Echinoderes aff. angustus Higgins & Kristensen, 1988

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 : 73-79

Echinoderes aff. angustus Higgins & Kristensen, 1988

Figs 25–26 , Tables 19–20

Material examined

ANTARCTICA – Antarctic Peninsula • 1 ♂ (mounted for LM in Fluoromount G on HS slide); CRS 1698; 64°51.60′ S, 62°33.80′ W; 541 m b.s.l.; 28 Nov. 2015; FjordEco 1; soft sediment; NHMD 1790691 • 1 ♀ (mounted for SEM); CRS 1702; 64°51.15′ S, 62°34.44′ W; 502 m b.s.l.; 30 Nov. 2015; FjordEco 1; soft sediment; MVS • 1 ♂ (mounted for SEM); CRS 1773; 64°52.35′ S, 62°25.88′ W; 553 m b.s.l.; 6 Apr. 2016; FjordEco 2; soft sediment; MVS • 1 ♂ (mounted for SEM); CRS 1776; 64°52.53′ S, 62°33.90′ W; 551 m b.s.l.; 7 Apr. 2016; FjordEco 2; soft sediment; MVS • 1 ♀ (mounted for LM in Fluoromount G on HS slide); CRS 1792; 64°51.40′ S, 62°34.01′ W; 525 m b.s.l.; 11 Apr. 2016; FjordEco 2; soft sediment; NHMD 1790692 • 1 ♂, 1 ♀ (mounted for SEM); CRS 1793; 64°39.53′ S, 62°55.03′ W; 701 m b.s.l.; 11 Apr. 2016; FjordEco 2; soft sediment; MVS • 1 ♂ (mounted for LM in Fluoromount G on HS slide); CRS 1809; 64°39.59′ S, 62°55.09′ W; 694 m b.s.l.; 15 Apr. 2016; FjordEco 2; soft sediment; NHMD 1790693 • 2 ♂♂ (mounted for SEM); CRS 1809; 64°39.59′ S, 62°55.09′ W; 694 m b.s.l.; 15 Apr. 2016; FjordEco 2; soft sediment; MVS .

Concise description

Except when clearly specified, the following concise description applies to both the Antarctic population of Echinoderes aff. angustus and to the Arctic type material, as well as supplementary specimens of E. angustus mounted for SEM.

GENERAL. An overview of measurements and dimensions is given in Table 19. Distributions of cuticular structures, i.e., sensory spots, glandular cell outlets, spines and tubes, are summarized in Table 20.

SEGMENT 1. Consists of a complete cuticular ring. Sensory spots are present in subdorsal, laterodorsal, and ventromedial positions; sensory spots are minute, and consist of relatively few, very short micropapillae arranged around two pores. Glandular cell outlets type 1 are present in middorsal and lateroventral positions. Cuticular hairs are arranged in three to four rows: anterior two to three rows are present only on the dorsal side, between midlateral positions, whereas the posteriormost row extends around the entire segment. The posterior segment margin is straight and terminates in a pectinate fringe with broad and well-developed slender fringe tips; fringe tips on ventral side are slightly longer than those on the lateral and dorsal sides ( Figs 25A–D , 26A–C ).

SEGMENT 2. Consists of a complete cuticular ring. However, a partially developed, midventral fissure is visible in the Arctic type specimens of E. angustus ( Fig. 25F ). Indications of such a fissure were never observed in any of the Antarctic specimens ( Fig. 25E ). Glandular cell outlets type 2 are present in subdorsal, laterodorsal, sublateral, and ventrolateral positions. Sensory spots are present in middorsal, laterodorsal, midlateral, and ventromedial positions; the micropapillary areas around the sensory spots on this, and all following segments, are even smaller than those on segment 1 and form a slightly oval ring around a single pore; one or two long and rigid hairs (extremely extended micropapillae?) stick out from the micropapillary area. Glandular cell outlets type 1 are present in middorsal and ventromedial positions. Fairly long bracteate cuticular hairs are arranged in three to four transverse rows on the dorsal and lateral sides; ventromedial and paraventral areas without hairs. The posterior segment margin is straight, terminating in uniform, well-developed fringe tips ( Figs 25A–F , 26A–C ).

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. Cuticular hairs present in four rows on the tergal and lateral halves of the sternal plates, except in hair-less midlateral areas; paraventral areas and most ventral parts of ventromedial areas completely free of hairs and hair-like structures. Posterior segment margin straight and pectinate fringe as on preceding segment ( Figs 25A–B , 26A–B, D ).

SEGMENT 4. With spine in middorsal position. Glandular cell outlets type 2 are present in subdorsal positions. Sensory spots are not present. Glandular cell outlets type 1 are present in paradorsal and ventromedial positions. Cuticular hairs as on preceding segment, but now arranged in five rows. Posterior segment margin and pectinate fringe as on preceding segment ( Figs 25A–B , 26A–B, D ).

SEGMENT 5. With spine in middorsal position and tubes in lateroventral positions. Glandular cell outlets type 2 are present in midlateral positions. Sensory spots present in subdorsal and ventromedial positions, and glandular cell outlets type 1 in paradorsal and ventromedial positions. Cuticular hairs, posterior segment margin, and pectinate fringe as on preceding segment ( Figs 25G–L , 26E–G ).

SEGMENT 6. With spines in middorsal and lateroventral positions. Sensory spots present in paradorsal, subdorsal, midlateral, and ventromedial positions; ventromedial sensory spots situated closer to midventral articulation than those on preceding segment. Glandular cell outlets type 1 present in paradorsal and ventromedial positions. Cuticular hairs, posterior segment margin, and pectinate fringe as on preceding segment ( Figs 25G–J , 26E–G ).

SEGMENT 7. With spines in middorsal and lateroventral positions. Sensory spots present in paradorsal, midlateral, and ventromedial positions; ventromedial sensory spots situated more lateral than those on preceding segment and aligned with those on segment 5. Glandular cell outlets type 1 present in paradorsal and ventromedial positions. Cuticular hairs, posterior segment margin, and pectinate fringe as on preceding segment ( Figs 25G–J , 26E–I ).

SEGMENT 8. With spines in middorsal and lateroventral positions. Glandular cell outlets type 2 are present in sublateral 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 the midlateral hairless areas have moved to more laterodorsal positions, and middorsal to paradorsal positions are also devoid of hairs. Posterior segment margin and pectinate fringe as on preceding segment ( Figs 25G–J, M–N , 26G–I ).

SEGMENT 9. With spines in lateroventral positions. Sensory spots present in paradorsal, subdorsal, midlateral, and ventrolateral positions.Glandular cell outlets type 1present in paradorsal and ventromedial positions. Small rounded sieve plates located in sublateral positions. Cuticular hairs, posterior segment margin, and pectinate fringe as on preceding segment ( Figs 25O–P, R–S , 26H–J ).

SEGMENT 10. With glandular cell outlets type 2 in laterodorsal positions near posterior segment margin. Sensory spots present in subdorsal and ventrolateral positions. Glandular cell outlets type 1 present as two longitudinally arranged outlets in middorsal position and in ventromedial positions. Cuticular hair covering reduced to a few (4 to 6) hairs in subdorsal positions and otherwise only hairs in laterodorsal to ventromedial areas. The posterior segment margin of the tergal plate is straight, with minute fringe tips. Sternal plate margins oblique, with longer fringe tips in ventromedial and paraventral areas ( Figs 25O, Q, R–S, U , 26J–M ).

SEGMENT 11. With lateral terminal spines and a middorsal protuberance emerging from the intersegmental zone between segments 10 and 11. The segment appears to consist of two tergal and two sternal plates. The visualisation of the middorsal fissure between the tergal plates easily gets obscured by the overlaying protuberance, but the presence of the fissure was nevertheless confirmed in both Antarctic ( Fig. 25T ) and Arctic ( Fig. 25U ) specimens. Females with lateral terminal accessory spines; males with thin, tubular dorsal and ventral penile spines; medial pair of penile spines cone-shaped and well-developed. Males in addition with short tubes inserted near bases of dorsal and medial penile spines; the tubes resemble an extra set of penile spines, but are much shorter, roughly 5 µm estimated from SEM; due to their minute size and the numerous other structures in this area, the tubes are difficult to visualise with LM. The presence of these tubes is only confirmed for the Antarctic specimens. Sensory spots present in paradorsal positions only. The segment is devoid of cuticular hairs, but has scattered hair-like extensions over the tergal extensions. Tergal extensions are elongate triangular, with pointed tips. Sternal extensions short, broadly triangular, and not extending beyond tergal extensions ( Figs 25R–U , 26K–M ).

Distribution

Antarctic Peninsula: Gerlache Strait and Andvord Bay MBA and IBB, 502 to 701 m b.s.l. See Fig. 1 for geographic overview of stations and Table 1 for station and specimen information. Possibly also South Orkney Trench, 5 251 m b.s.l. (see Sánchez et al. 2024).

Diagnostic remarks

Echinoderes angustus was described from Disko Island in West Greenland ( Higgins & Kristensen 1988). It has subsequently been the subject of redescriptions and additional important notes have been added to the diagnosis of the species. Grzelak & Sørensen (2018) documented the presence of glandular cell outlets type 2 in subdorsal, laterodorsal, sublateral, and ventrolateral positions on segment 2, in subdorsal positions on segment 4, in midlateral positions on segment 5, and (very likely) in laterodorsal positions on segment 10. Shortly after, Herranz et al. (2018) reported the presence of a partial midventral fissure in segment 2 and noted that E. angustus is most easily distinguished from the highly similar species E. pennaki by its longer (about 20%) middorsal and laterodorsal spines. Re-examinations of type material and fresh material collected close to the type locality was carried out during the present study. They confirm these observations, add a complete mapping of sensory spots ( Table 20) in E. angustus , and confirm the presence of laterodorsal type 2 outlets on segment 10. In addition, the examinations revealed that the species has a middorsal protuberance projecting from the intersegmental zone between segments 10 and 11, and that the tergal plate of the terminal segment is split into two halves by a middorsal fissure. Visualisation of this middorsal fissure is often obscured by the overlaying protuberance.

Since its discovery, E. angustus has also been recorded from the Barents Sea and the fjords of Svalbard, where it is one of the most frequently observed species ( Grzelak & Sørensen 2019a, 2019b).This suggests that the species’ distributional range at least covers the Arctic extension of the Atlantic Ocean. In a recent study of kinorhynchs in the Subantarctic South Orkney Trench, not too distant from the Antarctic Peninsula, Sánchez et al. (2024) surprisingly found a specimen that almost matched the morphology of E. angustus . Its distribution of spines and glandular cell outlets type 2 matched the emended diagnoses provided by Grzelak & Sørensen (2018) and Herranz et al. (2018), and the sensory spot distribution largely followed the pattern reported in the present study. The only two notable differences regarded its longer lateral terminal spines and the absence of a partial midventral fissure on segment 2. Based on the great level of similarity, but also taking the minor differences and the considerable geographic distance into account, Sánchez et al. (2024) reluctantly reported the species as Echinoderes cf. angustus .

Echinoderes aff. angustus , recorded in the present study, fits the Subantarctic specimen reported by Sánchez et al. (2024): its morphology is highly similar to that of E. angustus , but its lateral terminal spines are slightly longer, and there is no indication of a midventral fissure on segment 2. The difference in spine lengths is less pronounced though. Whereas the lateral terminal spines of the Subantarctic female specimen of Sánchez et al. (2024) measured 223 µm, unlike the only 140–180 µm in the female E. angustus types ( Higgins & Kristensen 1988), the lengths of these spines in the Antarctic specimen are only an extension of the size range in the male types (140–198 µm in male types vs 197–214 µm in male Antarctic specimens). The Antarctic specimens are certainly conspecific with the South Orkney Trench specimen reported by Sánchez et al. (2024); this is supported by morphology, morphometrics and geography. The open question is obviously whether these specimens can be considered as conspecific with E. angustus . The lateral terminal spine lengths can hardly be used as an argument to separate the Antarctic specimens from E. angustus , and even though the missing midventral fissure on segment 2 could indicate that they represent a distinct species, it is hard to accept this difference as the sole diagnostic character. The only other potential differential character is the short set of midlateral tubes found in male specimens on their terminal segment. Since these tubes are only visible with SEM, the type material of E. angustus is not useful to confirm or reject the presence of such tubes in E. angustus . However, a single male E. angustus specimen from Svalbard did not seem to have such tubes; thus, their presence in the Antarctic specimens could support that this is a different species.

The geographic distance between the Arctic and (Sub-)Antarctic populations obviously also speaks in favour of considering them as two different species, but we have recently seen indications of other species with a potential bipolar distribution. For instance, the high Arctic, north Atlantic, and Mediterranean species Echinoderes pterus has also been reported from the Atacama Trench, off Chile ( Yamasaki et al. 2018a; Grzelak et al. 2021), and Grzelak & Sørensen (2022) reported specimens from New Zealand showing a close resemblance to Echinoderes beringiensis , which, as the name indicates, was described from the Bering Strait ( Adrianov & Maiorova, 2022) (see also the following section for an additional discussion of this species). With these indications of potential bipolar kinorhynch distributions taken into account, we cannot rule out that E. angustus could also be present in both the Arctic and Antarctic. Thus, the only fair conclusion at this stage seems to be that it would require comparison of molecular barcodes to solve the question. It is intriguing, though, to experience how we keep observing extremely wide distributions of conspecific populations, or at least very closely related species.

Figures

Fig. 25. Comparative light micrographs showing details of Echinoderes aff. angustus Higgins & Kristensen, 1988 from the Antarctic Peninsula (first and third column from the left) and holo- and paratypes of E. angustus (second and fourth column from the left). A, C, E, G, I, K, M, O, R, T. Male Echinoderes aff. angustus (NHMD 1790692). B, D, H, J, P–Q, S. Female, holotype of E. angustus (USNM 233200). F, L, N. Female, paratype of E. angustus (NHMD 99369). U. Male, paratype of E. angustus (NHMD 99368). A–B. Segments 1 to 4, dorsal view. C–D. Segments 1 to 2, ventral view. E–F. Segment 2, ventral view, focused at intracuticular level. G–H. Segments 5 to 8, dorsal view. I–J. Segments 5 to 8, ventral view. K–L. Segment 5, focused at midlateral level. M–N. Segment 8, focused at sublateral level. O. Segments 9 to 10, dorsal view. P. Segment 9, dorsal view. Q. Segment 10, dorsal view. R–S. Segments 9 to 11, ventral view. T–U. Segment 11, focused at midtergal fissure. Scale bars: A–B, G–N, R–S = 30 µm; C–F, O–Q, T–U= 20 µm.

Fig. 26. Scanning electron micrographs showing overviews and details of Echinoderes aff. angustus Higgins & Kristensen, 1988. A. Segments 1 to 4, dorsal view. B. Segments 1 to 4, left lateral view. C. Segments 1 to 2, ventral view. D. Segments 3 to 4, ventral view. E. Segments 4 to 6, dorsal view. F. Segments 4 to 6, left lateral view. G. Segments 5 to 8, ventral view. H. Segments 7 to 9, ventral view. I. Segments 7 to 9, left lateral view. J. Segments 9 to 10, ventral view.K. Segments 10 to 11, dorsal view, showing female sexual dimorphism. L. Segments 10 (posterior part) to 11, left lateral view, showing male sexual dimorphism. M. Segments 10 to 11, ventral view, showing male sexual dimorphism. Scale bars: A–C, E–I = 20 µm; D, J–K, M = 10 µm; L = 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.