Polacanthoderes grzelakae, 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 : 20-31

Polacanthoderes grzelakae sp. nov urn:lsid:zoobank.org:act:3966DEF9-01C9-4294-AAFB-DCDB56648067

Figs 5–8 , Tables 7–8

Diagnosis

Polacanthoderes with regular acicular spines in middorsal positions on segments 4 to 8, and in lateroventral positions on segments 6 to 9; lateroventral spines on segments 8 (in particular) and 9 are conspicuously stronger than other acicular spines, and the one on segment 8 is the strongest. Tubes are present in subdorsal and ventrolateral positions on segment 2, midlateral positions on segment 4, lateroventral positions on segment 5, and laterodorsal positions on segment 10. Small acicular spines are present in the following series: subdorsal positions on segments 4 and 5 (but missing in some specimens), laterodorsal positions on segments 6 to 8, midlateral positions on segments 5 to 9, sublateral positions on segment 7, lateral accessory positions on segments 6 and 8 to 9, ventrolateral positions on segments 8 to 10, and in ventromedial positions on segments 4 to 7. Glandular cell outlets type 2 are present in midlateral positions on segments 6 and 8. Dorsal glandular cell outlets type 1 are present in middorsal positions on segments 1 to 3 and 10, and in paradorsal positions on segments 4 to 9. Small sieve plates present on segment 9 in sublateral positions.

Etymology

The species is dedicated to Katarzyna Grzelak – a fabulous kinorhynch taxonomist and meiofauna ecologist.

Material examined

Holotype ANTARCTICA • ♂ (mounted for LM in Fluoromount G on HS slide); Antarctic Peninsula, CRS 1706 ; 64°50.47′ S, 62°35.12′ W; 499 m b.s.l.; 1 Dec. 2015; FjordEco1; soft sediment; NHMD 1784249 .

Paratypes

ANTARCTICA – Antarctic Peninsula • 1♀ (mounted as holotype); CRS 1769; 64°52.37′ S, 62°25.27′ W; 547 m b.s.l.; 5 Apr. 2016; FjordEco2; soft sediment; USNM 1740027 • 1 ♂ (mounted as holotype); CRS 1773; 64°52.35′ S, 62°25.88′ W; 553 m b.s.l.; 6 Apr. 2016; FjordEco2; soft sediment; NHMD 1784250 • 1 ♂ (mounted as holotype); CRS 1776; 64°52.53′ S, 62°33.90′ W; 551 m b.s.l.; 7 Apr. 2016; FjordEco2; soft sediment; USNM 1740028 • 3 ♀♀ (mounted as holotype); CRS 1793; 64°39.53′ S, 62°55.03′ W; 701 m b.s.l.; 11 Apr. 2016; FjordEco2; soft sediment; NHMD 1784251 , 1784284 , 1784303 • 2 ♂♂, 1 ♀ (mounted as holotype); CRS 1809; 64°39.59′ S, 62°55.09′ W; 694 m b.s.l.; 15 Apr. 2016; FjordEco2; soft sediment; NHMD 1784304 to 1784306 .

Additional material

ANTARCTICA – Antarctic Peninsula • 1 ♀ (mounted for SEM); same data as for holotype; MVS • 1 ♂ (mounted for SEM); CRS 1776; 64°52.53′ S, 62°33.90′ W; 551 m b.s.l.; 7 Apr. 2016; FjordEco2; soft sediment; MVS • 4 ♂♂, 1 ♀ (mounted for SEM); CRS 1793; 64°39.53′ S, 62°55.03′ W; 701 m b.s.l.; 11 Apr. 2016; FjordEco2; soft sediment; MVS • 3 ♂♂, 1 ♀ (mounted for SEM); CRS 1809; 64°39.59′ S, 62°55.09′ W; 694 m b.s.l.; 15 Apr. 2016; FjordEco2; soft sediment; MVS • 4 ♂♂, 5 ♀♀ (mounted for SEM); 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 5A–B , 6 , 7A , 8B ). The species is large for an Echinoderidae , 467 to 488 µm in trunk length, and the segments are completely devoid of regular, cuticular hairs. An overview of measurements and dimensions is given in Table 7. Distributions of cuticular structures, i.e., sensory spots, glandular cell outlets, spines, and tubes, are summarized in Table 8.

HEAD. Consists of a retractable mouth cone and an introvert ( Figs 6 , 8A ). Inner oral styles of mouth cone are arranged in three rings: 10 styles are present in the outermost ring and 5 in the following. The innermost ring could not be examined. The external mouth cone armature consists of nine outer oral styles; bases of outer oral styles each flanked by a transverse fringe row consisting of very short spikes and a V-shaped row with considerably longer tips.

INTROVERT. The sectors are defined by the ten primary spinoscalids in Ring 01. Each primary spinoscalid consists of a basal sheath and a distal end piece with a blunt tip. The sheaths have, described from proximal towards distal parts: a transverse fringe with ca 10 long fringe tips; a slightly more distal fringe with four, terminally bifurcated fringe tips; and numerous short fringe tips along the distal margin of the sheath. End pieces are flexible and smooth. Rings 02 and 04 have 10 spinoscalids, and Rings 03 and 05 have 20 spinoscalids. All spinoscalids in these rings are well-developed, and consist of a basal sheath and a pointed end piece. Ring 06 has only 6 spinoscalids, located in sectors 1, 3, 5, 6, 7, and 9; they resemble those in preceding sectors, but without the distinct differentiation into sheath and end piece. Ring 07 has 8 spinoscalids, located as pairs in sectors 1, 3, and 9, and unpaired but laterally displaced in sectors 5 and 7 (trichoscalids take up the space in the opposite side of each sector); ring 07 spinoscalids appear very simplified and resemble thin fringes rather than actual scalids ( Figs 6 , 8A ). Described sector-wise ( Fig. 6 ), sectors 1, 3, and 9 are similar, having spinoscalids arranged as two double diamonds anterior to an additional pair of Ring 07 spinoscalids. Sectors 2, 4, 8, and 10 all have spinoscalids arranged as a quincunx, located in between an anterior spinoscalid in Ring 02 and a trichoscalid plate. Sectors 5 and 7 have spinoscalids forming double diamonds, anterior to an unpaired, lateral spinoscalid; the lateral spinoscalid is unpaired because a trichoscalid plate takes up the space in the opposite side of the sector. Sector 6 has its trichoscalids arranged as double diamonds. Regular trichoscalids with trichoscalid plates are present in sectors 2, 4, 5, 7, 8, and 10.

NECK. Consists of 16 placids. Midventral placid broadest, 17 µm in width and 18 µm in length, whereas all others are narrower, measuring 9 µm in width at their bases ( Fig. 7B–C ). The trichoscalid plates are well-developed.

SEGMENT 1. Consists of a complete cuticular ring. Sensory spots are present in subdorsal, laterodorsal sublateral, and ventromedial positions; sensory spots on this and following segments are small, slightly depressed into the cuticle, rounded, and composed of a central pore surrounded by a few micropapillae. Glandular cell outlets type 1 are present in middorsal and ventromedial positions. Posterior segment margin with very fine denticles in middorsal to midlateral positions and hardly any denticles or fringe tips at all in sublateral to ventromedial positions; ventromedial to midventral positions though with 11 to 13 strongly developed, dagger-shaped fringe tips ( Figs 5A–B , 7B–C , 8C–D ).

SEGMENT 2. As remaining segments, consists of a tergal and two sternal plates. Tubes are present in subdorsal and ventrolateral positions. Sensory spots are present in middorsal position, as two pairs in laterodorsal positions and in ventromedial positions. Glandular cell outlets type 1 are present in middorsal position and ventromedial positions; ventromedial outlets are located close, but anterior to the ventromedial sensory spots. The posterior segment margin is straight, with small denticles along the tergal plate and slightly longer but also thinner fringe tips along the margins of the sternal plates ( Figs 5A–B , 7B–C , 8C–D ).

SEGMENT 3. With sensory spots present in subdorsal, laterodorsal, midlateral and ventromedial positions, and glandular cell outlets type 1 in middorsal and ventromedial positions. A single specimen had small acicular spines in ventromedial positions. Posterior segment margin as on preceding segment ( Figs 5A–B , 7C , 8C–E ).

SEGMENT 4. With regular, acicular spine in middorsal position and small acicular spines in subdorsal and ventromedial positions; small subdorsal acicular spines were missing in five specimens. Tubes present midlateral positions; tubes missing in one specimen. Sensory spots present in subdorsal and ventromedial positions; subdorsal sensory spots slightly more lateral than subdorsal acicular spines.

Glandular cell outlets type 1 present in paradorsal and ventromedial positions. Posterior segment margin as on preceding segment ( Figs 5A–B , 7B–C , 8D–F ).

SEGMENT 5. With regular, acicular spine in middorsal position and small acicular spines in subdorsal, midlateral, and ventromedial positions; small subdorsal acicular spines were missing in five specimens. Tubes present lateroventral positions. Sensory spots present in subdorsal, midlateral, and ventromedial positions; subdorsal sensory spots slightly more lateral than subdorsal acicular spines, as on preceding segment. Glandular cell outlets type 1 present in paradorsal and ventromedial positions. Posterior segment margin as on preceding segment ( Figs 5A–B , 7D–E , 8D–F ).

SEGMENT 6. With regular, acicular spines in middorsal and lateroventral positions, and small acicular spines in laterodorsal, midlateral, lateral accessory, and ventromedial positions; one specimen had an extra set of laterodorsal small acicular spines, whereas another specimen lacked short lateral accessory acicular spines. Well-developed glandular cell outlets type 2, located in midlateral positions, slightly posterior to the short acicular spines. Females with ventromedial female papillae, forming small funnel-shaped intracuticular structures. Sensory spots present in paradorsal, laterodorsal and ventromedial positions; laterodorsal sensory spots slightly more dorsal than laterodorsal acicular spines. Glandular cell outlets type 1 present in paradorsal and ventromedial positions. Posterior segment margin as on preceding segment ( Figs 5A–B, D , 7D–E, H , 8F, H–I ).

SEGMENT 7. With regular, acicular spines in middorsal and lateroventral positions, and small acicular spines in laterodorsal, midlateral, sublateral and ventromedial positions; one specimen also with a pair of small acicular spines in lateral accessory positions. Females with ventrolateral female papillae, forming small funnel-shaped intracuticular structures. Sensory spots present in paradorsal, laterodorsal, midlateral, and ventromedial positions. Glandular cell outlets type 1 and posterior segment margin as on preceding segment ( Figs 5A–B, D , 7F–H , 8F, H–I ).

SEGMENT 8. With regular, acicular spines in middorsal and lateroventral positions, and small acicular spines in laterodorsal, midlateral, lateral accessory, and ventrolateral positions; lateroventral acicular spines are conspicuously strong on this segment and nearly twice as thick as the lateroventral spines on the two preceding segments; small laterodorsal acicular spines were missing in two specimens. Well-developed glandular cell outlets type 2, located in midlateral positions, slightly posterior to the short acicular spines. Sensory spots present in paradorsal, laterodorsal, and ventrolateral positions. Glandular cell outlets type 1 and posterior segment margin as on preceding segment ( Figs 5A–B , 7F–G, J , 8G, I ).

SEGMENT 9. With regular, acicular spines in lateroventral positions and small acicular spines in midlateral, lateral accessory and ventrolateral positions; lateroventral spine considerably stronger than those in same positions on segments 6 and 7, although not as strong as the spine on segment 8. Small rounded sieve plates present in sublateral positions. Sensory spots present in paradorsal, subdorsal, midlateral, and ventrolateral positions. Glandular cell outlets type 1 and posterior segment margin as on preceding segment ( Figs 5A–B , 7I–J , 8G ,).

SEGMENT 10. With laterodorsal tubes located near, but not at, posterior segment margin and small acicular spines in ventrolateral positions. Sensory spots present in subdorsal and ventrolateral positions. Glandular cell outlets type 1 present as two longitudinally arranged outlets in middorsal position and a pair in ventromedial positions. Posterior segment margin with thin but longer fringe tips between the laterodorsal tubes and along the concave margins of the sternal plates ( Figs 5A–C , 7I, K , 8J–K ).

SEGMENT 11. With lateral terminal spines. Females with thin lateral terminal accessory spines; males with three pairs of conspicuously long penile spines; especially the flexible and pointed dorsal and ventral pair are long, occasionally exceeding 100 µm, whereas the truncate and slightly more rigid median pair is shorter, around 54 µm. Sensory spots present in subdorsal positions, on inferior margins of tergal extensions. Glandular cell outlets type 1 were not observed. As is the case with all preceding segments, regular cuticular hairs are absent, but patches of very short triangular hairs are present on the sternal extensions. Tergal and sternal extensions are triangular with fine marginal fringes; tergal extensions are slightly longer than the sternal ones. A pair of rigid setae attaches on the outer lateral margins of the tergal extensions ( Figs 5A–C , 7K–M , 8J–K ).

Distribution

Antarctic Peninsula: Gerlache Strait and Andvord Bay MBA and IBB, 499 to 701 m b.s.l. See Fig. 1 for geographic overview of stations and Table 1 for station and specimen information.

Polymorphism

The species shows a relatively high level of polymorphism, expressed in the relatively frequent absence of small acicular spines and, less frequently, in the presence of additional small acicular spines. It is not possible to provide a complete overview of the polymorphism in the 30 examined specimens of P. grzelakae sp. nov., because some specimens were damaged and others (SEM specimens) were mounted in a way that prevented observation of all relevant characters. It is, however, still possible to get some indications. Out of the 30 specimens, eight showed a confirmed, identical distribution of tubes and spines (except for the sexually dimorphic ones). The two most frequent ‘abnormalities’ were the lack of small acicular spines in subdorsal positions, on either segment 4 or 5. Interestingly, the spines were never missing on both segments in the same specimen. Five specimens (paratype NHMD 1784250 from stn 1773, two SEM specimens from stn 1793, and two from stn 1832) were lacking subdorsal spines on segment 5, but showed otherwise no variation from the most common spine pattern. Another five (SEM specimens from stn 1706, 1793, 1809 and 1832) lacked subdorsal spines on segment 4, but only one of these (a male from stn 1793) showed additional variation by also lacking small acicular spines in midlateral positions of segment 4. Two SEM specimens from stn 1832 lacked small acicular spines in laterodorsal positions on segment 8, but otherwise all remaining observed variation was restricted to three specimens with their own unique spine combinations, expressed either as lack of spines or presence of additional spines. For example, one specimen (paratype USNM 1740027 from stn 1769) had a set of small acicular spines in ventromedial positions on segment 3, although such spines were absent in all other specimens, and another (SEM specimen from stn 1909) was missing small acicular spines in lateral accessory positions on segment 6, but had instead double pairs of small acicular spines in laterodorsal positions on this segment. A third specimen (SEM specimen from stn 1793) followed the most common spine pattern for P. grzelakae , but had in addition – as the only Polacanthoderes in all samples – small acicular spines in lateral accessory positions on segment 7. The seven remaining specimens also appeared to follow the common pattern but had, due to their condition or mounting orientation, characters that could not be confirmed visually.

Diagnostic remarks on Polacanthoderes grzelakae sp. nov.

With the addition of P. grzelakae sp. nov., Polacanthoderes now accommodates three species that all are restricted to the Antarctic continent ( Sørensen 2008a; Yamasaki et al. 2022). The genus and its first described species, Polacanthoderes martinezi Sørensen, 2008 , was described from the South Shetland Islands ( Sørensen 2008a), and a phylogenetic analysis supported that Polacanthoderes represents a separate evolutionary linage within Echinoderidae ( Sørensen 2008b) . One of the characters that makes Polacanthoderes stand out from other echinoderids is the numerous small acicular spines in rather unusual positions, i.e., subdorsal, laterodorsal, midlateral, and ventromedial ( Sørensen 2008a).

Some years later, Yamasaki et al. (2022) described the second species of the genus, P. shiraseae . The species was described from Lützow-Holm Bay, Cape Damley, and near Totten Glacier, i.e., from Antarctic areas that geographically are pretty much opposite to the Antarctic Peninsula and South Shetland Island. Besides contributing with a new species, Yamasaki et al. (2022) also provided a redescription of P. martinezi and, in addition, shed light on the exceptionally high level of morphological variation within the two species (see following section for further discussion of polymorphism in species of Polacanthoderes ). In a group like Echinoderidae , where much of the taxonomy traditionally has been based on the presence and position of spines and tubes, such variation can obviously lead to some taxonomic challenges. Yet, Yamasaki et al. (2022) explained how P. martinezi and P. shiraseae fairly easily could be distinguished by the conspicuously stronger lateroventral spines on segments 8 and 9 in P. shiraseae , and by the position of short acicular spines on segment 7, which appear in lateral accessory positions in P. martinezi and in sublateral positions in P. shiraseae . The latter character might seem like a very subtle alteration, open for subjective interpretation, but when observed the difference is in fact very distinct, and since both species otherwise have a series of short lateral accessory acicular spines on segments 6 to 9, it is easy to detect the sublateral displacement of the spines on segment 7 in P. shiraseae .

The new species, P. grzelakae sp. nov., very clearly shares most characters with P. shiraseae , including the strong lateroventral spines on segments 8 and 9, and the sublateral short acicular spines on segment 7. Thus, P. grzelakae is easily distinguished from P. martinezi . The characters that separate P. grzelakae from both congeners are the presence of subdorsal and ventrolateral tubes on segment 2. In the present study, a total of 38 specimens of Polacanthoderes were examined. Out of these, four specimens had no tubes on segment 2 and were thus identified as P. shiraseae . Thirty other specimens had both subdorsal and ventrolateral tubes on segment 2, and were assigned to P. grzelakae . Of the remaining four specimens, three had only subdorsal tubes on segment 2, whereas a single specimen had only ventrolateral tubes.

In light of the known morphological variation within species of Polacanthoderes (and without access to molecular barcoding data), it is obviously not straightforward to determine whether P. grzelakae sp. nov. is a distinct species or a morphological variation of P. shiraseae . However, a strong argument favours the first option. Recently, Anguas-Escalante et al. (2023) demonstrated with molecular barcoding that the presence or absence of tubes on segment 2 in otherwise very similar species of Echinoderes actually should be seen as species diagnostic. In P. grzelakae the species diagnostic trait is exactly expressed as the presence of tubes on segment 2, and these tubes are consistently present within the type series of the species, whereas the morphological variation we observe is mostly expressed in the presence/absence of short acicular spines in the dorsal series on segments 4, 5, and 8 (see below for further discussion of this).

Based on these arguments, we propose P. grzelakae sp. nov. as a new species showing close resemblance with P. shiraseae , but distinguished by the presence of subdorsal and ventrolateral tubes on segment 2. This proposal seems fair, but it is admittedly challenged by the four remaining specimens, Polacanthoderes sp. 1 and P. sp. 2, that show variation in their tubes on segment 2. With tubes only in either subdorsal or ventrolateral positions on segment 2, the four specimens fall in between the two species, and the number of potential explanations are too numerous to allow a final conclusion. The two morphotypes could represent the bridging between P. shiraseae and P. grzelakae , suggesting that they are all conspecific; they could represent another two distinct species; or they could be results of hybridisation between P. shiraseae and P. grzelakae . It would require molecular barcoding to solve this question, and any other conclusion at this point would be nothing but speculation. However, based on the arguments above, we find it justified to consider P. grzelakae as a new, easily distinguishable species of Polacanthoderes .

The distribution pattern of glandular cell outlets type 1 in P. grzelakae sp. nov. follows the MD Seg. 1–3, PD 4–9 pattern (see table with summary of species with this pattern described up to 2020 in Sørensen et al. 2020). The same pattern is present in both Polacanthoderes congeners ( Sørensen 2008a; Yamasaki et al. 2022).

Figures

Fig. 5. Line art illustrations of Polacanthoderes grzelakae sp. nov. A. Male, dorsal view. B. Male, ventral view. C. Female, segments 10 to 11, dorsal view, with lateral terminal spines drawn to full length. D. Female, segments 6 to 7, ventral view.

Fig. 6. Diagram of mouth cone (grey area), introvert, and placids in Polacanthoderes grzelakae sp. nov., showing distribution of inner oral styles (closed dots), outer oral styles, scalids, and trichoscalids. Table shows the scalid arrangement by sector (S1 to S10); single-lined boxes mark quincunxes, double-lined boxes mark “double diamonds”.

Fig. 7. Light micrographs showing overviews and details of Polacanthoderes grzelakae sp. nov. A–G, I–J, L–M. Male, holotype (NHMD 1784249). H, K. Female, paratype (NHMD 1784284). A. Ventral overview. B. Segments 1 to 3, dorsal view. C. Segments 1 to 3, ventral view. D. Segments 5 to 6, dorsal view. E. Segments 5 to 6, ventral view. F. Segments 7 to 8, dorsal view. G. Segments 7 to 8, ventral view. H. Tergal plates on segments 6 to 7, showing female sexual dimorphism. I. Segments 9 to 10, dorsal view. J. Segments 8 to 9, ventral view. K. Segments 10 to 11, ventral view, showing female sexual dimorphism. L. Segment 11, dorsal view, showing male sexual dimorphism. M. Segment 11, ventral view, showing male sexual dimorphism. Scale bars: A = 100 µm; B–M = 25 µm.

Fig. 8. Scanning electron micrographs showing overviews and details of Polacanthoderes grzelakae sp. nov.A. Introvert focused on the middorsal sector 6.B. Lateroventral overview of female.C. Segments 1 to 2, right lateral view. D. Segments 1 to 3, ventrolateral view. E. Segments 3 to 5, subdorsal view. F. Segments 4 to 6, right lateral view. G. Segments 8 to 9, ventrolateral view. H. Segments 8 to 9, subdorsal view. I. Segments 6 to 8, right lateral view. J. Segments 10 to 11, right lateral view, showing male sexual dimorphism. K. Segments 10 to 11, dorsocaudal view, showing female sexual dimorphism. Scale bars: A, C–K = 20 µm; B = 100 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.