Chaetozone setosa Malmgren 1867

Chaetozone setosa Malmgren, 1867

Figures 1‒2; Table 1

Malmgren 1867: 96, Pl. 14, Fig. 84; Petersen 1999: 111; Chambers 2000: 589‒591, Fig. 1.

Material examined. Norwegian Arctic, Spitsbergen, Isfjord, 30 fathoms (55 m), coll. 0 6 June 1864, A.J. Malmgren syntype lot SMNH 1493: Lectotype, female, SMNH 1493-03 examined by Dr. Mary E. Petersen and designated in print (Petersen 1999); 38 Paralectotypes from SMNH 1493: 1493-01, 1493-02, 1493-04‒36; data on these collections provided with illustrations, sketches, and descriptive data from preliminary manuscript by Dr Petersen, dated 0 3 April 2001; eight prepared slides of parapodia and illustrations from paralectotype 1493-33 also provided by Dr. Petersen; additional study of prepared slides including preparation of photomicrographs, by J.A. Blake, September 2014.

Description. A moderately sized species, lectotype a complete ovigerous female, 20.2 mm long, 1.7 mm wide for 90 setigerous segments; complete paralectotypes up to 28 mm long, 2 mm wide for 94 setigerous segments (Table 1). Body of most preserved specimens curled into a C-shape, but not strongly coiled. Body thickened in middle, narrowing anteriorly and posteriorly. Anterior setigers short, wide, becoming up to 2 times longer in middle body segments, but always narrower than wide except for some segments on ovigerous specimens. Dorsal groove weakly developed, narrow, often limited to anterior setigers; ventral groove well-developed, visible along most of body, absent in far posterior cinctured segments. Color in alcohol brown or grey; no distinct pigmentation.

Prostomium conical, narrow, bluntly pointed anteriorly (Fig. 1 A); eyes absent; nuchal organs narrow diagonal slits, not pigmented. Peristomium with two large, distinct rings best visible laterally, overlain dorsally by swollen peristomial crest with peristomial annulations weakly developed or not apparent on crest (Fig. 1 A), crest overlapping prostomium anteriorly, narrowing posteriorly, extending to near anterior margin of achaetous segment 1. Dorsal tentacles arising from notch at posterior margin of peristomium (Fig. 1 A); first pair of branchiae typically positioned posterior to tentacles on posterior margin of incomplete achaetous segment; second pair of branchiae on setiger 1 (Fig. 1 A).

1. Deceased 0 7 July 2014.

Setiger 1 of approximately same size as preceding achaetous segment and subsequent setigers (Fig. 1 A); parapodial lobes reduced, inconspicuous ridges in anterior and middle setigers; enlarged with elevated ridges in posterior cinctured segments bearing conspicuous armature; posterior segments separated by deeply cut intersegmental furrows and with highly elevated membranous podial lobes from which spines and capillaries emerge, forming full cinctures; notopodial spines directed ventrally, neuropodial spines directed dorsally (Fig. 2 A).

Noto- and neurosetae from setiger 1, setae of anterior segments all limbate capillaries, numbering about 7–10 per fascicle (Fig. 1 D); long, natatory-like notosetae present from about setiger 18‒21, continuing posteriorly. Capillaries thin throughout, some with fibrils along edge, but not consistent; natatory-like setae capillaries, very long, flattened in cross section numbering 2‒5 per notopodium mainly restricted to lower part of setal fascicle (Fig. 1 E‒F). Based on data from 16 types in Table 1, with 63‒93 setigerous segments (mean = 83.9 ± SD 7.7), acicular spines begin from setiger 35‒65 in neuropodia (mean = 51.7 ± SD 7.6) and setiger 43‒71 in notopodia (mean = 58.7 ± SD 7.1). Lectotype with neuroacicular spines from setiger 57 and notoacicular spines from setiger 63 (Table 1). Spines numbering 1‒3 at first, accompanied by narrow limbate capillaries, increasing to 10‒13 in each ramus in fully developed and complete posterior cinctures, with 20‒26 spines on a side with alternating capillaries (Fig. 1 F, 2A); spines sometimes overlapping at dorsal midline; when long natatory-like setae occur within posterior cinctures, they accompany ventral-most notopodial spines and sometimes dorsal-most neuropodial spines (Fig. 1 E‒F). Spines brownish or brassy in appearance, round in cross section with weak narrow notch at point of emergence, with slightly curved or sigmoidal shape narrowing to a bluntly pointed tip (Figs. 1 B‒C, 2B‒D); shafts with thick borders and fine internal striations.

Last few cinctured setigers tapering to narrow posterior end; pygidium with terminal anus and small flattened ventral lobe.

Methyl Green staining pattern. MG stains the peristomial area with heavy to sparse concentrations of small rows or stripes of stained cells, sometimes very intense; a pattern of stripes on anterior thoracic segments and posterior cinctured segments are most prominent on the ventral surface, but are not intersegmental.

Biology. The syntype collection comes from coastal fjords and shelf depths around Spitsbergen of 20‒60 fathoms (36.5‒110 m), in clayey or muddy sediments. Numerous specimens were observed with oocytes in various stage of maturity (Table 1); largest ova observed were 120‒125 µm in diameter (SMNH 1493-09). Some males confirmed with sperm platelets (Table 1).

Remarks. Chaetozone setosa is readily distinguished from most related species by having a separate achaetous segment anterior to setiger 1 that bears the first pair of branchiae in combination with an enlarged dorsal swelling or crest on the peristomium. In addition, acicular spines of C. setosa begin in the posterior third of the body and have fully developed cinctures in the far posterior parapodia with 22‒24 spines on each side. Further, C. setosa has a MG staining pattern that includes stain on the peristomium and ventral segmental stripes in anterior and far posterior parapodia. Other species of Chaetozone described with an inflated lobe or crest overlying the peristomium as in C. setosa include: C. corona Berkeley & Berkeley, 1941, C. pugettensis Blake n. sp., C. platycera Hutchings & Murray, 1984, and C. carpenteri McIntosh, 1911.

Chaetozone corona is readily distinguished from C. setosa and other species by having the neuropodial acicular spines from setiger 1 and a pair of black eyes. Chaetozone pugettensis n. sp., described in the present study, most closely resembles C. setosa in the nature of the two large peristomial rings that are overlain dorsally by a variably inflated dorsal ridge or crest (See comments for C. pugettensis n. sp.). C. setosa differs from C. pugettensis n. sp. in that long natatory-like capillaries are present on all specimens examined including the posterior cinctures, whereas no specimens of C. pugettensis n. sp. have been observed with these setae. Another important difference is that C. setosa has a distinct MG staining pattern, whereas C. pugettensis n. sp. has none. Chaetozone platycera from New South Wales, Australia is another species with an inflated lobe or crest overlying the peristomium, however, in this species distinct peristomial annuli were not observed (Hutchings & Murray 1984). Unlike C. setosa, the noto- and neuroaciculars of C. platycera begin on anterior setigers (~ 23 in notopodia; ~ 47 in neuropodia on specimens with ~200 setigers) instead of the posterior third and have 11–13 spines per side posteriorly instead of 22–24. Chaetozone carpenteri is an unusual Mediterranean species in that long, enlarged and elongate noto- and neuropodial acicular spines first appear in anterior setigers 6‒9, continuing to about the middle body segments; from there the spines become narrower, but remain long and alternate with thin capillaries in far posterior cinctures (Chambers et al. 2011).

Distribution. Pending further study of historical records, C. setosa is here limited to Arctic and subarctic areas around Spitsbergen and other areas of northern Europe in shelf depths of 30‒ 110 m. Dr. Petersen’s notes indicate that the species was also found offshore East Greenland in muddy sediments.