae caestus with small projection at levels of leg I and II and developed projection at posterior end; postero-lateral alae caestus with parallelogram-shaped projection at posterior end; caudal ala divided into four lobes; caudal alae caestus with pair of small projections; longitudinally elongate secondary clava with internally directed weak swelling.

Material examined

Holotype: KUZ Z705: adult female found in sediment sample 3 (Table 1).

Paratypes: KUZ Z706: adult female found in sediment sample 2 (Table 1); KUZ Z707: adult male found with holotype; KUZ Z708: four-clawed specimen of undetermined status found from sediment sample 5 (Table 1); KUZ Z709: four clawed juvenile found from sediment sample 2 (Table 1).

Type locality

Water depth of 6 m, off Camp Schwab, Oura Bay, Okinawajima, one of the Ryukyu Islands, Japan (26°31'51.78"N, 128°3'10.74"E). Collected by the author on 25th January 2014.

Type depository

The type series is deposited in the Zoological Collection of Kyoto University (KUZ).

Description of holotype

Adult female, body length: 257 μm, excluding alae (Fig. 2A, 3A). Cephalic region clearly separated from trunk. Dorsal surface smooth with folds. Ventral surface smooth. Lateral margin of body surrounded by aliform expansions with continuous caestus, which consists of frontal ala, pair of antero-lateral alae, pair of postero-lateral alae and caudal ala. Frontal ala spreads across entire anterior margin of cephalic region. Scapi of internal cirri continuous with ala. Base of lateral cirri and primary clavae enveloped together in ala. Antero-lateral ala spreads from approximately level with median cirrus to level of leg III with four slight indentations. Antero-lateral alae caestus with small projections at level of leg I and leg II and developed projection at posterior end. Slightly overlapping antero-lateral ala, postero-lateral ala spreads from level of leg III to level of cirrus E with two indentations: anterior slight indentation and posterior relatively strong indentation. Postero-lateral alae caestus with developed projection parallelogram-shaped at posterior end. Caudal ala spread between pair of cirri E with pair of lateral indentations (26 μm deep) and medial indentation (40 μm deep). Caudal caestus with pair of small projections. Unpaired median cirrus (36 μm) with scapus (10 μm), tubular portion (22 μm) and flagellum (4 μm) inserted dorsally 27 μm from frontal margin. Pair of internal cirri (46 μm) each with scapus (13 μm) tubular portion (30 μm) and flagellum (3 μm) inserted at anterior margin. Internal structure of internal cirrus arise 25 μm from frontal margin. Pair of external cirrus (44 μm) with scapus (18 μm), tubular portion (20 μm) and flagellum (6 μm) inserted ventrally 30 μm from frontal margin. Internal structure of external cirrus arise 39 μm from frontal margin. Lateral cirrus (43 μm) with scapus (15 μm), tubular portion and flagellum and primary clava arise from same cirrophore. Boundary between tubular portion and flagellum of lateral cirrus indistinct in holotype. Lateral cirrus inserted dorso-posteriorly to primary clava. Primary clava (101 μm) thicker near base with basal van der Land’s body. Secondary clavae in shape of longitudinally elongated, flat sac with internally directed weak swelling on each side of ventrally protruded mouth cone (Figs 2B, 3B). Bucco-pharyngeal apparatus not visible except for pharyngeal bulb (32 μm × 24 μm). No bacterial vesicles visible. Leg I sense organ (29 μm) consists of tapering spine and distal flagellum. Leg II and leg III sense organ (28 μm and 24 μm respectively) each consists of unsegmented tapering spine. Leg IV sense organ (29 μm) consists of tapering spine with basal van der Land’s body, distal constricted portion and distal pore. Pair of cirri E (46 μm) each with proximal portion and flagellum arise from between postero-lateral and caudal alarum caesti. Rosette-like female gonopore opens 27 μm anterior to anus. Pair of seminal receptacles sited laterally at a level between gonopore and anus. Seminal receptacle consists of sinuous duct, which opens 29 μm laterally from gonopore and terminates in spherical vesicle 9 μm in diameter. Each leg terminates in four digits with proximal wrinkles and distal claws. External digits with hook-shaped peduncle. Internal digits longer than external digits. Internal claws with dorsal spur. External claws with calcar and avicularia. Internal claws smaller than external claws.

Figure 2.

Drawing of Florarctus wunai sp. n., holotype KUZ Z705. A dorsal view B ventral view of cephalic region. a anus; AL anterolateral ala; C caudal ala; ca caestus; cE cirrus E; ec external cirrus; F frontal ala; g gonopore; ic internal cirrus; lc lateral cirrus; mc median cirrus; mo mouth; pb pharyngeal bulb; pc primary clava; PL postero-lateral ala; sc secondary clava; sr seminal receptacle; s1–4 leg I–IV sense organs.

Figure 3.

Phase contrast micrograph of Florarctus wunai sp. n. A dorsal view, holotype KUZ Z705, scale bar = 100 μm B secondary clavae (white arrowhead), holotype KUZ Z705, scale bar = 20 μm C bacterial vesicles (white, hollow arrowhead), paratype KUZ Z709, scale bar = 20 μm D caudal alae caestus with pair of small projections (black arrowhead), paratype KUZ Z708, scale bar = 20 μm.

Etymology

The specific epithet, wunai, is a Ryukyuan word for “sister” (Nakamoto 1981) referring to the new species as a sister of Florarctus antillensis Van der Land, 1968, a species with similar morphology.

Remarks on paratypes

The adult male, KUZ Z707, was smaller than adult females, KUZ Z705 and Z706, but had longer primary clavae relative to its body length (Table 2). The male gonopore of KUZ Z707 opens 10 μm anterior to the anus. The precise shape of male gonopore was not visible but spermatozoa were present inside the body. Excluding the lack of the genital structure, the paratypic four-clawed juvenile, KUZ Z709, was identical to the adults. A pair of bacterial vesicles is present in the paratypic four-clawed juvenile (Fig. 3C). For antero-lateral and postero-lateral alae, the number of slight indentations varied among specimens. There are two projections at the level of leg II in the paratypic specimen, KUZ Z708. The pair of small projections of the caudal alae caestus was better observed in the paratypes (Fig. 3D).

Morphometrics of the three new species (measurements in μm). Dashes indicate unmeasured trait.

Species Florarctus wunai sp. n. Halechiniscus churakaagii sp. n. Halechiniscus yanakaagii sp. n.
Holotype Paratype Paratype Paratype Paratype Holotype Paratype Holotype
KUZ No. Z705 Z706 Z707 Z708 Z709 Z710 Z711 Z712
Status Female Female Male ? Four-clawed juvenile Female Female Female
Body length 257 241 125 132 122 170 183 170
Median cirrus 36 34 22 19 19 66 - 35
Internal cirrus 46 49 32 29 24 37 40 26
External cirrus 44 42 32 28 19 23 24 18
Lateral cirrus 43 46 32 - 32 68 52 41
Primary clava 101 101 78 - - 34 38 21
Cirrus E 46 54 37 36 39 86 - 41
Leg I sense organ 29 31 19 18 14 11 12 14
Leg II sense organ 28 29 14 16 12 36 36 21
Leg III sense organ 24 28 15 - 17 32 33 21
Leg IV sense organ 29 25 17 17 15 16 17 15
Scapular process absent absent absent absent absent 42 44 absent
Double process (anterior) absent absent absent absent absent absent absent 23
Double process (posterior) absent absent absent absent absent absent absent 26

Differential diagnosis

The presence of the continuous caestus and the absence of dorsal mammilla-like ornamentation, are shared by Florarctus antillensis, F. glareolus Noda, 1987, F. pulcher De Zio Grimaldi, Lamarca, D’addabbo Gallo & Pietanza, 1999 and F. wunai sp. n. The new species is distinguished from these three species by the two small projections of the caudal alae caestus, which are long projections in F. glareolus, long projections with swollen tips in F. pulcher and absent in F. antillensis (using Renaud-Mornant [1970] for information on the caestus morphology of F. antillensis).

Subfamily Halechiniscinae Thulin, 1928
Genus Halechiniscus Richters, 1908

Halechiniscus churakaagii sp. n.

Figs 4, 5, Table 2

Diagnosis

Halechiniscus with cephalic region consisting of antero-medial lobe and dorsal lobe; median cirrus inserted on long robust cirrophore; dorsal internal cirrus inserted on robust cirrophore; ventral external cirrus inserted on short cirrophore; lateral cirrus and primary clava inserted on large lateral cirrophore; large scapular process with flat oval tip; cirrus E with proximal portion with distal dark portion and distal flagellum; bipartite leg I sense organ; large, unsegmented legs II and III sense organs; papillate leg IV sense organ; all claws with calcar.

Material examined

Holotype: KUZ Z710: adult female found in sediment sample 1 (Table 1).

Paratype: KUZ Z711: adult female found in sediment sample 4 (Table 1).

Type locality

Water depth of 6 m, off Sedake, Oura Bay, Okinawajima, one of the Ryukyu Islands, Japan (26°32'39.06"N, 128°2'52.8"E). Collected by the author on 24th January 2014.

Type depository

The type series is deposited in the Zoological Collection of Kyoto University (KUZ).

Description of holotype

Adult female, body length: 170 μm (Figs 4, 5A, B). Dorsal and ventral surface smooth. Cephalic region divided into two lobes: antero-ventrally protruded round medial lobe and dorsal lobe. Unpaired median cirrus with scapus (42 μm) constricted at distal end, tubular portion (20 μm) and flagellum (4 μm) inserted on robust cirrophore (28 μm); positioned dorsally 28 μm from frontal margin on dorsal lobe. Pair of internal cirri each with scapus (16 μm), tubular portion (16 μm) and flagellum (5 μm) inserted on cirrophore; positioned dorsally on basal margin of medial lobe. Base of internal cirri arise postero-internal to base of scapi beneath cuticle. Pair of external cirri each with scapus (9 μm), tubular portion (9 μm) and flagellum (4 μm) inserted on cirrophore; positioned ventrally on medial lobe. Base of external cirri arise posterior to scapi beneath cuticle. Lateral cirrus with scapus (35 μm), tubular portion (23 μm) and flagellum (10 μm) and elongate primary clava (34 μm) inserted on each large, lateral cirrophore; positioned slightly anterior to level of median cirrus on dorsal lobe. Primary clava with basal van der Land’s body inserted antero-ventrally to lateral cirrus. Secondary clava absent. Mouth cone protruded antero-ventrally. Bucco-pharyngeal apparatus not visible except for pharyngeal bulb (17 μm × 19 μm). Laterally protruded scapular process (42 μm) with flat oval tip (Figs 4, 5C). No other process present. Cirrus E (86 μm) segmented into proximal portion and distal flagellum. Under phase contrast microscopy distal end of proximal portion appears dark (Fig. 5D). This could be a distinct portion with accordion-like folds but scanning electron microscopy is required for confirmation. Rosette-like female gonopore opens ventrally 20 μm anterior to anus. Seminal receptacle ducts open postero-lateral to gonopore. Vesicles of seminal receptacles not visible. Leg I sense organ (11 μm) consists of scapus and flagellum. Leg II and III sense organs (36 μm, 32 μm) each consists of unsegmented, large spine. Papillate leg IV sense organ (16 μm) with basal van der Land’s body and terminal constriction inserted on slender cirrophore. Each leg terminates in digits with wrinkles and distal claws. All claws with small calcar.

Figure 4.

Drawing of Halechiniscus churakaagii sp. n., holotype KUZ Z710 (dorsal view). a anus; cE cirrus E; d seminal receptacle duct; ec external cirrus; g gonopore; ic internal cirrus; lc lateral cirrus; mc median cirrus; mo mouth; pb pharyngeal bulb; pc primary clava; sc scapular process; s1–4 leg I–IV sense organs.

Figure 5.

Phase contrast micrograph of Halechiniscus churakaagii sp. n., holotype KUZ Z710. A dorsal view, scale bar = 50 μm B ventral view, scale bar = 50 μm C scapular process (white arrowhead), scale bar = 10 μm D cirrus E with dark region (white, hollow arrowhead), scale bar = 20 μm.

Etymology

The specific epithet, churakaagii, is a Ryukyuan word for “beautiful woman” (Tojo 1930) referring to the well-defined cephalic morphology of the new species.

Differential diagnosis

The robust cirrophores of the median and lateral cirri are present in Halechiniscus chafarinensis De Zio Grimaldi & Villora Moreno, 1995, H. macrocephalus Grimaldi de Zio, D’Addabbo Gallo & Morone De Lucia, 1988, H. paratuleari Grimaldi de Zio, D’Addabbo Gallo & Morone De Lucia, 1988, H. tuleari Renaud-Mornant, 1979 and H. churakaagii sp. n. Among these species, H. paratuleari and the new species are the only species with large scapular processes. The new species is distinguished from H. paratuleari by the flat oval tip of the scapular process, which is acute in the latter and by the dark portion on cirrus E, which is absent in the latter.

I have interpreted the dorsally positioned cirrus as internal cirrus and the ventrally positioned cirrus as external cirrus in the new species, which is opposite to the interpretation of these features in the original descriptions of H. tuleari and H. paratuleari.

Halechiniscus yanakaagii sp. n.

Figs 6, 7, Table 2

Diagnosis

Halechiniscus with no distinct cephalic lobes; only lateral cirrus and primary clava inserted on cirrophore; laterally protruded arched, double processes with acute tips at level of leg I; unsegmented cirrus E; bipartite leg I sense organ; unsegmented legs II and III sense organs; papillate leg IV sense organ; claws of internal digits with dorsal spur.

Material examined

Holotype: KUZ Z712: adult female found in sediment sample 5 (Table 1).

Type locality

Water depth of 6 m, Off Thima, Oura Bay, Okinawajima, one of the Ryukyu Islands, Japan (26°32'0.81"N, 128°3'49.61"E). Collected by the author on 28th January 2014.

Type depository

The holotype is deposited in the Zoological Collection of Kyoto University (KUZ).

Description of holotype

Adult female, body length: 170 μm (Figs 6, 7A). Dorsal and ventral surface smooth. Cephalic region not divided into distinct lobes. Tubular portions of cephalic cirri indistinct from flagellum for median, internal and external cirri. Unpaired median cirrus with scapus (11 μm) and flagellum (24 μm); inserted dorsally 16 μm from frontal margin. Pair of internal cirri each with scapus (6 μm) and flagellum (20 μm); inserted dorsally close to frontal margin. Pair of external cirri each with scapus (7 μm) and flagellum (11 μm); inserted ventrally close to frontal margin. Lateral cirrus with scapus (11 μm), tubular portion (22 μm) and flagellum (8 μm) and elongate primary clava (21 μm); inserted on each lateral cirrophore positioned at same level as median cirrus. Primary clava with basal van der Land’s body inserted antero-ventrally to lateral cirrus. Secondary clava absent. Mouth cone protruded antero-ventrally. Bucco-pharyngeal apparatus not visible except for pharyngeal bulb (14 μm × 20 μm). Laterally protruding arched, double processes (23 μm, 26 μm) with acute tip situated dorsally at level of leg I (Figs 6, 7B). Unsegmented cirrus E (41 μm) inserted on short cirrophore. Rosette-like female gonopore present. Seminal receptacles not visible. Leg I sense organ (14 μm) consists of tapering spine and distal flagellum. Leg II and III sense organs (both 21 μm) each consists of unsegmented spine. Papillate Leg IV sense organ (15 μm) with basal van der Land’s body inserted on small cirrophore. Each leg terminates in digits with wrinkles and distal claws. Claws of internal digits with dorsal spur.

Figure 6.

Drawing of Halechiniscus yanakaagii sp. n., holotype KUZ Z712 (dorso-lateral view). cE cirrus E; dp1–2 anterior and posterior double processes; ec external cirrus; g gonopore; ic internal cirrus; lc lateral cirrus; mc median cirrus; mo mouth; pb pharyngeal bulb; pc primary clava; s1–4 leg I–IV sense organs.

Figure 7.

Phase contrast micrograph of Halechiniscus yanakaagii sp. n., holotype KUZ Z712. A dorso-lateral view, scale bar = 50 μm B double processes (white arrowhead), scale bar = 10 μm.

Etymology

The specific epithet, yanakaagii, is a Ryukyuan word for “ugly woman” (Tojo 1930) referring to dirty appearance of the holotype.

Differential diagnosis

Halechiniscus yanakaagii sp. n. and H. tuleari are the only two Halechiniscus species with double processes at the level of leg I. The new species is distinguished from H. tuleari by the absence of distinct cephalic lobes and robust cephalic cirrophores (which are present in the latter species), the similar length, arched, double processes (23 μm, 26 μm) in contrast with a short, straight, anterior process (holotype female: 8 μm; paratype male: 4 μm) and a long, straight, posterior process (holotype female: 19 μm; paratype male: 10 μm) (see: Renaud-Mornant 1979), and the absence of processes at level of leg II and III, which are present in H. tuleari.

Subfamily Styraconyxinae Kristensen & Renaud-Mornant, 1983
Genus Styraconyx Thulin, 1942

Styraconyx sp.

Fig. 8

Material examined

One female adult and one four-clawed juvenile found in sediment sample 4 (Table 1).

Remarks

The individuals found resemble Styraconyx nanoqsunguak Kristensen & Higgins, 1984 by the dorsal ridges (Fig. 8A, B). However, these specimens are distinguished by the lateral cirrus with no scapus (which is present S. nanoqsunguak), longer peduncles of the external digits and leg IV sense organs consisting of a spherical papilla and a distal spine (which is an elongate papilla and a shorter distal spine in S. nanoqsunguak). While I believe this is a new undescribed species, lack of visible taxonomic characters has hindered providing a complete species description.

Figure 8.

Phase contrast micrograph of Styraconyx sp. A ventral view, scale bar = 50 μm B dorsal cuticle with ridges, scale bar = 20 μm.

Discussion

With the addition of the three new species and two unidentified species reported in this study, two orders, five families, 15 named and two unidentified genera, 13 named and 14 unidentified species of marine heterotardigrades are known from the Ryukyu Islands (Table 3). Sudzuki (1979) reported the first species as Actinarctus sp., which I deem a misidentification of Florarctus sp. according to the micrograph in his paper. Subsequently, Noda (1993, 1994a,b,c, 1998) reported 21 species but, with the exception of four species, with neither exact sampling localities nor remarks on species morphology. He noted that Renaudarctus psammocryptus Kristensen & Higgins, 1984 accorded well with the original description (Noda 1994b) and considered three species to be undescribed: Stygarctidae gen. (?) sp. (Noda 1993), Renaudarctidae gen. (?) sp. (Noda 1994b) and Anisonyches sp. (Noda 1994c). Recently, Fujimoto and Miyazaki (2013) described a new species from a submarine cave off Shimoji Island, Miyako Islands.

Marine tardigrades reported from the Ryukyu Islands.

Taxon Sampling locality References
Order ARTHROTARDIGRADA
Family Batillipedidae
Batillipes pennaki Marcus, 1946 Kabira, Ishigakijima and/or Tsunami, Okinawajima Noda (1994a,b)
B. similis Schulz, 1955 Kabira, Ishigakijima and/or Tsunami, Okinawajima Noda (1994a,b)
Family Halechiniscidae
Dipodarctinae Dipodarctus borrori Pollock, 1995 Kuroshima Noda (1998)
D. sp. Oura Bay, Okinawajima This study
Florarctinae Florarctus wunai sp. n. Oura Bay, Okinawajima This study
Florarctus sp. 1 (Originally reported as Actinarctus sp.) Taketomijima and Okinawajima Sudzuki (1979)
F. sp. 2 Kabira, Ishigakijima and/or Tsunami, Okinawajima Noda (1994a,b)
F. sp. 3 Kabira, Ishigakijima and/or Tsunami, Okinawajima Noda (1994a,b)
Wingstrandarctus sp. Tsunami, Okinawajima Noda (1994a,b)
Halechiniscinae Halechiniscus chafarinensis De Zio Grimaldi & Villora Moreno, 1995 Kuroshima Noda (1998)
H. churakaagii sp. n. Oura Bay, Okinawajima This study
H. yanakaagii sp. n. Oura Bay, Okinawajima This study
H. sp. 1 Kabira, Ishigakijima and/or Tsunami, Okinawajima Noda (1994a,b)
H. sp. 2 Tsunami, Okinawajima Noda (1994a)
Styraconyxinae Angursa clavifera Noda, 1985 Tsunami, Okinawajima Noda (1994a)
Raiarctus sp. Kabira, Ishigakijima and/or Tsunami, Okinawajima Noda (1994a,b)
Styraconyx nanoqsunguak Kristensen & Higgins, 1984 Kabira, Ishigakijima and/or Tsunami, Okinawajima Noda (1994a,b)
S. sp. Oura Bay, Okinawajima This study
Tholoarctus natans Kristensen & Renaud-Mornant, 1983 Kabira, Ishigakijima and/or Tsunami, Okinawajima Noda (1994a,b)
Tanarctinae Tanarctus sp. Tsunami, Okinawajima Noda (1994a)
Family Renaudarctidae
Renaudarctus psammocryptus Kristensen & Higgins, 1984 Kabira, Ishigakijima Noda (1994b)
Gen. (?) sp. Kabira, Ishigakijima Noda (1994b)
Family Stygarctidae
Stygarctinae Neostygarctus lovedeluxe Fujimoto & Miyazaki, 2013 Twin Cave, off Shimojijima Fujimoto and Miyazaki (2013)
Parastygarctus higginsi
Renaud-Debyser, 1965
Kabira, Ishigakijima and/or Tsunami, Okinawajima Noda (1994a,b)
Stygarctus sp. Kabira, Ishigakijima and/or Tsunami, Okinawajima Noda (1994a,b)
Gen. (?) sp. Off Kuroshima Noda (1993)
Order ECHINISCOIDEA
Family Echiniscoididae
Anisonyches sp. Kabira, Ishigakijima Noda (1994b,c)

As noted above, at best the identifications are ambiguous, and verifying the identity of the species across published papers is difficult. Nonetheless, the data shows that Ryukyu Islands harbour a rich marine tardigrade fauna. With more research we can expect further species discoveries as many of the islands are unexplored and there are currently only seven species reported for the usually species-rich sub-littoral zone.

Acknowledgments

The author’s thanks are due to the participants of the first Umisawa-kai, especially the two hosts, Daisuke Uyeno and Hiroshi Yamasaki, and the members of the diving team Snack Snufkin for facilitating the opportunity to survey Oura Bay. Thanks are also due to Tsutomu Hikida for helpful comments on the manuscript. This work was supported by Japan Society for the Promotion of Science Grant-in-Aid for JSPS fellows (Grant No. 25987).

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