Aphelochaeta abyssorum (Hansen, 1879) comb. nov.

Figs 10–16

Cirratulus (?) abyssorum Hansen, 1879: 10, pl. VII fig. 1.

Cirratulus abyssorum – Hartman 1959: 402.

Diagnosis

Body long and slender, segments never beaded, anterior dorso-ventrally flattened; three peristomial rings, dorsal crest present, pigmentation absent; latero-ventral bands anteriorly with MB.

Type material

Lectotype

NORWEGIAN SEA • anterior fragment; Vøringen; 64.2° N, 5.35° E; depth 911 m; 1876–1878; N.N.H.E. stn 87; clay; ZMBN 1970.

Paralectotypes

NORWEGIAN SEA • 7 intermediate fragments; same data as for lectotype; ZMBN 1970.

Other material examined

ARCTIC OCEAN • 1 spec.; 81.89659° N, 9.85533° E; depth 834 m; 8 Jun. 2017; collected with box corer; R/V Polarstern sample PS 106-1; fixed in 96% ethanol; DNA voucher: MG751; no repository (specimen used for SEM) .

GREENLAND SEA • 1 spec.; 67.63266° N, 26.76650° W; depth 316 m; 14 Sep. 2011; collected with CliSAP sledge; IceAGE sample M 85-3 1136; fixed in 96% ethanol; SMF 32821 • 1 spec.; 67.21383° N, 26.22516° W; depth 697 m; 14 Sep. 2011; collected with CliSAP sledge; sample IceAGE M85-3 1119; fixed in 96% ethanol; SMF 32810, 32856 • 1 spec.; 67.83700° N, 23.70200° W; depth 1241 m; 15 Sep. 2011; collected with box corer; IceAGE sample M85-3 1142; fixed in 96% ethanol; SMF 32857 • 3 specs; 67.63800° N, 26.75466° W; depth 318 m; 14 Sep. 2011; collected with epibenthic sledge; IceAGE sample M85-3 1132; fixed in 96% ethanol; SMF 32871, 32872, 32808 .

NORTH ATLANTIC • 1 spec.; 63.33333° N, 23.16666° W; depth 305 m; 4 Sep. 2011; collected with box corer; sample IceAGE M85-3 1031; fixed in 96% ethanol; SMF 32839 .

NORWEGIAN SEA • 1 spec.; 63.38933° N, 8.157° W; depth 687 m; 31 Jul. 2013; collected with brenke sledge; IceAGE 2 sample POS456 880; fixed in 96% ethanol; DNA voucher: MG597; no repository (specimen used for SEM) • 1 spec.; 61.77600° N, 3.87333° W; depth 834 m; 28 Jul. 2013; collected with grab; IceAGE 2 sample POS456 880; fixed in 96% ethanol; SMF 32822 • 2 specs; 62.75533° N, 0.89900° W; depth 1571 m; 26 Jul. 2013; collected with grab; sample IceAGE 2 POS456 871; fixed in 96% ethanol; SMF 32829, 32835 • 1 spec.; 66.30100° N, 12.37333° W; depth 732 m; 22 Sep. 2011; collected with box corer; IceAGE sample M85-3 1217; fixed in 96% ethanol; SMF 32836 • 1 spec.; 62.20866° N, 0.26200° E; depth 669 m; 25 Jul. 2013; collected with box corer; IceAGE sample POS456 868; fixed in 96% ethanol; DNA voucher: MG779; no repository (specimen used for SEM) • 1 spec.; 66.53816° N, 12.86483° W; depth 316 m; 22 Sep. 2011; collected with RP sledge; IceAGE sample M85-3 1209; fixed in 96% ethanol; SMF 32837 • 1 spec.; 67.07866° N, 13.06383° W; depth 1575 m; 21 Sep. 2011; IceAGE sample M85-3 1191; fixed in 96% ethanol; SMF 32838 • 1 spec.; 63.57766° N, 7.74650° W; depth 1043 m; 1 Aug. 2013; collected with epibenthic sledge; IceAGE 2 sample POS456 881; fixed in 96% ethanol; SMF 32867 • 12 specs; 63.09366° N, 8.57200° W; depth 511 m; 31 Jul. 2013; DZMB exped; collected with epibenthic sledge; IceAGE 2 sample POS456 879; fixed in 96% ethanol; SMF 32823, 32806, 32873, 32874, 32887, 32848 to 32854 • 1 spec.; same data as for preceding; DNA voucher: MG1055; no repository (specimen used for SEM) • 26 specs; 63.41733° N, 10.96633° W; depth 441 m; 2 Aug. 2013; collected with epibenthic sledge; IceAGE 2 sample POS456 882; fixed in 96% ethanol; SMF 32855, 32812, 32840 to 32845, 32814 to 32820, 32824 to 32828, 32875, 32830, 32831, 32858 to 32860 • 1 spec.; same data as for preceding; DNA voucher: MG738; no repository (specimen used for SEM) • 1 spec.; same data as for preceding; DNA voucher: MG839; no repository (specimen used for SEM) • 1 spec.; same data as for preceding; DNA voucher: MG950; no repository (specimen used for SEM) • 1 spec.; same data as for preceding; DNA voucher: MG953; no repository (specimen used for SEM) • 1 spec.; same data as for preceding; DNA voucher: MG765; no repository (specimen used for SEM) • 1 spec.; 63,64533° N, 7,7858°3 W; depth 1080 m; 1 Aug. 2013; collected with grab; sample IceAGE 2 sample POS456 881; fixed in 96% ethanol; SMF 32861 .

Description

Mostly based on lectotype and 10 newly collected complete specimens. Lectotype ovigerous female in eight fragments including anterior and posterior end for about 30 mm, up to 1 mm wide (Fig. 10). Ten complete specimens 13 to 20 mm long, 0.5 to 0.9 mm wide for 87 to 139 segments (Fig. 11A–F). Colour in ethanol light tan or cream to light grey, venter often lighter than dorsum. Body elongate; short anterior region distinct, flattened, nearly twice as wide as midbody; midbody segments cylindrical, never longer than wide or high, never beaded; short posterior region enlarged, tapering towards pygidium. Anterior 22–27 segments 12–14 times as wide as and 4–9 times as high as long, oval in cross section. Midbody segments progressively lengthening to 2–3 times as wide and high as long, round in cross section, with distinct, thin and shallow transversal intersegmental grooves, never beaded or moniliform. Posterior 20–50 segments 5–8 times as wide as and 3–6 times as high as long, oval to flattened in cross section, sometimes distinctly enlarged over anteriormost segments and tapering towards pygidium. A thin, shallow dorsal groove sometimes present over posterior enlarged region. A wide, low ventral ridge present along entire body (Figs 12A, 15D).

Prostomium one third to half as long as peristomium, broad, conical, without rings; eyespots absent; nuchal organs simple slits on posterolateral margins (Figs 12A–C, 13A–C). Peristomium as long as 6–7 anterior segments, generally longer than wide, with three distinct rings visible laterally and ventrally, incomplete over dorsum due to broad, elongate dorsal crest, overlapping chaetiger 1 between dorsal tentacles, often higher than first 2–4 chaetigers (Figs 12A–C, 13A–C). Dorsal tentacles arising from posterior margin of peristomium, medial to parapodium of chaetiger 1, well separated. First pair of branchiae arising lateral to dorsal tentacles on anterior margin of chaetiger 1 (Figs 12A–C, 13A–C). Second pair of branchiae arising from chaetiger 1, directly above and slightly posterior to parapodia (Figs 12B–C, 13A–C). Subsequent branchiae similarly placed, present on nearly all anterior chaetigers, occasionally along first half of body, and rarely along posterior half.

Parapodia biramous, low mounds to inconspicuous, placed high over anterior region forming distinct shoulders (Figs 12B–C, 13A–C), shifting to a medium position in midbody (Fig. 14A–B) and a ventral position along posterior enlarged region (Fig. 12D). Chaetae all capillaries 7–10 per neuropodium, 16– 19 per notopodium, typically arranged on two rows; neurochaetae short, 1–2 per neuropodium as long as notochaetae in midbody, basally strongly curved upwards in anterior segments, gradually straightening but retaining a light curvature throughout, covered in fine, short, dense scales on concave side made by protruding fibrils; notochaetae 1–2 times as long as body width in anterior and midbody, directed nearly upwards anteriorly, shifting laterally in midbody, covered in long, pointy, flat scales along one side (Fig. 14C–F); posterior neuro- and notochaetae short.

Pygidium with terminal anus, reduced with large rounded ventral lobe and 5 small dorsal lobes (Fig. 15E).

Methylene blue

Prostomium and peristomium retain a light stain without any particular pattern, although the dorsal crest is generally of a lighter colour and some longitudinal lines stay unstained. Distinct latero-ventral bands are present from segment 10–13 and fade in midbody where only faint lines of dark dots remain. More anterior segments retain a light stain laterally and ventrally over the posterior half of each segment. Bands of dark dots can be present dorsally over anterior segments (Fig. 15A–C).

Distribution and habitat

Aphelochaeta abyssorum comb. nov. is widely distributed at shelf and slope depths (300–1500 m deep) from the Arctic Ocean around Svalbard to the Norwegian Sea and Denmark Strait (Fig, 16).

Remarks

Hansen (1879) reported 3 specimens from the same station (N.N.H.E stn 87) in the original description of C. abyssorum . The type material for this taxon (ZMBN 1970) has been located in the collections of the University Museum of Bergen (Oug et al. 2014). However, the sample ZMBN 1870 includes only a single anterior fragment and 7 midbody fragments. The anterior fragment is in close agreement with the original description given by Hansen (1879, 1882) and is here designated the lectotype. The remaining midbody fragments may belong to other specimens and are considered paralectotypes.

In some specimens, the posterior end is not particularly enlarged. In smaller specimens, the anterior part is not always much wider, but nearly always flatter and with the notopodium forming distinct shoulders. In the smallest fragments (possibly juveniles) the prostomium is as long as the peristomium, the peristomial rings are not as distinct and the prechaetiger area is overall more elongated and cylindrical.

The genus Aphelochaeta is one of the least studied genera among the bitentaculate Cirratulidae . Some species have been studied in the Pacific (Blake 1991, 2018, 2019; Dean & Blake 2016), the Southern Ocean (Blake 2023) and to a small extent the West Atlantic (Elías & Rivero 2009; Blake & Dean 2019). However, these studies mostly concern species living in coastal waters or abyssal depths and very little is known about species living at shelf and slope depths. In the North-East Atlantic, very little is known about the genus in general. Only a few species have been described, all from coastal waters. In addition, as was the case in the present study, most of these descriptions are old, inadequate and need to be updated. Therefore, diversity of the genus Aphelochaeta in the area has never been properly surveyed and assessed. A revision of the genus in Europe is necessary to both properly re-describe known species and identify potential new species.