Raricirrus arcticus Buzhinskaja & Smirnov, 2017

Raricirrus arcticus Buzhinskaja & Smirnov, 2017 View in CoL

Figs 8–9 View Fig View Fig

Diagnosis

Heart body from chaetiger 2; branchiae cylindrical; distinct ciliary field near mouth; long serrate chaetae with straight tips in midbody neuropodia.

Material examined

ARCTIC MID-OCEAN RIDGE • 1 spec.; Loki’s Castle hydrothermal vent field; 73.5662° N, 8.1585° E; 2300 m depth; 14 Jul. 2008; collected with ROV; sample BIODEEP08-LC-ROV11; fixed in 96% ethanol; DNA voucher: LC06; ZMBN 157600 View Materials GoogleMaps • 5 specs; same data as for preceding; ZMBN 157689 View Materials GoogleMaps • 3 specs; Loki’s Castle hydrothermal vent field; 73.5662° N, 8.1583° E; 2330 m depth; 15 Jul. 2008; collected with ROV; sample BIODEEP08-LC-ROV12; fixed in 96% ethanol; ZMBN 157601 View Materials GoogleMaps • 17 specs; Loki’s Castle hydrothermal vent field, barite field; 74.5661° N, 8.1585° E; 2357 m depth; 7 Aug. 2009; collected with ROV; sample H2 DEEP09 -ROV08; fixed in 96% ethanol; ZMBN 157686 View Materials GoogleMaps • 72 specs; same data as for preceding; ZMBN 157602 View Materials GoogleMaps • 1 spec.; same data as for preceding; DNA-voucher: LC08; ZMBN 157621 View Materials GoogleMaps • 1 spec.; same data as for preceding; DNA-voucher: LC09; ZMBN 157687 View Materials GoogleMaps • 4 specs; same data as for preceding; ZMBN 157688 View Materials GoogleMaps • 1 spec.; same data as for preceding; ZMBN 157604 View Materials GoogleMaps • 5 specs; same data as for preceding; ZMBN 157610 View Materials GoogleMaps • 1 spec.; same data as for preceding; ZMBN 157605 View Materials GoogleMaps • 2 specs; same data as for preceding; mounted on SEM-stub; ZMBN 157685 View Materials GoogleMaps • 1 spec.; same data as for preceding; ZMBN 157603 View Materials GoogleMaps • 1 spec.; Loki’s Castle hydrothermal vent field, barite field; 73.5665° N, 8.1618° E; 2385 m depth; 15 Jul. 2010; collected with ROV; sample GS10-ROV04; fixed in 96% ethanol; DNA-voucher: LC07; ZMBN 157509 View Materials GoogleMaps • 1 spec.; same data as for preceding; DNA-voucher: LC10; ZMBN 157510 View Materials GoogleMaps • 1 spec.; same data as for preceding; DNA-voucher: LC11; ZMBN 157511 View Materials GoogleMaps • 24 specs; same data as for preceding; ZMBN 157606 View Materials GoogleMaps • 28 specs; same data as for preceding; ZMBN 157607 View Materials GoogleMaps • 46 specs; same data as for preceding; fixed in formaldehyde and transferred to 75% ethanol; ZMBN 157608 View Materials GoogleMaps • 2 specs; Loki’s Castle hydrothermal vent field; 73.5673° N, 8.1573° E; 2314 m depth; 17 Jul. 2010; collected with ROV; sample GS10-ROV07; fixed in 96% ethanol; ZMBN 157611 View Materials GoogleMaps • 1 spec.; same data as for preceding; mounted on SEM-stub; ZMBN 157684 View Materials GoogleMaps • 4 specs; Loki’s Castle hydrothermal vent field; 73.5662° N, 8.1610° E; 2335 m depth; 17 Jul. 2010; collected with ROV; sample GS10-ROV06-b; fixed in 96% ethanol; ZMBN 157609 View Materials GoogleMaps • 5 specs; Loki’s Castle hydrothermal vent field, eastern mound, Joao; 73.5658° N, 8.1610° E; 2385 m depth; 18 Jul. 2010; collected with ROV; sample GS10-ROV09; fixed in formaldehyde and transferred to 75% ethanol; ZMBN 157622 View Materials GoogleMaps • 1 spec.; same data as for preceding; fixed in 96% ethanol; DNA-voucher: LC12; ZMBN 157512 View Materials GoogleMaps • 14 specs; same data as for preceding; fixed in 96% ethanol; ZMBN 157612 View Materials GoogleMaps • 1 spec.; Loki’s Castle hydrothermal vent field; 73.5668° N, 8.1605° E; 2373 m depth; 19 Jul. 2010; fixed in 96% ethanol; DNA voucher: LC13; ZMBN 157613 View Materials GoogleMaps • 2 specs; Loki’s Castle hydrothermal vent field; 73.5663° N, 8.1610° E; 2350 m depth; 11 Jul. 2015; collected with ROV; sample GS10-ROV10; fixed in 96% ethanol; ZMBN 157614 View Materials GoogleMaps • 4 specs; Loki’s Castle hydrothermal vent field, barite field; 73.5664° N, 8.1618° E; 2342 m depth; 10 Jul. 2017; fixed in 96% ethanol; ZMBN 157617 View Materials GoogleMaps • 3 specs; Loki’s Castle hydrothermal vent field, eastern mound, base of chimney; 73.5664° N, 8.1616° E; 2329 m depth; 11 Jul. 2017; collected with ROV; sample GS15-AGR09; fixed in 96% ethanol; ZMBN 157645 View Materials GoogleMaps • 6 specs; same data as for preceding; ZMBN 157616 View Materials GoogleMaps • 5 specs; Loki’s Castle hydrothermal vent field, barite field, chimney; 73.5666° N, 8.1624° E; 2344 m depth; 12 Jul. 2017; collected with ROV; sample GS17-ROV21- R1 ; fixed in 96% ethanol; ZMBN 157615 View Materials GoogleMaps • 1 spec.; Loki’s Castle hydrothermal vent field, barite field, worm forest; 73.5663° N, 8.1624° E; 2342 m depth; 18 Jul. 2018; collected with ROV; sample GS18-107-ROV26; fixed in 96% ethanol; ZMBN 157619 View Materials GoogleMaps • 1 spec.; same data as for preceding; ZMBN 157620 View Materials GoogleMaps • 4 specs; Loki’s Castle hydrothermal vent field, barite field, worm forest; 73.5662° N, 8.1621° E; 2341 m depth; 22 Jul. 2018; collected by ROV; sample GS18- 107-ROV28; fixed in 96% ethanol; ZMBN 157451 View Materials GoogleMaps • 2 specs; Loki’s Castle hydrothermal vent field, barite field, worm forest; 73.5665° N, 8.1623° E; 2342 m depth; 26 Jun. 2019; collected with ROV; sample GS19-108-ROV19; fixed in 96% ethanol; ZMBN 147555 View Materials GoogleMaps • 2 specs; same data as for preceding; 73.5664° N, 8.1623° E; 2345 m depth; 29 Jun. 2019; collected with ROV; sample GS19-108-ROV22; fixed in 96% ethanol; ZMBN 138221 View Materials GoogleMaps • 2 specs; same data as for preceding; ZMBN 138223 View Materials GoogleMaps • 2 specs; Loki’s Castle hydrothermal vent field, oasis, worm forest; 73.5671° N, 8.1617° E; 2356 m depth; 1 Jul. 2019; collected with ROV; sample GS19-108-ROV25; fixed in 96% ethanol; ZMBN 138217 View Materials GoogleMaps • 1 spec.; same data as for preceding; ZMBN 138218 View Materials GoogleMaps • 15 specs; Loki’s Castle hydrothermal vent field, oasis, worm forest; 73.567° N, 8.1617° E; 2356 m depth; 14 Jul. 2022; collected with ROV; sample GS22-108- ROV575; fixed in 96% ethanol; ZMBN 157500 View Materials GoogleMaps • 1 spec.; Loki’s Castle hydrothermal vent field, eastern mound, base of chimney; 73.5664° N, 8.16° E; 2313 m depth; 10 Jul. 2017; collected with ROV; sample GS17-ROV18-SS5; fixed in 96% ethanol; ZMBN 157618 View Materials GoogleMaps .

Description

Complete specimens 5–8 mm long and 0.2–0.9 mm wide for 27–35 segments. Colour in ethanol dark brown to black, rarely light tan. Wide and convoluted dark heart body distinctly visible from segment 11–12 to 16–18. Body short, anterior and median chaetigers bead-shaped, transitioning in posterior half or third in distinct flattened region with short and wide segments. Anterior and middle segments twice as high and wide as long, or up to as wide and high as long; posterior segments up to four times wider and three times higher than long. Dorsal groove or ridge absent. Ventral ridge present along posterior part of body ( Fig. 8 View Fig ). Large portion of individuals regenerating anterior or posterior parts of body.

Prostomium short, rounded, dorsoventrally flattened; nuchal organs large rounded ciliary fields at posterior dorsolateral margins connected by transversal ventral ciliary bands. Peristomium short, fused with chaetiger 1 in single ovoid bead, with ventral groove separating them; ciliary field present ventrally, continuing as longitudinal band through several anterior segments ( Fig. 9B View Fig ). Branchiae arising from posterior segmental margins, slightly above notopodia, present as early as segment 6 and as far as segment 16.

Parapodia reduced, chaetae emerging directly from body wall. Anterior and midbody neuropodia with 2–5 capillaries; anterior notopodia with 5–7 capillaries; midbody notopodia with up to 11 chaetae; posterior notopodia with 1–3 chaetae. Capillary chaetae long, in mid-body notopodia few capillaries longer than body width, flat, finely serrated along one edge with distinctly round tip. Neuropodia with 3–5 pectinate chaetae from chaetiger 1–3, finely to coarsely serrated along one edge with distinct rounded tip. Neuropodial pectinate chaetae progressively transitioning to spines, 1–2 per neuropodium; 1–3 per notopodium from posterior third of body; straight along most of their length and curving at tip, pointed, with fine serration on concave side becoming coarser posteriorly ( Fig. 9A View Fig , C-H).

Pygidium with terminal anus, rounded.

Methylene blue staining pattern

Specimens do not retain any colour except for the tip of the prostomium that stains dark blue.

Comparative remarks

The specimens presented here agree well with the original description, with slight differences in chaetal numbers or distribution and in characters related to the heart body. Differences in chaetal numbers can be attributed to intraspecific variability. In the ethanol fixed specimens available for this study, the heart body was always much darker than described in the original description, and usually only the wide, convoluted part was visible. However, the location of this part of the heart body observed in the material matches well the original description.

Raricirrus arcticus is most similar to Raricirrus beryli , the only other species of the genus reported from the area. Differences include morphology, the segment where the heart body starts (segment 2 in R. arcticus or 7 in R. beryli, Buzhinskaja & Smirnov 2017 ), and the depth range where they occur: 60–300 m for R. beryli (Petersen & George 1991) vs 2000–2500 m for R. arcticus . The morphology of nuchal organs has been proposed as a diagnostic character between both species, R. beryli was described with nuchal organs without cilia surrounded by ciliary patches and R. arcticus was described with nuchal organs inconspicuous among ciliary patches ( Buzhinskaja & Smirnov 2017). Nuchal organs in cirratulids have been described as ciliated pits or pits surrounded by cilia and the possibility of eversion of these structures has been proposed as an explanation for the observed variation in this structure ( Blake & Magalhães 2017). Personal observations of several species indeed show variation in the shape of nuchal organs due to retraction which could be natural or a consequence of fixation. In particular, specimens of R. beryli have been observed with simple rounded ciliary patches on the prostomium without conspicuous non-ciliated nuchal organs (M. Grosse pers. obs.). Therefore, we propose that this character is an artefact and not diagnostic.

Distribution and habitat

Raricirrus arcticus has been recorded from only two locations: the Laptev Sea ( Buzhinskaja & Smirnov 2017) and Loki’s Castle vent field ( Eilertsen et al. 2024), from around 2000 to 2500 m deep. At LCVF, it was most common in the Barite Field and the Oasis, but also found on the Eastern Mound at the base of a chimney. Raricirrus arcticus is considered to be a chemosynthetic-based ecosystem specialist ( Eilertsen et al. 2024).