Chaetocirratulus abranchiatus Grosse & Alvestad & Nygren & Kongsrud 2025, comb. nov.
Creators: name: Grosse, Maël
name: Alvestad, Tom
name: Nygren, Arne
name: Kongsrud, Jon Anders
Publisher: name: Zenodo
Year: 2025
Subjects: Biodiversity
Taxonomy
Animalia
Annelida
Polychaeta
Terebellida
Cirratulidae
Chaetocirratulus
Chaetocirratulus abranchiatus

description: Chaetocirratulus abranchiatus (Hansen, 1879) comb. nov.
Figs 3–9
Cirratulus (?) abranchiatus Hansen, 1879: 10, pl. VII figs 3–7.
Chaetozone abranchiata – Hartman 1959: 401.
Diagnosis
In adults, short anterior region distinctively narrower than following segments; prostomium broadly triangular, three rings visible dorsally, two ventrally, dorsal crest absent; wide ventral ridge.
Type material
Neotype (here designated)
NORWEGIAN SEA • 64.10522° N, 5.72433° E; depth 636 m; 27 Jun. 2007; collected with RP sledge; MAREANO sample R932-75, RP; fixed in 96% ethanol; ZMBN 136930.
Other material examined
ARCTIC OCEAN • 4 specs; 81.60734° N, 33.40031° E; depth 518 m; 27 Jun. 2022; collected with grab; MAREANO sample R2915-55, GR; fixed in 96% ethanol; DNA voucher: MG1126; ZMBN 149490 • 1 spec.; 81.34727° N, 21.99029° E; depth 458 m; 2 Jul. 2022; collected with grab; MAREANO sample R2963-79, GR; fixed in 96% ethanol; ZMBN 149491 • 1 spec.; 81.41596° N, 21.67928° E; depth 704 m; 5 Jul. 2022; collected with grab; MAREANO sample R2983-90, GR; fixed in 96% ethanol; ZMBN 149492.
BARENTS SEA • 1 spec.; 71.92061° N, 20.38130° E; depth 349 m; 1 Jun. 2019; MetaMon project; collected with grab; sample GE-09; fixed in 96% ethanol; ZMBN 136914.
GREENLAND SEA – Denmark Strait • 4 specs; 67.86783° N, 23.69633° W; depth 1281 m; 15 Sep. 2011; collected with RP sledge; IceAGE sample M85-3 1144; fixed in 96% ethanol; SMF 32846, 32847, 32832, 32833 • 1 spec.; same data as for preceding; DNA voucher: MG810; no repository (specimen used for SEM) • 2 specs; 67.21383° N, 26.22516° W; depth 700 m; 14 Sep. 2011; collected with CliSAP sledge; IceAGE sample M85-3 1119; fixed in 96% ethanol; SMF 32876, 32877 • 2 specs; 67.837° N, 23.702° W; depth 1241 m; 15 Sep. 2011; collected with box corer; IceAGE sample M85-3 1142; fixed in 96% ethanol; SMF 32809, 32863 • 1 spec.; same data as for preceding; DNA voucher: MG978; no repository (specimen used for SEM) • 3 specs; 67.8465° N, 23.696° W; depth 1249 m; 15 Sep. 2011; collected with epibenthic sledge; IceAGE sample M85-3 1148; fixed in 96% ethanol; SMF 32864 to 32866 • 1 spec.; same data as for preceding; DNA voucher: MG984; no repository (specimen used for SEM) • 2 specs; 67.84° N, 23.696° W; depth 1250 m; 15 Sep. 2011; collected with epibenthic sledge; sample IceAGE M85-3 1148; fixed in 96% ethanol; SMF 32884, 32885 • 1 spec.; 67.86783° N, 23.69616° W; depth 1270 m; 15 Sep. 2011; collected with epibenthic sledge; IceAGE sample M85-3 1144; fixed in 96% ethanol; SMF 32886 • 1 spec.; 67.21383° N, 26.2075° W; depth 716 m; 14 Sep. 2011; collected with RP sledge; IceAGE sample M85-3 1123; fixed in 96% ethanol; SMF 32834. – Fram Strait • 1 spec.; 79.03412° N, 7.26576° E; depth 1275 m; 27 Oct. 2019; collected with grab; MAREANO sample R2099-2155, GR; fixed in 96% ethanol; ZMBN 136920 • 1 spec.; 79.1385° N, 6.08217° E; depth 1282 m; 10 Jul. 2017; Hausgarten project; collected with UNSEL box corer; fixed in 96% ethanol; ZMBN 144610 • 1 spec.; 78.9888° N, 5.432° E; depth 995 m; 2 Jul. 2016; Hausgarten project; fixed in 96% ethanol; ZMBN 130912 • 1 spec.; 79.08467° N, 6.45138° E; depth 1235 m; 29 Sep. 2019; collected with grab; MAREANO sample R2107-2163, GR; fixed in 96% ethanol; ZMBN 144613 • 1 spec.; 79.10622° N, 6.15371° E; depth 1235 m; 29 Sep. 2019; collected with grab; MAREANO sample R2108-115, GR; fixed in 96% ethanol; mounted on SEM-stub; ZMBN 144614 • 12 specs; 79.11608° N, 6.15363° E; depth 1229 m; 29 Sep. 2019; collected with beam trawl; MAREANO sample R2108-18, BT; fixed in 96% ethanol; ZMBN 144615 • 5 specs; 79.10649° N, 6.15297° E; depth 1262 m; 29 Sep. 2019; collected with RP sledge; MAREANO sample R2108-21, RP; fixed in 96% ethanol; DNA vouchers: MG1173, MG1174; ZMBN 144616 • 1 spec.; same data as for preceding; mounted on SEMstub; ZMBN 144616 -SEM • 12 specs; 79.10189° N, 6.15166° E; depth 1238 m; 30 Sep. 2019; collected with RP sledge; MAREANO sample R2108-22, RP; fixed in 96% ethanol; ZMBN 144617 • 8 specs; 79.10372° N, 6.15409° E; depth 1236 m; 30 Sep. 2019; collected with RP sledge; MAREANO sample R2108-23, RP; fixed in 96% ethanol; DNA voucher: MG1172; ZMBN 144618, 144619 • 1 spec.; same data as for preceding; mounted on SEM-stub; ZMBN 144618 -SEM • 1 spec.; 78.98620° N, 7.24162° E; depth 1232 m; 30 Sep. 2019; collected with grab; MAREANO sample R2114-2170, GR; fixed in 96% ethanol; ZMBN 144620.
NORWEGIAN SEA • 3 specs; 63.41033° N, 8.187° W; depth 687 m; 31 Jul. 2913; collected with epibenthic sledge; IceAGE 2 sample POS456 880; fixed in 96% ethanol; SMF 32868 to 32870 • 5 specs; 63.09366° N, 8.572° W; depth 500 m; 31 Jul. 2013; collected with epibenthic sledge; IceAGE 2 sample POS456 879; fixed in 96% ethanol; SMF 32881 to 32883, 32862, 32811 • 1 spec.; 63.57766° N, 7.7115° W; depth 1044 m; 1 Aug. 2013; collected with brenke sledge; IceAGE 2 POS456 881; fixed in 96% ethanol; SMF 32803 • 1 spec.; 66.84333° N, 7.8945° E; depth 735 m; 2 Sep. 2015; collected with RP sledge; IceAGE 2 sample R1569-179, RP; fixed in 96% ethanol; ZMBN 120504 • 1 spec.; 64.10311° N, 5.73214° E; depth 626 m; 27 Jun. 2013; collected with beam trawl; MAREANO sample R932-454, BT; fixed in 96% ethanol; ZMBN 144621 • 4 specs; 62.553° N, 0.981° E; depth 800 m; 16 Aug. 1981; R/V “ H. Mosby ” sample HM81.08.16.7; fixed in formaldehyde and transferred to 75% ethanol; ZMBN 144594 • 3 specs; 62.491° N, 1.721° E; depth 604 m; 21 Jan. 1982; R/V “ H. Mosby ” sample HM82.01.21.2; fixed in formaldehyde and transferred to 75% ethanol; ZMBN 144595 • 4 specs; 62.56° N, 0.981° E; depth 804 m; 21 Jan. 1982; R/V “ H. Mosby ” sample HM82.01.21.4; fixed in formaldehyde and transferred to 75% ethanol; ZMBN 144596 • 2 specs; 62.198° N, 0.003° W; depth 708 m; 2 Jun. 1983; R/V “ H. Mosby ” sample HM83.06.02.1; fixed in formaldehyde and transferred to 75% ethanol; ZMBN 144597 • 10 specs; 61.343° N, 3.185° W; depth 1338 m; 3 Jun. 1983; R/V “ H. Mosby ” sample HM83.06.03.1; fixed in formaldehyde and transferred to 75% ethanol; ZMBN 144598 • 1 spec.; 65.168° N, 9.493° W; depth 784 m; 8 Jun. 1983; R/V “ H. Mosby ” sample HM83.06.08.1; fixed in formaldehyde and transferred to 75% ethanol; ZMBN 144599 • 11 specs; 62.593° N, 1.233° E; depth 781 m; 17 Jun. 1983; R/V “ H. Mosby ” sample HM83.06.17.3; fixed in formaldehyde and transferred to 75% ethanol; ZMBN 144600 • 2 specs; 62.585° N, 1.793° E; depth 656 m; 23 May 1984; R/V “ H. Mosby ” sample HM84.05.23.1; fixed in formaldehyde and transferred to 75% ethanol; ZMBN 144601 • 20 specs; 62.508° N, 1.851° E; depth 576 m; 23 May 1984; R/V “ H. Mosby ” collected with RP sledge; sample HM84.05.23.3; fixed in formaldehyde and transferred to 75% ethanol; ZMBN 144602 • 5 specs; 62.603° N, 2.233° E; depth 576 m; 23 May 1984; Brattegards exped.; collected with RP sledge; sample HM84.05.23.5; fixed in formaldehyde and transferred to 75% ethanol; ZMBN 144603 • 1 spec.; 62.553° N, 1.82° E; depth 652 m; 21 Nov. 1984; R/V “ H. Mosby ” collected with RP sledge; sample HM84.11.21.2; fixed in formaldehyde and transferred to 75% ethanol; ZMBN 144604 • 3 specs; 62.706° N, 1.186° E; depth 897 m; 8 Nov. 1885; R/V “ H. Mosby ” collected with RP sledge; sample HM85.01.08.2; fixed in formaldehyde and transferred to 75% ethanol; ZMBN 144605 • 2 specs; 63.045° N, 7.028° W; depth 1022 m; 13 Jun. 1986; R/V “ H. Mosby ” collected with RP sledge; sample HM86.06.13.4; fixed in formaldehyde and transferred to 75% ethanol; ZMBN 144606 • 9 specs; 62.948° N, 7.002° W; depth 748 m; 13 Jun. 1986; R/V “ H. Mosby ” collected with D sledge; sample HM86.06.13.5; fixed in formaldehyde and transferred to 75% ethanol; ZMBN 144607 • 1 spec.; 62.855° N, 5.698° W; depth 750 m; 16 Jun. 1986; R/V “ H. Mosby ” collected with D sledge; sample HM86.06.16.1; fixed in formaldehyde and transferred to 75% ethanol; ZMBN 144608 • 2 specs; 62.61° N, 1.573° E; depth 654 m; 15 Jun. 1986; R/V “ H. Mosby ” collected with RP sledge; sample HM86.08.15.5; fixed in formaldehyde and transferred to 75% ethanol; ZMBN 144609 • 1 spec.; 63.59067° N, 5.57533° E; depth 763 m; 20 Jun. 2014; collected with beam trawl; MAREANO sample R1349-274, BT; fixed in 96% ethanol; ZMBN 144611 • 1 spec.; 66.84067° N, 7.88867° E; depth 734 m; 2 Sep. 2015; collected with beam trawl; MAREANO sample R1569-553, BT; fixed in 96% ethanol; ZMBN 144612 • 1 spec.; 63.59067° N, 5.57533° E; depth 763 m; 20 Jun. 2014; collected with beam trawl; MAREANO sample R1349-274, BT; fixed in 96% ethanol; ZMBN 162355 • 1 spec.; 68.28796° N, 10.70476° E; depth 689 m; 28 Apr. 2012; collected with beam trawl; MAREANO sample R763-001; fixed in 96% ethanol; ZMBN 162356 • 1 spec.; 67.59864° N, 9.31061° E; depth 918 m; 5 May 2012; collected with RP sledge; MAREANO sample R818-9, RP; fixed in 96% ethanol; ZMBN 162357 • 4 specs; 62.42324° N, 1.20725° E; depth 654 m; 2 May 2021; collected with grab; MAREANO sample R2531-101, GR; fixed in 96% ethanol; DNA voucher: BeMG457; ZMBN 162348 • 2 specs; 64.74521° N, 5.19276° E; depth 707 m; 9 May 2021; collected with grab; MAREANO sample R2574-216, GR; fixed in 96% ethanol; DNA voucher: BeMG460; ZMBN 162349 • 1 spec.; 64.73235° N, 5.41943° E; depth 631 m; 9 May 2021; collected with grab; MAREANO sample R2577-220, GR; fixed in 96% ethanol; DNA voucher: BeMG458; ZMBN 162350.
Description
Neotype incomplete, 12 mm long, 2.5 mm wide, 50 segments (Fig. 3). Other complete specimens range from 3 to 21 mm long, 0.5 to 5 mm wide and 34 to 59 segments (Fig. 4A–N). Colour in ethanol light tan to light grey, old formalin fixed specimens with light pink colour. Body short and stout, of uniform width and height abruptly tapering at both extremities or with a distinct enlarged anterior half and posterior tail; anterior up to twice as wide as posterior, tapering towards pygidium (Fig. 4A–N). In larger specimens, first 6–8 segments not enlarged, slightly wider than pre-chaetiger area, and approximately as wide as tall. Smaller specimens often round in cross section, larger specimen oval to dorso-ventrally flattened. Specimens fixed in tubes with much thinner shape, with constant width (Fig. 4M). Dorsal groove or ridge nearly always absent, rarely a thin dorsal groove anteriorly. A wide ventral ridge made from segmental pads along entire body.
Prostomium half as long as peristomium, broadly triangular, without rings; eyespots absent; nuchal organs simple slits at posterior lateral margin (Figs 5A–E, 6A–C). Peristomium as long as four anterior chaetigers, with three distinct rings, anterior one longer with prominent rounded dorsum slightly overlapping prostomium anteriorly, anterior and middle rings fused ventrally with, middle and posterior rings each slightly wider and as long as anterior chaetigers (Figs 5A–E, 6A–C). Dorsal tentacles arising between last peristomial ring and chaetiger 1, well separated (Figs 5A–B, D–E, 6A–C). First pair of branchiae arising between last peristomial ring and chaetiger 1, lateral to tentacles (Figs 5A–B, D–E, 6A–C). Second pair of branchiae arising from chaetiger 1, dorsal and slightly posterior to parapodia. Subsequent branchiae similarly placed, branchiae or branchial stubs present on all chaetiger on enlarged anterior half (Fig. 6A, D), absent from posterior region (Figs 6E, 7A).
Parapodia biramous, low mounds or ridges distinct in larger specimens. Capillary chaetae 4–6 per neuropodium; 5–10 per notopodium, arranged in two rows; 2–4 capillary chaetae per neuropodium and notopodium are longer than body width in anterior region, each finely serrated along one edge (Fig. 7C–E). Short capillary chaetae progressively transition to thicker chaetae to 2–3 straight spines per neuropodium and notopodium in posterior half, with 2–4 alternating capillary chaetae, long, straight and slender (Fig. 7F).
Pygidium with terminal anus and short, rounded ventral lobe (Fig. 7A–B).
Methylene blue
Use of methylene blue results in variable patterns. All specimens retain a uniform light blue colour, branchiae retain a dark colour usually accompanied by dark blue spots spread at least over the proximal half. On some specimens sparse to very dense dark blue spots can be observed on the prechaetigerous area as well as on the first 5–6 parapodia.
Biology
Some specimens have been observed partially inside sandy tubes (Fig. 4). This phenomenon has previously been observed in Chaetocirratulus gayheadius (Hartman, 1965) where the tubes were assumed to be made by C. gayheadius which was considered unusual among cirratulids where tubes, when present, are usually muddy (Blake 2022). However, when examining bulk samples, such tubes were found in various sizes and occupied by a variety of animals and seem more likely to be made by some other organism such as Foraminifera d’Orbigny, 1826 and opportunistically occupied or temporarily crawled through by C. abranchiatus comb. nov.
A few of the larger specimens were found with eggs in the body cavity. While numerous examples of asexual reproduction and budding were found in C. gayheadius (Blake 2022), none was observed in the material examined for C. abranchiatus comb. nov.
Distribution
This species is distributed all around the North-East Atlantic on continental shelves and slopes (Fig. 8).
Comparative remarks
So far, Chaetocirratulus abranchiatus comb. nov. is the only member of its genus recorded from the North-East Atlantic. There are several other species occurring in the North-West Atlantic. Among them, Chaetocirratulus gayheadius is, as far as we can tell, closely related to C. abranchiatus. The North-East Atlantic species agree well with the description of the West Atlantic species (Blake 2022), except for the methyl green/methylene blue staining pattern. Indeed, while some specimens examined in this study show the same pattern as described for the North American species, this pattern is variable as not all specimens retain a dark stain. This variation in staining pattern is seen among specimens collected together in the same sample, which partially excludes collection, fixation or preservation artefacts and could be due instead to intraspecific variability, specimen maturity or sexual dimorphism.
However, cryptic diversity is common in Cirratulidae (Grosse et al. 2020), and the known distribution of the two species is restricted to different biogeographical regions (Costello et al. 2017). In addition, asexual reproduction and budding are well documented for C. gayheadius (Blake 2022), but not observed in C. abranchiatus comb. nov. (this study). Therefore, we do not at this stage propose to synonymize the two species until genetic information can be obtained from specimens identified as C. gayheadius.
Remarks on the original description and illustrations and on the neotype
No type material could be found for this species and no mention is made of type specimens in the original description. All the existing material from the N.N.H.E. expeditions is kept at the Bergen University Natural History Museum and the Natural History Museum in Oslo. However, no specimens of Chaetocirratulus abranchiatus comb. nov. was found in either collection, and there are no records of it in the archives and the protocols of the museums. Therefore, we conclude that the original specimens were never deposited in any collection and a neotype has to be designated. The specimen designated as neotype in this study was chosen as it was collected the closest to the type locality, about 135 km, and sequences were available for it. While it is incomplete (part of the posterior region is missing), none of the specimens in better condition were collected so close to the type locality.
The original description was short, as was often the case in those times. It was however informative enough with regards to the body shape, the shape of the prechaetiger area and the shape of the parapodia in particular. One point of disagreement between the original description and our observations is on the capillary chaetae that were described as perfectly smooth. This can be explained by the techniques available at the time as the characteristic serrations present on the capillary chaetae of Chaetocirratulus abranchiatus comb. nov., while easily observed with SEM, are difficult to see even with modern light microscopy (Fig. 7E). Compared to the description, the original illustrations were particularly informative, as they show very well the distinctive body shape and anterior morphology of this species (Fig. 9). In addition to the fact that some specimens studied in this work were collected close to the type locality, this leaves no doubt about the fact that the specimens examined by Hansen for his description and the specimens examined for this study belong to the same species.
description: Published as part of Grosse, Maël, Alvestad, Tom, Nygren, Arne & Kongsrud, Jon Anders, 2025, Phylogenetic analyses elucidate the identity and distribution of two early-described species of Arctic Cirratulidae (Annelida, Sedentaria), pp. 189-220 in European Journal of Taxonomy 987 on pages 196-206, DOI: 10.5852/ejt.2025.987.2869, http://zenodo.org/record/15297612