Echinoderes antarcticus, Sørensen & Macheriotou & Braeckman & Smith & Ingels, 2025

Sørensen, Martin V., Macheriotou, Lara, Braeckman, Ulrike, Smith, Craig R. & Ingels, Jeroen, 2025, Antarctic Kinorhyncha: Seven new species from the Antarctic Peninsula, European Journal of Taxonomy 1000 (1), pp. 1-102 : 58-65

Echinoderes antarcticus sp. nov.

urn:lsid:zoobank.org:act:940A9763-763D-4DBE-9A81-66F7A97312B8

Figs 19–21 , Tables 15–16

Diagnosis

Echinoderes with acicular spines in middorsal position on segments 4, 6, and 8, and in lateroventral positions on segments 6 to 9. Tubes present in subdorsal, sublateral, and ventrolateral positions on segment 2, in lateroventral positions on segment 5, in sublateral positions on segment 8, and in laterodorsal positions on segment 10. Terminal segment with middorsal fissure, splitting the tergal plate in two. Male morphology unknown. Female papillae or glandular cell outlets type 2 not present. Dorsal glandular cell outlets type 1 are present in middorsal positions on segments 1 to 3, 5, 7 and 10, and in paradorsal positions on segments 4, 6, 8, and 9. Sieve plates present on segment 9 in lateral accessory positions.

Etymology

The species name ‘ antarcticus ’ is derived from the Greek ανταρκτικός (antarcticos), meaning ‘opposite to north’ (masculine).

Material examined

Holotype ANTARCTICA • ♀ (mounted for LM in Fluoromount G on HS slide); Antarctic Peninsula, CRS 1778; 64°47.01′ S, 62°43.90′ W; 567 m b.s.l.; 8 Apr. 2016; FjordEco2; soft sediment; NHMD 1786932 .

Paratype

ANTARCTICA • 1 ♀ (mounted as holotype); Antarctic Peninsula, CRS 1809; 64°39.59′ S, 62°55.09′ W; 694 m b.s.l.; 15 Apr. 2016; FjordEco2; soft sediment; USNM 1740040 .

Additional material

ANTARCTICA • 1 ♀ (mounted for SEM); Antarctic Peninsula, CRS 1832; 64°39.30′ S, 62°55.98′ W; 631 m b.s.l.; 21 Apr. 2016; FjordEco2; soft sediment; MVS .

Description

GENERAL. Adults with head, neck and eleven trunk segments ( Figs 19A–B , 20A , 21A–B ). The trunk is nearly parallel-sided from segment 2 to 7, and the cuticle of a thickness that makes all cuticular structures appear very distinct. An overview of measurements and dimensions is given in Table 15 . Distributions of cuticular structures, i.e., sensory spots, glandular cell outlets, spines and tubes, are summarized in Table 16 .

HEAD. The available SEM specimen and the paratype had their heads fully retracted, whereas the head of the holotype was only partly extended; thus, information on head morphology is very limited. The presence of nine outer oral styles, each composed of two units, is evident though. The neck consists of 16 placids. Midventral placid broadest, 14 µm in width and 16 µm in length, whereas all others are narrower, measuring 9 µm in width at their bases. The trichoscalid plates are well-developed and hat-shaped.

SEGMENT 1. Consists of a complete cuticular ring. Sensory spots are present in subdorsal, laterodorsal, and ventromedial positions; the sensory spots are rounded to droplet-shaped, with numerous micropapillae around a central pore. Glandular cell outlets type 1 are present in middorsal and lateroventral positions. Cuticular hairs emerge from rounded perforation sites and are arranged in three diffuse rows on the dorsal side, a single row in the sublateral positions, and two rows ventrally, between the sensory spots. The posterior segment margin is almost straight and terminates in a pectinate fringe with tripartite tips ( Figs 19A–B , 20B–C , 21C–D ).

SEGMENT 2. Consists of a complete cuticular ring. Tubes are located in subdorsal, sublateral, and ventrolateral positions; all tubes are very slender, and the proximal thickenings are hardly visible. Sensory spots present in middorsal, midlateral, and ventromedial positions; the micropapillary areas around the sensory spots on this, and all following segments, are rounded, with up to five longer micropapillae extending from the posterior part of the papillated areas. Glandular cell outlets type 1 are present in middorsal and ventromedial positions. Bracteate cuticular hairs are arranged in five transverse rows; hairs in the first anterior row are short, whereas those of the more posterior rows are considerably longer. The posterior segment margin is nearly straight, except at the small, midventral V-shaped extension. Pectinate fringe with well-developed, pointed fringe tips along dorsal and lateral margins; ventromedial fringe tips similar, but shorter and narrower ( Figs 19A–B , 20B–C , 21C–D ).

SEGMENT 3. As following seven segments, consisting of one tergal and two sternal plates. Sensory spots are present in subdorsal and sublateral positions, and glandular cell outlets type 1 in middorsal and ventromedial positions. The hair covering of the tergal and lateral halves of sternal plates is dense on the anterior half of the segment, except in hair-less midlateral areas, with bracteate cuticular hairs as on preceding segment. Paraventral areas without bracteate hairs, but with shield-shaped patch of well-developed hair-like extensions. Posterior segment margin straight, and pectinate fringe as on preceding segment ( Figs 19A–B , 20B–C ).

SEGMENT 4. With spine in middorsal position. Sensory spots are not present. Glandular cell outlets type 1 are present in paradorsal and ventromedial positions. Cuticular hairs from paradorsal to ventromedial positions arranged in seven to eight rows, with very short hairs in the anteriormost row, and hairs of remaining rows getting progressively longer towards the more posterior rows; middorsal area with triangular patch of short, hair-like extensions; paraventral areas as on preceding segment. Segment margin and pectinate fringe as on preceding segment ( Figs 19A–B , 20B–C , 21E ).

SEGMENT 5. With tubes in lateroventral positions. Sensory spots present in subdorsal, sublateral and ventromedial positions, and glandular cell outlets type 1 in middorsal and ventromedial positions. Cuticular hairs, posterior segment margin, and pectinate fringe as on preceding segment ( Figs 19A–B , 20D–E , 21E–G ).

SEGMENT 6. With spines in middorsal and lateroventral positions. Sensory spots present in paradorsal, sublateral and ventromedial positions, and glandular cell outlets type 1 in paradorsal and ventromedial positions. Cuticular hairs as on preceding segment, except for the dorsal patch of hair-like extensions that covers both the middorsal and paradorsal areas. Posterior segment margin and pectinate fringe as on preceding segment ( Figs 19A–B , 20D–E , 21E–G ).

SEGMENT 7. With spines in lateroventral positions. Sensory spots present in paradorsal, sublateral, and ventromedial positions, and glandular cell outlets type 1 in middorsal and ventromedial positions. Cuticular hairs, posterior segment margin, and pectinate fringe as on preceding segment ( Figs 19A–B , 20D–E , 21G, I ).

SEGMENT 8. With spines in middorsal and lateroventral positions, and tubes in sublateral positions. Sensory spots present in paradorsal positions only, and glandular cell outlets type 1 in paradorsal and ventromedial positions. Cuticular hairs, posterior segment margin, and pectinate fringe as on preceding segment ( Figs 19A–B , 20F–H , 21I ).

SEGMENT 9. With spines in lateroventral positions. Sensory spots present in paradorsal, subdorsal, laterodorsal, and ventrolateral positions. Glandular cell outlets type 1 present in paradorsal and ventromedial positions. Small rounded sieve plates present in lateral accessory positions. Cuticular hairs as on preceding segment, but with midlateral hairless areas in more laterodorsal positions and dorsal area of hair-like extensions even broader, extending into the subdorsal areas. Posterior segment margin and pectinate fringe as on preceding segment ( Figs 19A–B , 20J–K , 21H ).

SEGMENT 10. With short laterodorsal tubes of females attaching at the posterior segment margin. Potential sexual dimorphism in length of these laterodorsal tubes unknown. Sensory spots present in subdorsal and ventrolateral positions. Glandular cell outlet type 1 present as two longitudinally arranged outlets in middorsal position, and in ventromedial positions. Cuticular hairs in three rows and only present from laterodorsal to ventromedial areas; middorsal to subdorsal areas with patch of short, hair-like extensions. The posterior segment margin of the tergal plate is straight, whereas sternal plate margins are deeply concave; all fringe tips along the margins are narrow and slender ( Figs 19A–B , 20J–K , 21H, J ).

SEGMENT 11. Consisting of two tergal and two sternal plates. Lateral terminal and lateral terminal accessory (the latter assumed female dimorphic) spines present. Sensory spots not observed. The segment is devoid of cuticular hairs, but has a dense covering of minute cuticular hair-like structures, only interrupted by smooth areas laterodorsally and lateroventral to ventromedially. Tergal extensions are triangular, and sternal extensions rounded, with deeply fringed margins ( Figs 19A–B , 20I–K , 21H, J ).

Distribution

Antarctic Peninsula: Gerlache Strait and Andvord Bay OBA, 567 to 694 m b.s.l. See Fig. 1 for geographic overview of stations and Table 1 for station and specimen information.

Diagnostic remarks

The new species can easily be assigned to Echinoderes based on the composition of its trunk segments. The middorsal fissure that divides the tergal plate of segment 11 into two halves is not common in the genus, but it does occur in various species, such as E. beringiensis , E. cernunnos Sørensen et al., 2012 , E. drogoni Grzelak & Sørensen, 2018 , E. galadrielae Grzelak & Sørensen, 2022 , the recently described species E. quasae Herranz et al., 2024 ( Sørensen et al. 2012; Grzelak & Sørensen 2018, 2022; Adrianov & Maiorova 2022; Herranz et al. 2024), and in E. angustus as discussed below under Echinoderes aff. angustus . Besides the divided tergal plate, the four species do not share any particular similarities.

The character that most easily distinguishes the new species is the configuration of tubes on segment 2. Tubes on segment 2 are quite common among species of Echinoderes , but the presence of tubes in subdorsal, sublateral, and ventrolateral positions, and not in laterodorsal positions as well, is only shared between six species ( Yamasaki et al. 2020b). Among these, just four species have middorsal spines on segments 4, 6 and 8 only. The four species in question are E. belenae , E. hispanicus , E. peterseni Higgins & Kristensen, 1988 , and E. xiphophorus Adrianov & Maiorova, 2021 . Among these species, E. belenae differs the most, with its short and stout lateral terminal spines and numerous tubes distributed over most trunk segments ( Pardos et al. 2016b). Also E. hispanicus is easily distinguished from the new species by having both sublateral and lateral accessory tubes on segment 8 ( Pardos et al. 1998) and E. xiphophorus by its long and slender tergal extensions ( Adrianov & Maiorova 2021).

Interestingly, the species showing the closest resemblance to E. antarcticus sp. nov. is the Arctic species E. peterseni , known from West Greenland, NE Canada, and Svalbard ( Higgins & Kristensen 1988; Sørensen & Kristensen 2000; Grzelak & Sørensen 2018). The two antipodean species basically share the same spine/tube distribution patterns, with the tube positions on segment 8 as the only minor difference. In E. antarcticus , the tubes are sublateral, whereas they are lateral accessory in E. peterseni , but this in itself is not sufficient to distinguish the two species. The easiest way to tell the species apart is morphometrically, where E. antarcticus generally is larger than E. peterseni . The trunk lengths of the two E. antarcticus types are 318 µm and 377 µm, whereas trunk lengths of E. peterseni , as reported by Higgins & Kristensen (1988), range between 250 µm and 325 µm. Likewise, the middorsal spines are longer in E. antarcticus , i.e., on segment 4: 48–55 µm vs 30–44 µm; on segment 6: 78–79 µm vs 45–60 µm; on segment 8: 92–105 µm vs 60–70 µm. Regarding cuticular structures, the sensory spot distribution patterns differ slightly between the two species, i.e., E. antarcticus has paradorsal sensory spots on segment 7, whereas these sensory spots in E. peterseni clearly sit in subdorsal positions (see Grzelak & Sørensen 2018: fig. 18f). Furthermore, E. antarcticus has three pairs of tergal sensory spots on segment 9, in paradorsal, subdorsal, and laterodorsal positions, whereas E. peterseni has only two pairs, in paradorsal and laterodorsal positions. Finally, the cuticular hair covering is generally denser in E. antarcticus , typically with hairs in seven to eight rows, as opposed to only five rows in E. peterseni , and the latter species also lacks patches of hair-like extensions in middorsal and paraventral positions, instead having regular bracteate hairs.

Thus, with help from these relatively subtle characters, it is possible to distinguish the two species. It is, however, still striking to observe two species with an antipodean distribution being so similar and putatively closely related. This clearly suggests that kinorhynch relationships are not always reflected in their biogeography.

The distribution of glandular cell outlets type 1 in E. antarcticus sp. nov. follows the MD Seg. 1–3, 5, 7, PD 4, 6, 8–9 pattern, as is the case with the all congeners described above.

Figures

Fig. 19. Line art illustrations of Echinoderes antarcticus sp. nov. A. Female, dorsal view. B. Female, ventral view.

Fig. 20. Light micrographs showing overviews and details of Echinoderes antarcticus sp. nov., female, holotype (NHMD 1786932). A. Dorsal overview. B. Segments 1 to 4, dorsal view. C. Segments 1 to 4, ventral view. D. Segments 5 to 7, dorsal view. E. Segments 5 to 7, ventral view. F. Segments 8 to 10, dorsal view. G. Segments 8 to 10, ventral view. H. Detail showing lateroventral positions on segments 8 to 9. I. Segments 10 (posterior part) to 11, dorsal view. J. Segments 9 to 11, dorsal view. K. Segments 9 to 11, ventral view. Scale bars: A = 50 µm; B–K = 20 µm.

Fig. 21. Scanning electron micrographs showing overviews and details of femaleEchinoderes antarcticus sp. nov. A. Right lateral overview. B. Ventral overview. C. Segments 1 to 2, right laterodorsal view. D. Segments 1 to 2, ventral view. E. Segments 4 to 6, right subdorsal view. F. Segments 5 to 6, right lateral view. G. Segments 5 to 7, ventral view. H. Segments 9 to 11, right subdorsal view. I. Segments 7 to 8, right lateral view. J. Segments 10 to 11, ventral view. Scale bars: A–B = 100 µm; C–H = 20 µm; I–J = 10 µm.

Fig. 1. Map showing the sampling stations. A. Overview of Antarctica, with the Antarctic Peninsula framed. Previous records of Polacanthoderes martinezi Sørensen, 2008 (green dots) and P. shiraseae Yamasaki et al., 2022 (red dots) are marked. B. Antarctic Peninsula with sampling area framed. C. Sampling area with station numbers and stations indicated as yellow dots. Study sites in Andvord Bay refer to OBA – Outer Basin A; MBA – Middle Basin A; IBB – Inner Basin B. Only stations yielding adult kinorhynch specimens are included.