Echinoderes okiensis Yamasaki, 2024

Echinoderes okiensis Yamasaki View in CoL , sp. nov.

[New Japanese name: Oki-togekawa]

( Figs. 3–7 View Fig View Fig View Fig View Fig View Fig ; Tables 3 and 4)

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3.2.1. Synonym list Echinoderes sp. 1 : Yamasaki et al. (2013); Dal Zotto et al. (2013).

3.2.2. Diagnosis

Echinoderes with middorsal acicular spines on segments 4–8; ventrolateral tubes on segment 2; lateroventral tubes on segment 5; lateroventral acicular spines on segments 6–9; midlateral tubes on segment 10; one type-1 glandular cell outlets present middorsally each on segments 1–3, 5, and 7, two present middorsally on segments 10 and 11, one pairs present paradorsally on segments 4, 6, 8, and 9, sublaterally on segment 1, and ventromedially on segments 2–10; type-2 glandular cell outlet absent; tergal extension narrowing abruptly into long acicular tips, constituting 5–7% of trunk length; males with three pairs of penile spines of which middle pair very short; females with papillae in ventrolateral position on segment 7 and ventromedial position on segment 8, as well as with lateral terminal accessory spines.

3.2.3. Etymology

The specific name “ okiensis ” is derives from its type locality (Oki Islands).

3.2.4. Material examined

Holotype: Adult female (catalogue no. NSMT-Ki 52), collected on 21 June 2022 from rocky sediment sample at Onbe, Oki Islands, Japan (station Onbe-1: 36 ◦ 9.185′N, 133 ◦ 10.962′E, 67–70 m depth), mounted in Fluoromount G ® on a H-S slide GoogleMaps . Paratype: three adult males and one adult female (catalogue no. NSMT-Ki 53–56), and exoskeleton of one adult female used for DNA extraction (catalogue no. NSMT-Ki 57), all collected with the holotype; exoskeletons of two adult females used for DNA extraction (catalogue no. NSMT-Ki 58–59) collected on 19 August 2022 from rocky sediment sample at Onbe , Oki Islands, Japan (station Onbe-2: 36 ◦ 9.218′N, 133 ◦ 10.781′E, 70 m depth) GoogleMaps ; two adult males and one adult female (catalogue no. NSMT-Ki 60–62), collected on 20 May 2011 from muddy sand sample off Iejima Island , Okinawa, Japan (station 9-1: 26 ◦ 40.283′N, 127 ◦ 45.433′E, 138 m depth) GoogleMaps ; three adult males and three adult females (catalogue no. NSMT-Ki 63–68), and exoskeleton of one adult female used for DNA extraction (catalogue no. ICHUM-04291: the acronym of the museum collection was changed from ZIHU to ICHUM), collected on 20 May 2011 from muddy sand sample off Iejima Island , Okinawa, Japan (station 9-2: 26 ◦ 40.517′N, 127 ◦ 45.333′E, 124 m depth) GoogleMaps ; all individually mounted in Fluoromount G® on H-S slides.

Non-type: one adult male and one adult female, collected with the holotype, mounted on aluminum stubs for SEM.

3.2.5. Type locality

Onbe , Oki Islands, Japan (36 ◦ 9.185′N, 133 ◦ 10.962′E), 67–70 m depth GoogleMaps .

3.2.6. Description

Adult with head, neck, and eleven trunk segments ( Fig. 3A, B View Fig , 4A–D View Fig , 5A View Fig ). See Table 3 for the positions of cuticular structures. Table 4 indicates measurements.

Head consists of retractable mouth cone and introvert ( Fig. 5A and B View Fig ). Mouth cone comprised of inner and outer oral styles, however, detail of inner oral styles not observable in available specimens. Outer oral styles alternating in length: slightly longer in odd sector than in even sector. Outer oral style in sector 6 missing. Introvert with at least six rings of spinoscalids and one ring of trichoscalids ( Fig. 5B View Fig ). Ring 01 with ten primary spinoscalids composed of one basal sheath and one distal end piece. Each basal sheath with proximal fringe composed of several projections between two long and slightly thicker lateral ones. Distal end pieces with proximal fringe with projections and long flexible spinose process, representing longest within all spinoscalids. Rings 02 and 04 with 10 spinoscalids; rings 03 and 05 with 20 spinoscalids. Each spinoscalid of rings 02–05 with basal sheath and end piece. Basal sheath of spinoscalids on ring 03 appeared to have a thick spinose process, whereas others composed of multiple thin and short spinose processes. Spinoscalids ring 06 not examined in detail, but with at least four spinoscalids distributed in section 4, 6, and 8. Six trichoscalids covered with long hairs on entire surface and each attached to trichoscalid plate. Number and arrangement of outer oral styles, and spinoscalids summarized in Fig. 6 View Fig .

Neck with 16 placids. Midventral placid wider than others, and remaining placids similar in width.

Trunk with eleven segments; segment 1 consists of complete cuticular ring; segment 2 also consists of single cuticular ring, but with midventral scar, segments 3–11 consist of one tergal and two sternal plates ( Fig. 4 View Fig E-N, 5C-E, and 6). Thickened cuticle forms pachycyclus at anterior margin and beside midventral and tergosternal articulations of segments 2–10 (e.g., Fig. 4A–H, J, K View Fig ).

Segment 1 with type-1 glandular cell outlet in middorsal position ( Fig. 3A View Fig and 4E View Fig ). Additional pair of type-1 glandular cell outlets in sublateral position ( Fig. 3B View Fig ). Sensory spots present subdorsally and laterodorsally ( Fig. 3A View Fig , 4E View Fig and 5C View Fig ). Non-bracteate cuticular hairs arising from perforation sites, covering whole segment ( Fig. 5C and D View Fig ). Posterior edge with primary pectinate fringe consisted of very short fringe tips.

Segment 2 with ventrolateral tubes ( Fig. 3B View Fig , 4F View Fig and 5D View Fig ). Type-1 glandular cell outlets, one in middorsal and one pair in ventromedial positions ( Fig. 3A, B View Fig , 4E, F View Fig ). One sensory spot present middorsally, two pairs laterodorsally, and one pair ventromedially ( Fig. 3A, B View Fig , 4E, F View Fig , 5C, D View Fig ). Bracteate cuticular hairs from perforation sites covering whole segment except for muscular attachment sites in laterodorsal position as well as ventromedial–midventral area ( Fig. 3A, B View Fig , 5C, D View Fig ). Primary pectinate fringe similar to that on segment 1, but with slightly longer fringe tips especially on midlateral-lateroventral area.

Segment 3 with one middorsal and one pair of ventromedial type-1 glandular cell outlets ( Fig. 3A, B View Fig , 4E, F View Fig ). Sensory spots present in subdorsal and midlateral positions ( Fig. 3A, B View Fig , 4E, F View Fig , 5C View Fig ). Cuticular hairs and primary pectinate fringe as on preceding segment.

Segment 9 with lateroventral acicular spines ( Fig. 3A, B View Fig , 4I View Fig , 7A, D, E View Fig ). One pair of type-1 glandular cell outlets present each in paradorsal and ventromedial position ( Fig. 3A, B View Fig , 4H, I View Fig ). Sensory spots present in paradorsal, subdorsal, midlateral, and ventrolateral positions ( Fig. 3A, B View Fig , 4I View Fig , 7C View Fig ). Small rounded sieve plate present in lateral accessory position ( Fig. 3B View Fig and 4I View Fig ). Cuticular hairs and primary pectinate fringe as on segment 5.

Segment 10 with relatively long midlateral tubes ( Figs. 3 View Fig and 4L, N View Fig , 7C–E View Fig ). Two type-1 glandular cell outlets present in middorsal position, and one pair in ventromedial position ( Fig. 3 View Fig ). Sensory spots present subdorsally and ventrolaterally ( Figs. 3 View Fig and 7C View Fig ). Cuticular hairs and primary pectinate fringe as on segment 5.

Segment 11 with lateral terminal spines ( Figs. 3 View Fig and 4L–N View Fig , 5A View Fig , 7C–E View Fig ). Males with three pairs of penile spines: most dorsal and most ventral penile spines long whereas middle penile spines short and visible only in SEM ( Fig. 3C, D View Fig , 4N View Fig , 7C, E View Fig ). Females with lateral terminal accessory spines ( Fig. 3A, B View Fig , 4L View Fig , 7D View Fig ). Two type-1 glandular cell outlets present middorsally, and sensory spots present subdorsally ( Figs. 3 View Fig and 7E View Fig ). Segment surface smooth and hairless. Posterior edge of tergal plate forming tergal extension narrowing abruptly into long acicular tips ( Figs. 3 View Fig and 4L–N View Fig , 5A View Fig , 7C, D View Fig ).

Segment 4 with middorsal acicular spine ( Fig. 3A View Fig , 4E, G View Fig , and 5A, C View Fig ). One pair of type-1 glandular cell outlets present each in paradorsal and ventromedial positions ( Fig. 3A, B View Fig , 4F, H View Fig ). Sensory spots present subdorsally and laterodorsally ( Fig. 3A View Fig , 4E, G View Fig , and 5C View Fig ). Cuticular hairs and primary pectinate fringe as on segment 2.

Segment 5 with middorsal acicular spine and lateroventral tubes ( Fig. 3A, B View Fig , 4E–G, I View Fig , 5E View Fig , 7A View Fig ). One pair of type-1 glandular cell outlets present each in middorsal and ventromedial positions ( Fig. 3A, B View Fig , 4F, H, I View Fig ). Sensory spots present subdorsally, midlaterally, and ventromedially ( Fig. 3A, B View Fig , 4E–G, I View Fig , 7A View Fig ). Cuticular hairs as on segment 2. Primary pectinate fringe with slightly longer fringe tips than those on segments 2–4.

Segment 6 with middorsal and lateroventral acicular spines ( Fig. 3A, B View Fig , 4G, I View Fig , 5A, E View Fig , 7A, B View Fig ). One pair of type-1 glandular cell outlets present each in paradorsal and ventromedial positions ( Fig. 3A, B View Fig , 4H, I View Fig ). Sensory spots present paradorsal, subdorsal, and midlateral positions ( Fig. 3A, B View Fig , 4G View Fig ). Cuticular hairs as on segment 2. Primary pectinate fringe as on segment 5.

Segment 7 with middorsal and lateroventral acicular spines ( Fig. 3A, B View Fig , 4G, I–K View Fig , 5A, E View Fig , 7A, B View Fig ). Type-1 glandular cell outlets, sensory spots, cuticular hairs, and primary pectinate fringe as on segment 5 ( Fig. 3A, B View Fig , 4G–K View Fig , 5E View Fig , 7A, B View Fig ). Females with papillae with tubular-shaped intracuticular substructure in ventrolateral position ( Fig. 3A, B View Fig , 4I–K View Fig , 7B View Fig ).

Segment 8 similar to segment 6, except for presence of ventromedial papillae in females ( Fig. 3A, B View Fig , 4G–K View Fig , 5E View Fig , 7A–C View Fig ).

3.2.7. Variation

All the major morphological features —such as spines, tubes, glandular cell outlets, female papillae, and tergal extension shape— were congruent in all the Echinoderes specimens from the Oki Islands (stations Onbe-1 and Onbe-2) and off Iejima Island (stations 9-1 and 9-2). As shown in Table 4, morphometric data also overlapped between the two populations, and no significant differences were observed. Regarding the DNA sequences, 1776 bp of 18S was determined for individuals from both the Oki Islands and off Iejima Island, and these sequences are completely identical. For 28S, 3300 bp was determined for the specimen from off Iejima Island, while 3296 bp was determined for the specimen from the Oki Islands, with two undetermined internal regions (one representing 5–10 bp, and the other 25–30 bp). The 28S sequences were compared after alignment, and except for the missing data regions, there were 7 nucleotide substitutions and 4 nucleotide insertions/deletions between the sequences of the two individuals.

3.2.8. Taxonomic remarks

As mentioned in the previous section, Echinoderes individuals from the Oki Islands (stations Onbe-1 and Onbe-2) and off Iejima Island (stations 9-1 and 9-2) could not be distinguished morphologically. Regarding the DNA sequences, there is little data available for intraspecific variation in 18S and 28S sequences in Kinorhyncha. For example, in 18S of Echinoderes songae Sørensen et al., 2020 , two nucleotide substitutions were found in a comparable region of 1783 bp between Japanese (LC831703) and Korean (GQ229038) individuals. For 28S, there is currently no comparable data available for intraspecific variation within any kinorhynch species. However, when comparing the differences between closely related species, for example, Echinoderes ohtsukai and Echinoderes rex (the two species are suggested to be closely related in both this study and a previous study using 18S and COI ( Randsø et al., 2019)), there are three substitutions within 1772 bp in 18S (LC096961 for E. ohtsukai and LC081126 for E. rex ) and 52 substitutions as well as 9 bases insertions/deletions within 2206 bp in 28S (LC096963 for E. ohtsukai and LC081131 for E. rex ). This interspecific variation is significant compared to the differences between the Echinoderes individuals from the Oki Islands and off Iejima Island. Furthermore, the results of the phylogenetic analysis based on 18S and 28S sequences in this study also supported that the individuals from the two localities are closely related, forming a clade together (see the next section). Therefore, despite the geographic distance between the Oki Islands and Iejima Island, it is plausible to treat these individuals as the same species, E. okiensis sp. nov.

In the family Echinoderidae , the spine/tube pattern of especially on segments 1–9 is the most easily recognized and is often the primary taxonomic trait used for the identification and classification of species ( Yamasaki et al., 2020b). However, the pattern in E. okiensis sp. nov. —the presence of middorsal acicular spines on segments 4–8, lateroventral/ventrolateral tubes on segment 2, lateroventral tubes/spines on segments 5–9— is one of the most common spine/tube patterns in the family, shared with 31 congeners ( Yamasaki et al., 2020b). To distinguish species with such common spine/tube patterns, focusing on the pattern of type 1/2 glandular cell outlets and the shape of the tergal extension has recently gained attention as valuable taxonomic traits ( Varney et al., 2019; Sørensen et al., 2020). These traits make it easy to distinguish E. okiensis sp. nov. from the other congeners.

E. okiensis View in CoL sp. nov. possesses the tergal extensions which are narrowing abruptly into long acicular tips. Of the 31 congeners that share the spine and tube pattern with E. okiensis View in CoL sp. nov., the following six species also share the similar tergal extension shape: Echinoderes kohni Varney et al., 2019 View in CoL , Echinoderes lupherorum Sørensen et al., 2018 View in CoL , Echinoderes microaperturus Sørensen et al., 2012 View in CoL , Echinoderes spinifurca Sørensen et al., 2005 View in CoL , Echinoderes sylviae Landers and Sørensen, 2018 View in CoL , Echinoderes yamasakii Sørensen et al., 2018 View in CoL ; Sørensen et al., 2005, 2012, 2018; Landers and Sørensen, 2018; Varney et al., 2019). These six species are also thought to compose a species group, the so-called “ E. spinifurca View in CoL species group” ( Sørensen et al., 2018). At first glance, E. okiensis View in CoL sp. nov. also resembles these species and appears to belong to the E. spinifurca View in CoL species group. However, E. okiensis View in CoL sp. nov. differs from these species in having only one type-1 glandular cell outlet on tergal plate of the segments 5 and 7 (in the middorsal position), whereas all the species of the E. spinifurca View in CoL species group have a pair of type-1 glandular cell outlets on the tergal plate of these segments (in the paradorsal or subdorsal position). In addition, E. okiensis View in CoL sp. nov. does not have any type-2 glandular cell outlets, while all species of the E. spinifurca View in CoL species group, except E. spinifurca View in CoL , have type-2 glandular cell outlets on some of their trunk segments ( Sørensen et al., 2005; 2012; 2018; Landers and Sørensen, 2018; Varney et al., 2019; Yamasaki et al., 2020b).

The presence of type-1 glandular cell outlet in middorsal position on segments 5 and 7 in E. okiensis View in CoL sp. nov. is very unusual compared to the congeners that share the spine/tube pattern and is only known in Echinoderes aureus Adrianov et al., 2002 View in CoL among the 31 species (Sørensen et al., 2020). E. okiensis View in CoL sp. nov. is also similar to E. aureus View in CoL in the absence of any type-2 glandular cell outlets as well as the presence of tubes on segment 10 not in the midlateral position rather than in the laterodorsal position (Sørensen et al., 2020). However, E. okiensis View in CoL sp. nov. can be distinguished from E. aureus View in CoL by the tergal extension shape, which is narrowing abruptly into long acicular tips in E. okiensis View in CoL sp. nov., whereas it is short and well-spaced triangular in E. aureus View in CoL . E. okiensis View in CoL sp. nov. also tends to have a shorter trunk length (227–268 μm) and longer lateral terminal spines (108–159 μm) than those in E. aureus View in CoL (277–313 μm and 98–113 μm, respectively), resulting in markedly different ratios of lateral terminal spine length to trunk length (41–59 % in E. okiensis View in CoL sp. nov. compared to 31–38 % in E. aureus View in CoL ) ( Adrianov et al., 2002).