Chaetocirratulus abranchiatus Grosse & Alvestad & Nygren & Kongsrud 2025, comb. nov.

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-vent