Chaetozone pigmentata Blake, 2015, new species

Chaetozone pigmentata new species

Figures 3–4

Chaetozone setosa: Pettibone 1954, p. 287‒288, in part (not Fig. 33d = Tharyx alaskensis n. sp); Pettibone 1956: p. 562; Blake & Dean 1973, p. 34, in part. Not Malmgren 1867.

Material examined. Canadian Arctic, offshore Baffin Island, East Angiak Island, R/V Hero Sta. 20, 132– 245 m, 65°43ʹN, 62°05ʹW, coll. J.A. Blake, 14 Aug 1968, holotype (USNM 51221), 45 paratypes (USNM 1263246).— Off Labrador, Hebron Fjord, Blue Dolphin Sta. 27, 174 m, 58°11.4ʹN, 62° 34.2ʹW, coll. D.C. Nutt, 8 Aug 1949, Otter trawl, mud bottom, 16 specimens (USNM 22815); Sta. 28, 229 m, 58°09ʹN, 62°45.7ʹW, coll. D.C. Nutt, 8 Aug 1949, Otter trawl, 30+ specimens (USNM 22816).— Alaskan Arctic, Off Point Barrow, 104 m, offshore, coll. G.E. McGinitie, 11 Oct 1949, dredged, rocks, stones, gravel, 1 specimen (USNM 22803); 138 m, offshore, coll. G.E. McGinitie, 11 Oct. 1949, dredged, rocks and stones, 1 specimen (USNM 22804); 128 m, 7 mi offshore, coll. G.E. McGinitie, 0 9 Aug 1949, dredged, stones and gravel, 3 specimens (USNM 22805); 138 m, 8 mi offshore, coll. G.E. McGinitie, 11 Oct 1949, dredged, rocks, stones, gravel, 4 specimens (USNM 22806); 66 m, 7.5 mi offshore, coll. G.E. McGinitie, 6 September 1949, dredged, removed from rock and growth on rock, 4 specimens (USNM 22807); 226 m, 12.1 mi offshore, coll. G.E. McGinitie, 17 Aug 1950, dredged, mass of worm tubes, 1 specimen (USNM 22808); 38 m, offshore, coll. G.E. McGinitie, 0 9 Sep 1948, dredged, 1 specimen (USNM 22809); 40 m, offshore, 4 miles out, coll. G.E. McGinitie, 0 9 Aug 1949,stones, gravel, from surface of tunicates, 1 specimen (USNM 22811); 36.5 m, offshore, coll. G.E. McGinitie, 15 Sep 1948, 2 specimens (USNM 22813).

Description. A moderately sized species, holotype complete, 8.2 mm long, 0.8 mm wide across setigers 15–30, for 80 setigerous segments. Some complete paratypes up to 14 mm long, 1.2 mm wide for about 100 setigers; largest specimens from Baffin Island collection. Body widest in anterior 15–30 setigers, narrowing posteriorly; all anterior thoracic segments short, crowded, at least 15–20 times wider than long; middle body segments of some specimens larger, inflated on some specimens; posterior segments narrow, about 4–5 times wider than long. A weakly developed narrow dorsal groove runs along body from about setiger 30 (Fig. 4 A); a prominent ventral ridge formed of ventromedial bulges arising from each segment runs along entire length of body (Figs. 3 C, 4C, E); this feature present in all specimens examined. Color in alcohol light brown with body segments covered with numerous brown to black pigment speckles imparting distinctive background coloration to most specimens (Figs. 3 A–B; 4A–C, F); in some paratypes this pigment becomes prominent on prostomium and peristomium, forming bands across dorsum of some anterior segments, and often concentrated between segments; some specimens very darkly pigmented.

Prostomium triangular, narrowing anteriorly to rounded tip (Fig. 3 A); without eyes, with nuchal organ narrow slit on posterior margin of prostomium, sometimes pigmented resembling eyes (Fig. 3 B); peristomium with one large and one narrow achaetous ring, followed by an achaetous segment between peristomium and setiger 1; narrow peristomial ring incomplete dorsally, overlain medially by posterior extension of large ring; tentacles arising from narrow peristomial ring; achaetous segment similar in form to setiger 1, bearing first pair of branchiae on posterior margin; second pair of branchiae on setiger 1, dorsal to notosetae, branchiae continuing on subsequent setigers (Fig. 3 A).

Anterior setae all capillaries arranged in single rows in both noto- and neuropodia; notosetae numbering 6–9 per row, neurosetae numbering 7–10 per row; about half of specimens with additional long, natatory notosetae along most of body, these specimens sexually mature with many specimens having coelom full of eggs (Fig. 4 B, D). Notopodial acicular spines from setigers 30–50 in all specimens; neuropodial acicular spines from anterior third of body or setigers 12–26 in Baffin Island specimens having 50–85 total setigers and setigers 5–15 in specimens from Point Barrow and Labrador having total 29–65 setigers; neuroacicular spines 1–2 per neuropodium at first, increasing to 6–7 near posterior end of body; each neuropodial acicular spine with curved blunt-tips or weakly pointed (Figs. 3 E, 4G), these alternating with capillaries; notoacicular spines 2–5 per segment, narrower and more pointed than neuroacicular spines (Fig. 1 F), also with additional capillaries; noto- and neuroacicular spines and capillaries of posterior segments hooked, forming weakly developed cinctures with low membranes, leaving broad dorsal gap between opposite parapodia (Fig. 3 C).

Pygidium simple, with terminal anus and single ventral lobe (Fig. 3 D).

Variability. Some specimens from Point Barrow and Labrador with posterior part of first peristomial ring enlarged, bulbous, extending dorsally over two rings bearing the tentacles and first pair of branchiae; this variation is believed due to contraction during preservation. All specimens with distinct ventral line of ridges and reduced development of posterior segments; posterior segments bearing noto- and neuropodial acicular spines never developed into full cinctures as in related species, with only low parapodial membranes or none evident. Neuropodial acicular spines begin earlier in the Point Barrow and Labrador specimens, on setigers 5–15 vs. setigers 12–26 in the Baffin Island specimens. This difference appears to be size related, with larger Baffin Island specimens having spines beginning later, suggesting a replacement of spines by capillaries with growth. However, all specimens have acicular spines first appearing in the anterior third of the body.

Methyl Green staining pattern. No pattern.

Remarks. Based on available collections, Chaetozone pigmentata n. sp. is the only species found to range across the entire North American Arctic. While there is some variability, all of these specimens share several unique features not found in other species encountered as part of this study. These features include a heavily pigmented body, a prominent mid-ventral ridge line that extends along the entire body, a very weak mid-dorsal groove in middle body segments, the first occurrence of the neuropodial acicular spines in the anterior one-third of the body (setigers 5–26) with the smallest specimens having spines in anteriormost locations and the overall distribution being size dependent, and most importantly a reduced posterior armature of spines where the parapodia have only weakly developed cinctures and parapodial membranes. C. pigmentata n. sp. bears some resemblance to C. brunnea Blake, 2009 from deep-sea sediments off California in the nature of the peristomium and presence of body pigment. However, C. brunnea has an unusual body shape denoted by an enlarged and often dark mid-body stomach; in addition, C. brunnea has an achaetous segment bearing two pairs of branchiae instead of one pair and the posterior spines are more numerous and formed into well-developed parapodial cinctures more typical of most Chaetozone species. Some specimens of C. pigmentata n. sp. also have enlarged mid-body segments, but these appear to be associated with reproductive development. C. pigmentata n. sp. is readily distinguished from other Chaetozone species by the heavily pigmented body, weakly developed posterior cinctures with a reduced number of spines, the unusual ventral ridge with mid-ventral segmental bulges, and separate achaetous segments with the first bearing tentacles and the second bearing the first pair of branchiae.

Biology and ecology. Some Baffin Island specimens have eggs 120–125 µm in diameter (Fig. 4 D). Sediments at the collecting locality consisted of well-sorted brown-grey sticky mud with a mean particle size of 150 µm. In addition to C. pigmentata n. sp., 16 other polychaete species were identified (Blake & Dean 1973). Of these, Galathowenia oculata (Zachs, 1923) was the most abundant followed by C. pigmentata n. sp. and Myriochele heeri (Malmgren, 1867). Labrador specimens were found within mud-filled Pectinaria tubes; Point Barrow specimens were dredged from bottoms with stones and gravel.

Etymology. The species name is based on the pigment spots found on the bodies of all specimens.

Distribution. Widely distributed across the North American Arctic, Baffin Island, Labrador, and the Beaufort Sea, offshore Point Barrow, in shelf depths of 38– 245 m.