Myxicola polychroma, Darbyshire, 2023

Darbyshire, Teresa, 2023, Designation of a neotype for Myxicola infundibulum (Montagu, 1808) (Annelida: Sabellidae) and a new species from the UK, European Journal of Taxonomy 900 (1), pp. 106-137 : 122-127

Myxicola polychroma sp. nov.

urn:lsid:zoobank.org:act:ECEE8088-72FD-4C8F-9F4E-C579193E04A5

Figs 1–2 , 7–11 , Table 1

Myxicola sarsi – Knight-Jones et al. 2017: 226, fig. 6.21b.

Diagnosis

Body with 8 thoracic chaetigers and over 180 abdominal chaetigers; glandular ridge on chaetiger 2. Crown with 17–32 pairs of orange, green or white radioles with non-differentially-coloured tips; pinnulae long, fine, terminating below the lower margin of the basal membrane, 0.22–0.3 × yhr total length of the radiole. Dorsal lips with large, fleshy triangular radiolar appendages. Thoracic notochaetae dense, fine, narrow-hooded, over 100 per notopodium; abdominal neurochaetae similar. Abdominal uncini form almost complete cincture around the body. Interramal eyespots present, lateral eyespots present or absent, pygidial eyespots present or absent. Tube very soft, gelatinous.

Etymology

As the generic name Myxicola is derived from Greek stems, the specific epithet is also taken from Greek rather than Latin; a noun in apposition derived from the Greek ‘ poly ’ (= ‘many’) and ‘ chromos ’ (= ‘colour’) referring to the three different colour variations, not just shades of a single colour, found within the species.

Material examined

Holotype SCOTLAND • Loch Fyne ; 56.2594° N, 4.9347° W; depth 12 m; 12 Nov. 2022; L. Baldock leg.; sandy mud; NMW.Z.2019.023.0015 .

Paratypes SCOTLAND – Loch Fyne • 1 spec.; same collection data as for holotype; NMW.Z.2019.023.0016 . – Loch Sunart • 1 spec.; 56.6677° N, 5.8910° W; depth 26 m; 23 Oct. 2019; G. Brown leg.; mud; GenBank: OQ343682 (16S), OQ341622 (COI); NMW.Z.2019.023.0017 • 1 spec.; same collection data as for preceding; depth 22 m; L. Baldock leg.; mud; GenBank: OQ343684 (16S), OQ341623 (COI); NMW.Z.2019.023.0018 • 1 spec.; same collection data as for preceding; depth 24 m; 24 Oct. 2019; G. Brown leg.; NMW.Z.2019.023.0019 .

WALES – Pembrokeshire • Martins Haven ; 1 spec.; 51.7376° N, 5.2443° W; depth 14 m; 29 Jun. 2019; T. Darbyshire leg.; sandy mud; GenBank: OQ343683 (16S), OQ341624 (COI); NMW.Z.2019.023.0023 • 1 spec.; same collection data as for preceding; depth 12.7 m; GenBank: OQ343685 (16S), OQ341625 (COI); NMW.Z.2019.023.0024 .

Other material

SCOTLAND – Loch Duich • 1 spec.; 57.2612° N, 5.5142° W; depth 20 m; 3 May 2019; C. Rickard leg.; GenBank: OQ343679 (16S), OQ341620 (COI); NMW.Z.2019.023.0020 • 1 spec.; same collection data as for preceding; depth 19.5 m; 14 Oct. 2019; sandy mud; GenBank: OQ343680 (16S), OQ341621 (COI); NMW.Z.2019.023.0021 • 1 spec.; same collection data as for preceding; 17 May 2019; NMW.Z.2019.023.0022 . – Shetland • 2 specs; Sullom Voe ; depth 24 m; 1985; fine sand; NMW.Z.1986.112.0091 to 0092 • 1 spec.; near Sullom Voe; 31 m; Apr. 1984; medium sand; NMW.Z.1986.112.0486 .

WALES – Pembrokeshire • 1 spec.; Martins Haven ; 51.7376° N, 5.2443° W; depth 13.4 m; 29 Jun. 2019; T. Darbyshire leg.; sandy mud; GenBank: OQ343681 (16S), OQ341626 (COI); NMW.Z.2019.023.0025 .

ISLE OF MAN – Irish Sea • 1 spec.; 7 mi. W of Sloc; 54.1294° N, 4.9156° W; depth 55 m; muddy sand and shell gravel; NMW.Z.2009.038.0678 .

Description

Holotype complete with 104 chaetigers, 8 thoracic and 96 abdominal; body length 55 mm, crown length 22.7 mm. Paratypes with 86–191 chaetigers, body length 41–140 mm, crown length 19.7–31 mm. Preserved colour uniformly cream for both crown and body. Colour when alive, cream/pinkish body with white, orange or pale green radioles ( Fig. 7A–E ). Body flattened dorso-ventrally, widest around chaetiger 4–5 ( Fig. 8A–B ), then tapering posteriorly to a blunt pygidium ( Fig. 7E ).

Radiolar crown with semicircular radiolar lobes ( Fig. 8A–B, D ) bearing 29 pairs of radioles. Radioles connected by basal membrane along 85% of their length ( Fig. 8C ; measured from base to lower margin of connection between radioles); radioles with broad radiolar flanges and digitiform tips ( Figs 8C , 10A ; measured from end of pinnulae to tip of radiole), 18% of total radiole length. Radioles supported by two rows of vacuolated cells ( Fig. 10B ). Radiolar eyes absent. Pinnulae slender with blunt ends in basal region, becoming finer, tapered and longest in distal part of radiole, 0.24 × total length of radiole, then rapidly decreasing in length, terminating below lower margin of basal membrane.

Dorsal lips with large, fleshy, triangular radiolar appendages, arising centrally and oriented dorsalventrally on a central line, surrounded by enlarged, lobate ventral lips connected to radiolar lobes dorsally ( Figs 8D , 10C–D ); pinnular appendages absent; ventral radiolar appendages absent; parallel lamellae and ventral sacs both absent.

Posterior peristomial ring collar absent. Anterior peristomial ring with ventral triangular lobe, high with concave indentation ( Fig. 7B ). Lateral notches present, shallow, not impacting chaetiger 1 ( Fig. 8E ). Glandular girdle present, toward posterior boundary of chaetiger 2 ( Fig. 8A, E ). Ventral shields inconspicuous. Interramal eyespots present, one or two, minute, from chaetiger 3 or 4 to chaetiger 8, absent from abdominal region.

First notopodia as oval cushions with dense, fine, narrow-hooded notochaetae, up to 100. Second notopodia twice the size of the first, then reducing in size gradually down thorax; notochaetae of same form as on first notopodium, 100–150 per fascicle ( Fig. 8E , 9A , 10E ). Thoracic neuropodia situated at posteroventral border of notopodia, with few (up to 5) acicular uncini with long handles, strongly hooked, with long main fang and no clear teeth above ( Figs 9B , 10F ). Companion chaetae absent. Abdominal neuropodia small, oval, with narrow-hooded neurochaetae ( Fig. 9C ). Abdominal notopodial tori forming almost complete cinctures around body, with avicular uncini with one large tooth and 0–4 smaller teeth, variable in size, over main fang; upper teeth 0.6–0.8× length of main fang ( Figs 9D–E , 10G ); breast well-developed, with angled profile, slightly longer than main fang, handle absent ( Fig. 10G ).

Pygidium rounded, eyespots not seen. Pygidial cirrus absent. Tube semi-transparent, gelatinous, with few fine sand grains attached, slight colouration toward proximal end ( Fig. 8F ).

Methyl green staining pattern

Peristomial ring and ventral lobe with solid staining ( Fig. 8A, E ) followed by complete rings on the posterior border of each segment.

Variation

Radiolar pinnulae 0.22–0.3 × total length of radioles. Interramal eyespots present from chaetigers 3 or 4 and on rest of thorax; lateral eyespots absent or on first 1–4 segments of abdomen; the larger the animal the fewer the eyespots. Pygidium generally without eyespots although a row of 7 minute eyespots laterally on either side found on one small specimen. Radiole count 17–32 pairs, determined from a combination of divers’ photos and collected specimens.

Remarks

Myxicola polychroma sp. nov. has been recorded around the UK for many years, although it has not been recognised as a separate species. Other names that have previously been recorded from British waters are M. sarsii , M. steenstrupi and M. viridis , all of which were synonymised with M. infundibulum many years ago. As it is likely that more species of Myxicola will be identified in the future, possibly with the re-validation of previously synonymised names, it is worth making distinctions here from those other, currently invalid, species described from European waters. The original description of M. sarsii does not provide any information on which to base a comparison; however, Sars (1862) provided more detail, finding a radiole count only ranging from 12 to 21 pairs, accounting for both juvenile and mature specimens ( Myxicola polychroma sp. nov. has 17–32 pairs of radioles). Myxicola steenstrupi , as detailed later, is represented by type specimens that themselves appear to represent different taxa; however, the type from Greenland has slender, digitiform radiolar appendages (large, triangular radiolar appendages in M. polychroma sp. nov.) and the Faroe Island type has short radiolar tips and small, blunt radiolar appendages ( M. polychroma sp. nov. has long radiolar tips and large, triangular radiolar appendages); all specimens of M. steenstrupi also possess only 12–21 pairs of radioles, as opposed to 17–32 pairs in M. polychroma sp. nov. Myxicola grubii was described with dark tips to the radioles ( Krøyer 1856) and M. viridis with green banded radioles ( McIntosh 1874), and so neither could be potential candidates for the identity of M. polychroma sp. nov. The remaining two (invalid) European species M. modesta and M. parasites , were both clearly very small specimens, with only 9 and 11 pairs of radioles described, respectively ( Quatrefages 1866), and it is unknown whether they represented juvenile forms of larger taxa or not. If they were not juveniles, then the small body size and few radioles would distinguish them from M. polychroma sp. nov. immediately. In addition to the radioles though, M. parasites had only a short basal membrane, in comparison to the long membrane present in M. polychroma sp. nov., and M. modesta possessed no lateral or interramal eyes. Although it has been stated earlier in this paper that the presence or absence of lateral eyes is an unreliable character, interramal eyes are more consistent; both are always present in small specimens and it is only in larger and preserved specimens that lateral eyes become inconsistent or fade. Quatrefages’ specimens were described live, so preservation was not a factor, and either juvenile (so eyespots should be present if they were the same as M. polychroma sp. nov.) or a naturally small taxon, in which case they could also be discounted.

Myxicola polychroma sp. nov. is distinct from the other valid species of Myxicola in the UK, M. infundibulum and Myxicola aesthetica , by the lack of differential colour on the radiole tips (dark radiole tips in M. infundibulum ) and by having 8 thoracic chaetigers and 17–32 pairs of radioles, as opposed to 3–4 thoracic chaetigers and 7–10 pairs of radioles in M. aesthetica ( Soulier 1902) . All other known species are far removed geographically, but can be distinguished through a variety of characters: Myxicola violacea , the only other European species, has dark tips to the radioles ( M. polychroma sp. nov. has pale tips), M. nana has only 6 thoracic chaetigers ( Capa & Murray 2015) ( M. polychroma sp. nov. has 8), M. ommatophora has paired subdistal radiolar eyes and a clay tube ( Grube 1878) ( M. polychroma sp. nov. has no radiolar eyes and a gelatinous tube), M. fauveli has only 12 pairs of radioles with longitudinal green bands and sharply delineated filiform tips ( Potts 1928) ( M. polychroma sp. nov. has more than 12 pairs of radioles, without banding or sharply delineated tips) and M. sulcata also has only 12–17 pairs of radioles ( Tovar-Hernández et al. 2017) and few, inconspicuous notochaetae ( M. polychroma sp. nov. has more than 17 pairs of radioles and more than 100 fine but conspicuous notochaetae per notopodium).

Distribution

Found on the west and southwest coasts of the UK: west coast of Scotland, Isle of Man, west Wales and southwest England. A single record was received from the east coast of Scotland.

Ecology

Subtidal from 5–35 m (occasionally deeper) in sandy mud and muddy, soft sediments.

Figures

Fig. 1. Map showing the distribution of Myxicola infundibulum (Montagu, 1808) (closed circles) and M. polychroma sp. nov. (open circles) around the British Isles, based on validated specimens and photographs. Grey circles indicate an undetermined species of Myxicola in northeast England. Type locality of Myxicola infundibulum indicated by black arrow.

Fig. 2. Map showing the type localities of both valid (bold type) and invalid species of Myxicola in Europe.

Fig. 7. Images of live Myxicola polychroma sp. nov. demonstrating different colour morphs. A. White crown, Martin’s Haven, Wales. B. Orange crown, Martin’s Haven, Wales. C. Green crown, Loch Duich, Scotland. D. Orange body, Loch Duich, Scotland. E. White-cream body, Loch Duich, Scotland. Photos courtesy of Rhian Lewis James (A–B) and Chris Rickard (C–E).

Fig. 8. Myxicola polychroma sp. nov. A. Anterodorsal view (NMW.Z.2019.023.0023). B. Anterodorsal view (holotype, NMW.Z.2019.023.0015). C. Radiolar tips (NMW.Z.2019.023.0024). D. Dorsal and ventral lips (NMW.Z.2019.023.0023). E. Anterolateral view (NMW.Z.2019.023.0023). F. Tube. Scale bars: A, F = 10 mm; B–C = 5 mm; D–E = 1 mm.

Fig. 9. Myxicola polychroma sp. nov., NMW.Z.2019.023.0024. A. Thoracic chaetae, chaetiger 4, white arrow indicates thoracic uncini. B. Thoracic uncini, chaetiger 4. C. Abdominal chaetae, chaetiger 11. D. Abdominal uncini, top view. E. Abdominal uncini, lateral view. Scale bars: A = 100 µm; B, D–E = 10 µm; C = 50 µm.

Fig. 10. Myxicola polychroma sp. nov. A. Radiole tip illustrating basal membrane and extent of pinnulae (NMW.Z.2019.023.0024). B. Cross-section of radiole (NMW.Z.2019.023.0018). C–G. NMW.Z.2019.023.0024. C. Dorsal lips, ventral lips and radiolar lobes. D. Anterodorsal view showing ventral lobe and glandular ridge.E. Thoracic chaeta, chaetiger 4. F. Thoracic uncinus, chaetiger 4. G. Abdominal uncinus, chaetiger 12. Scale bars: A, C–D = 1 mm; B = 100 µm; E–F = 50 µm; G = 20 µm.

Fig. 11. Bayesian inference trees. A. 16S rRNA dataset. B. Cytochrome oxidase I gene dataset. The first value at each node represents maximum likelihood bootstrap support, the second the Bayesian posterior probabilities and the third the maximum parsimony bootstrap support.