Echinoderes crux, 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 : 66-73

Echinoderes crux sp. nov.

urn:lsid:zoobank.org:act:7F3ECD36-A974-404A-9476-D1A3435F29D3

Figs 22–24 , Tables 17–18

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 lateroventral positions on segment 5. Minute slit-like openings (glandular cell outlets type 2?) present near the anterior margin of segment 1 in subdorsal (two pairs), laterodorsal, sublateral, and ventromedial positions. Numerous glandular cell outlets type 2 are present throughout segments 2 to 9: paradorsal positions on segment 3; subdorsal positions on segments 2 to 9 (two pairs on segments 2 and 9); laterodorsal positions on segments 2 to 9 (two pairs on segment 2); sublateral positions on segments 2 to 3 and 5 to 9 (two pairs on segments 2 and 8); lateral accessory positions on segments 3 and 5 to 7; lateroventral positions on segment 4; and ventrolateral positions on segment 2. Dorsal glandular cell outlets type 1 are present at least in middorsal positions on segments 1, 3, 5, 7, and 10, and in paradorsal positions on segments 4, 6, 8, and 9. Female papillae not present; male morphology unknown. Sieve plates present on segment 9 in lateral accessory positions.

Etymology

The species is named ‘ crux ’, after the constellation Crux, also known as the Southern Cross.

Material examined

Holotype ANTARCTICA • ♀ (mounted for SEM); Antarctic Peninsula, CRS 1760; 64°47.86′ S, 65°21.09′ W; 593 m b.s.l.; 21 Dec. 2015; FjordEco1; soft sediment; NHMD 1790632 .

Paratype

ANTARCTICA • 1 ♀ (mounted for SEM); same data as for holotype; NHMD 1790633 .

Description

GENERAL. Adults with head, neck and eleven trunk segments ( Figs 22A–B , 23A–C ). The species is characterised by numerous series of minute glandular cell outlets type 2. An overview of measurements and dimensions estimated from SEM is given in Table 17. Distributions of cuticular structures, i.e., sensory spots, glandular cell outlets, spines, and tubes, are summarized in Table 18.

HEAD. The two available specimens both had their heads fully retracted; thus, information on head morphology is not available.

NECK. Consists of 16 placids. Midventral placid broadest, 9 µm in width and 10 µm in length, whereas all others are narrower, measuring 6 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 and laterodorsal positions; the sensory spots are large, rounded, with numerous micropapillae around two pores; seven to eight long cuticular hairs emerge from the margin of the micropapillary area. ‘Slit-like openings’ are located near the anterior segment margin in subdorsal (two pairs), laterodorsal, sublateral and ventromedial positions; the openings are perfectly round rather than slit-like, but the term ‘slit-like openings’ is chosen to stress their clear homology with the corresponding structures in Echinoderes aragorni Grzelak & Sørensen, 2022 . Glandular cell outlets type 1 are present in middorsal and lateroventral positions. Besides the cuticular hairs around the margins of the sensory spots, the segment is completely devoid of hairs. The posterior segment margin is straight along the dorsal and lateral sides, but has a broadly rounded ventral extension. The fringe tips are long and have a broader basis that narrows abruptly into a slender distal tip ( Figs 22A–B , 23D–F ).

SEGMENT 2. Consists of a complete cuticular ring. Glandular cell outlets type 2 are present in subdorsal (two pairs), laterodorsal (two pairs), sublateral (two pairs), and ventrolateral (one pair) positions; the outlets, on this and following segments, are minute (> 1 µm in diameter) and have a collar of fringes projecting from the openings. Sensory spots present in middorsal, laterodorsal, and ventromedial positions; the micropapillary areas around the sensory spots on this, and all following segments, are rounded, and longer micropapillae extend from the posterior part of the papillated areas. Glandular cell outlets type 1 are present in ventromedial positions; the most anterior part of the middorsal line could not be examined in any of the specimens; thus, type 1 outlets could potentially be present there as well. Bracteate cuticular hairs are arranged in three transverse rows; hairs in the first anterior row are short, whereas those of the more posterior rows are fairly long. The posterior segment margin is almost straight. Pectinate fringe with well-developed, long, triangular fringe tips along all margins ( Figs 22A–B , 23D–F ).

SEGMENT 3. As following seven segments, consisting of one tergal and two sternal plates. Glandular cell outlets type 2 are present in paradorsal, subdorsal, laterodorsal, sublateral, and lateral accessory positions. 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 22A–B , 23G–I ).

SEGMENT 4. With spine in middorsal position. Glandular cell outlets type 2 are present in subdorsal, laterodorsal, and lateroventral positions. Sensory spots are not present. Glandular cell outlets type 1 are present in paradorsal and ventromedial positions. Cuticular hairs, posterior segment margin, and pectinate fringe as on preceding segment ( Figs 22A–B , 23G–I ).

SEGMENT 5. With tubes in lateroventral positions. Glandular cell outlets type 2 are present in subdorsal, laterodorsal, sublateral, and lateral accessory 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 22A–B , 24A–C ).

SEGMENT 6. With spines in middorsal and lateroventral positions. Glandular cell outlets type 2 as on preceding segment. 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, but arranged in additional rows; middorsal area without hairs. Posterior segment margin and pectinate fringe as on preceding segment ( Figs 22A–B , 24A–C ).

SEGMENT 7. With spines in lateroventral positions. Glandular cell outlets type 2 and sensory spots as on preceding segment. Glandular cell outlets type 1 present in middorsal and ventromedial positions. Cuticular hairs as on proceeding segment, but with middorsal hairless areas now also reaching the paradorsal positions. Posterior segment margin and pectinate fringe as on preceding segment ( Figs 22A–B , 24D–F ).

SEGMENT 8. With spines in middorsal and lateroventral positions. Glandular cell outlets type 2 are present in subdorsal, laterodorsal, and sublateral (two pairs) positions. Sensory spots present in paradorsal positions only, and glandular cell outlets type 1 in paradorsal and ventromedial positions. Cuticular hairs as on preceding segment, but hairless dorsal area also reaches into the subdorsal positions and has a covering of hair-like extensions. Posterior segment margin and pectinate fringe as on preceding segment ( Figs 22A–B , 24F–H ).

SEGMENT 9. With spines in lateroventral positions. Glandular cell outlets type 2 are present in subdorsal (two pairs), laterodorsal, and sublateral positions. Sensory spots present in paradorsal, subdorsal, laterodorsal, and ventrolateral positions. Glandular cell outlets type 1 present in paradorsal and ventromedial positions. Very minute sieve plates present in lateral accessory positions, at base of spine. 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 well into the subdorsal areas. Posterior segment margin and pectinate fringe as on preceding segment ( Figs 22A–B , 24I–L ).

SEGMENT 10. Without spines, tubes, or glandular cell outlets type 2. Male morphology, and thus potential presence of tubes, is so far unknown. Sensory spots present in subdorsal and ventrolateral positions. Glandular cell outlet type 1 present as a single outlet in middorsal position and as a pair 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 between the sensory spots and as a distinct, oblique row below the laterodorsal cuticular hairs; paralateral areas without any kind of hairs or 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 22A–B , 24L–O ).

SEGMENT 11. With lateral terminal- and lateral terminal accessory (assumed female dimorphic) spines. Sensory spots present in ventromedial positions, at margins of sternal extensions. The segment is devoid of cuticular hairs, but has a dense covering of minute cuticular hair-like structures on tergal and sternal extensions. Tergal extensions are triangular and sternal extensions rounded, with fringed margins ( Figs 22A–B , 24M–O ).

Distribution

Antarctic Peninsula: Open continental shelf off the Antarctic Peninsula, 596 m b.s.l. See Fig. 1 for geographic overview of stations and Table 1 for station and specimen information.

Diagnostic remarks

We would usually hesitate to describe a species based only on SEM specimens. First of all, the SEM specimens deteriorate much faster than specimens mounted for LM, which shortens the available time span for re-examining the type material. Second, certain internal or intracuticular structures, such as pachycycli are difficult to visualise with SEM. However, we still chose to describe E. crux sp. nov. because of its very distinct morphology and clear phylogenetic affinities within the genus.

The most distinct character trait in E. crux sp. nov. is the numerous series of minute glandular cell outlets type 2, which sums up to 35 pairs in total. Such a morphology is only shared with a single congener, E. aragorni . Males of E. aragorni also have exactly 35 pairs of glandular cell outlets type 2, whereas the females have 34 ( Grzelak & Sørensen 2022). Besides the type 2 outlets on segments 2 to 9, they also share the presence of openings near the anterior margin of segment 1. In E. aragorni these openings are elongate, which is why Grzelak & Sørensen (2022) referred to them as ‘slit-like openings’. In E. crux the openings are perfectly round, but for the sake of terminological consistency, and in order to stress the homology between the structures in the two species, we also choose to refer to them as slit-like openings in E. crux . Grzelak & Sørensen (2022) discussed whether the openings on segment 1 should also be seen as a variation of glandular cell outlets type 2. It would require a comparative study of histological sections to confirm or reject this suggestion, and suitable material for a study like this is currently not available. However, we find it quite likely that there is a homology between the openings on segment 1, and the type 2 outlets on the following segments.

Thus, based on the presence of these openings on segment 1 and the numerous type 2 outlets on the following eight segments, the close relationship between the two species is undisputable. They are, however, still easily distinguished from each other. The most distinct difference between the two species is the lack of lateroventral spines on segments 6 and 7 in E. aragorni . In addition, there are several differences in the distribution of slit-like openings and glandular cell outlets type 2. Besides minor (interpretive) differences in longitudinal positioning, E. crux sp. nov. differs from E. aragorni by having one additional pair of slit-like openings on segment 1, and by having seven pairs of type 2 outlets on segment 2, unlike the ‘only’ five pairs in E. aragorni . In contrast, E. aragorni has four pairs of type 2 outlets on segment 4, whereas E. crux only has three. The number and approximate position of type 2 outlets on the remaining segments is the same for the two species and seems to be conserved. Males of E. aragorni have an additional pair of type 2 outlets in laterodorsal positions of segment 10, but since no males are yet available for E. crux , it is not possible to confirm whether this species shows the same sexual dimorphism.

The mapping of glandular cell outlets type 1 in the dorsal series of E. crux sp. nov. is not complete, but besides the uncertain appearance of a middorsal type 1 outlets on segment 2, the observed pattern fits the very common MD Seg. 1–3, 5, 7, PD 4, 6, 8–9 pattern, which is also present in most other species of the present study. The mapping of dorsal type 1 outlets in E. aragorni is very incomplete, but it is noteworthy that both E. aragorni and E. crux have only a single middorsal type 1 outlet on segment 10, unlike the otherwise extremely common presence of two, longitudinally aligned middorsal outlets.

Figures

Fig. 22. Line art illustrations of Echinoderes crux sp. nov. A. Female, dorsal view. B. Female, ventral view.

Fig. 23. Scanning electron micrographs showing overviews and details of anterior segments in female Echinoderes crux sp. nov. A. Female, paratype (NHMD 1790633). B–I. Female, holotype (NHMD 1790632). A. Dorsal overview. B. Right lateral overview. C. Lateroventral overview. D. Segments 1 to 2, dorsal view. E. Segments 1 to 2, right lateral view. F. Segments 1 to 2, ventrolateral view. G. Segments 3 to 4, dorsal view. H. Segments 3 to 4, right lateral view. I. Segments 3 to 4, ventral view. Scale bars: A–C = 50 µm; D–I = 10 µm.

Fig. 24. Scanning electron micrographs showing details of posterior segments in female, holotype (NHMD 1790632) of Echinoderes crux sp. nov. A. Segments 5 to 6, subdorsal view. B. Segments 5 to 6, right lateral view. C. Segments 5 to 6, ventral view. D. Segment 7, dorsal view. E. Segment 7, right lateral view. F. Segments 7 to 8, ventral view. G. Segment 8, dorsal view. H. Segment 8, right lateral view. I. Close-up showing sublateral to lateroventral details on segment 9. J. Segment 9, dorsal view. K. Segment 9, right lateral view. L. Segments 9 to 10, ventral view. M. Segments 10 to 11, dorsal view. N. Segments 10 to 11, right lateral view. O. Segments 10 to 11, ventral view. Scale bars: A–H, J–O = 10 µm; I = 5 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.