Orbinia swani Pettibone, 1957

Blake, James A., 2021, New species and records of Orbiniidae (Annelida, Polychaeta) from continental shelf and slope depths of the Western North Atlantic Ocean, Zootaxa 4930 (1), pp. 1-123 : 75-80

Orbinia swani Pettibone, 1957

Figures 38–39

Orbinia (Orbinia) swani Pettibone,1957: 161–162 , fig. 1; 1963: 284, fig. 75 d–e.

Orbinia swani: Maciolek-Blake et al. 1985 : B-5 (in part).

Scoloplos (? Leodamas ) sp. A: Maciolek-Blake et al. 1985: B-5.

Material examined. Northeastern USA. Maine, York Beach, coll. 12 Jul 1953, E. Swan & N. Riser, holotype ( USNM 28296 ) .— Georges Bank, Benthic Infauna Monitoring Program (1981–1984), coll. G.W. Hampson, Chief Scientist (25 specimens). Sta. 1: Cruise M-4, Rep. 6, 12 May 1982, 41°13.0′N, 67°15.3′W, 58 m (1, USNM 1622273 ) ; Cruise M-8, Rep. 3, 15 May 1983, 41°13.0′N, 67°15.3′W, 57 m (1, USNM 1622274 ) ; Cruise M-11, Rep. 4, Feb 1984, 41°13.0′N, 67°15.3′W, 55 m (1, USNM 1622275 ) . Sta. 2: Cruise M-4, Rep. 2, 12 May 1982, 40°59.1′N, 66°55.9′W, 66 m (1, USNM 1622276 ) ; Cruise M-11, Rep. 5, 03 Feb 1984, 40°59.1′N, 66°55.8′W, 79 m (1, USNM 1622277 ) . Sta. 10: Cruise M-10, Rep. 4, 14 Nov 1983, 40°42.0′N, 68°35.3′W, 66 m (1, USNM 1622278 ) ; Cruise M-11, Rep. 1, 02 Feb 1984, 40°42.0′N, 68°35.3′W, 66 m (1, USNM 1622279 ) ; Rep. 5, 02 Feb 1984, 40°42.0′N, 68°35.3′W, 66 m (2, USNM 1622280 ) . Sta. 12: Cruise M-12, Rep. 2, 03 Jun 1984, 40°22.2′N, 68°30.2′W, 108 m (1, USNM 1622281 ) . Sta. 5-1: Cruise M-2, Rep. 1, 19 Nov 1981, 40°39.5′N, 67°46.2′W, 81 m (1, USNM 1622282 ) ; Cruise M-4, Rep. 5, 14 May 1982, 40°39.5′N, 67°46.2′W, 80 m (1, USNM 1622283 ) ; Cruise M-8, Rep. 5, 17 May 1983, 40°39.5′N, 67°46.2′W, 81 m (1, USNM 1622284 ) . Sta. 5-2: Cruise M-3, Rep. 1, 15 Feb 1982, 40°39.6′N, 67°45.8′W, 81 m (1, USNM 1622285 ) . Sta. 5-4: Cruise M-4, Rep. 3, 14 May 1982, 40°39.5′N, 67°46.5′W, 80 m (1, USNM 1622286 ) . Sta. 5-9: Cruise M-3, Rep. 3, 16 Feb 1982, 40°39.9′N, 67°46.7′W, 82 m (1, USNM 1622287 ) ; Cruise M-6, Rep. 3, 23 Nov 1982, 40°39.9′N, 67°46.7′W, 78 m (1, USNM 1622288 ) . Sta. 5-12: Cruise M-6, Rep. 5, 23 Nov 1982, 40°39.6′N, 67°46.1′W, 80 m, (2, USNM 1622289 ) . Sta. 5-16: Cruise M-4, Rep. 4, 16 May 1982, 40°40.16′N, 67°46.1′W, 70 m (1, USNM 1622290 ) . Sta. 5-20: Cruise M-1, Rep. 6, Jul 1981, 40°38.5′N, 67°46.1′W, ~ 78 m (1, USNM 1622291 ) ; Cruise M-6, Rep. 6, 24 Nov 1982, 40°38.5′N, 67°46.1′W, 78 m (1, USNM 1622292 ) . Sta. 5-22: Cruise M-2, Rep. 6, 20 Nov 1981, 40°39.5′N, 67°43.3′W, 80 m (1 USNM 1622293 ) . Sta. 5-28: Cruise M-11, Rep. 3, 04 Feb 1984, 40°39.2′N, 67°46.6′W, 86 m (2, USNM 1622294 ) .— Atlantic Ocean, off Long Island, Port Liberty pipeline survey, coll. P.A. Neubert, Chief Scientist. Sta. 4, 15 Feb 2012, 40°32.186′N, 073°42.150′W, 17 m (1, JAB) .

Description. Holotype (USNM 28296) incomplete, with 58 setigers, 41 mm long, 2 mm wide across thorax; with 31 thoracic setigers, branchiae from setiger 5. Twenty-five newly collected specimens available from Georges Bank ranging from small to large with 16–31 thoracic setigers; all with branchiae from setiger 5. Only one specimen complete (USNM 1622294), with 151 setigers, 33.3 mm long and 1.1 mm wide across thorax; this specimen with 21 thoracic setigers and branchiae from setiger 5. Larger incomplete specimen (USNM 1622278) with 140 setigers, 60 mm long, 2.2 mm across thorax, with 31 thoracic setigers and branchiae from setiger 5. Smallest specimens with 16–19 thoracic segments. Description here based on larger specimens with up to 31 thoracic setigers. Difference with smaller specimens presented in Variability section. Color in alcohol light tan in larger specimens, opaque white in smaller or juvenile specimens; thoracic dorsal intersegmental glandular areas white.

Pre-setiger region and first three setigers enlarged, rounded dorsally ( Figs. 38B , 39 G–H), subsequent thoracic segments flattened dorsally ( Fig. 38A ) with parapodia elevated above mid-dorsal surface producing a dorsal channel between parapodia; venter of thoracic segments smooth, with weakly developed mid-ventral line or groove ( Fig. 38C ); intersegmental groove expanded mid-dorsally in middle and subsequent thoracic segments, appearing glandular ( Fig. 38A ). Thoracic segments about seven times wider than long ( Fig. 38A, C ). Abdominal segments crowded, weakly biannulate, short, rounded ventrally, flattened dorsally with parapodia shifted dorsally; dorsal and ventral grooves absent.

Prostomium triangular, narrow, tapering to acutely pointed tip ( Figs. 38 A–C, 39G–H); no eyespots; nuchal organs narrow slits on posterior lateral margin. Peristomium short, smooth, reduced dorsally, shorter than first setiger ( Figs. 38 A–C, 39G–H); ventrally forming upper and lower lips of mouth; anterior lips consisting of two thickened lobes; posterior lips consisting of several elongate lobes extending mid-ventrally on to setiger 1 ( Fig. 38C ); when everted, proboscis consisting of 3–4 lobes.

Thorax with up to 31 setigers in largest adults; smaller specimens or juveniles in present collections with as few as 16 setigers. Thoracic notopodia all similar with a triangular-shaped postsetal lobe. Abdominal notopodia becoming narrow and elongate. Thoracic neuropodia thick, elongate, oval-shaped, bearing rows of uncini and a few capillaries. On largest 31-setiger specimen, first setiger with single short postsetal lobe, increasing to two lobes on setiger 2, then up to 6–7 lobes by setigers 10–11 ( Fig. 38D ) and 12–13 postsetal lobes by setiger 25 ( Fig. 38E ); smaller specimens with a single postsetal lobe for up to 10 setigers ( Fig. 39 G–H), increasing gradually to about six lobes in posterior thoracic setigers; postsetal lobes arranged in a row posterior to rows of uncini ( Fig. 38 D–E). Individual subpodial lobes or papillae first present from about setiger 21–22, increasing to about 20–22 on a side by setiger 25 ( Fig. 38E ), encircling venter as fringe and continuing on to few anterior abdominal setigers, then absent along rest of body. Abdominal neuropodia with elongate flange ventral to neuropodium, initially rounded apically and with single lobe derived from thoracic postsetal lobes ( Fig. 38F ); this transitioning into a short ventral cirrus located on dorsal margin of narrow subpodial flange ( Fig. 38G ); ventral cirrus and flange continuing along most of body. Interramal cirrus or process absent.

Branchiae from setiger 5 on all specimens ( Fig. 38A ); branchiae mid-dorsal, well-developed at first with broad base tapering apically ( Fig. 38 D–E); branchiae of abdominal setigers longer, triangular, heavily ciliated ( Fig. 38 F–G).

Thoracic notosetae all crenulated capillaries, initially numbering about 75–90 per fascicle, then reduced to about 25–30 in last few thoracic setigers. Abdominal notosetae including a fascicle of camerated capillaries and 2–4 furcate setae; furcate setae with blunt-tipped unequal tynes each with an apical notch ( Fig. 39F ); tynes with thin needles directed medially; shaft smooth, transverse rows of barbs not apparent. Thoracic neurosetae including conspicuous spinous uncini and a few capillaries arranged in 4–6 narrow rows with spines of each row alternating in position with spine of adjacent row. Uncini of first three rows narrow blunt-tipped spines with transverse ribs across shaft ( Fig. 39A ); uncini of subsequent neuropodia of rows 4–6 with large, recurved spines with prominent ribs along one edge and a few narrower ribbed spines ( Fig. 39 B–C), usually in ventral most part of fascicle; uncini of last 8–10 thoracic setigers on a 31-setiger specimen, large with expanded apex notched and with transverse ribs not apparent ( Fig. 39D ). Capillaries more numerous in setigers 1–3, then reduced to a single dorsal fascicle and 1–2 groups of a few capillaries near middle of neuropodium on following setigers. Abdominal neurosetae consisting of 10–15 long serrated capillaries and 1–2 thin flail setae. One or two minute imbedded aciculae also present, usually not emergent. Flail setae with row of barbs along one edge, apex with a thin arista ( Fig. 39E ).

Pygidium consisting of four large rounded lobes around anal opening; with two thin dorsolateral anal cirri ( Fig. 39I ).

Variability. There is considerable variability in the sequence of development and number of neuropodial postsetal lobes and subpodial papillae of Orbinia swani based on the size and stage of development of individual specimens. Some data are presented in Table 2 , including a comparison with O. americana , which might actually be O. swani (see below). With increasing size, the number of postsetal lobes increases up to six in the largest specimens. Subpodial lobes or papillae do not appear until about 19 thoracic setigers are developed. With the development of 21 thoracic setigers, 1–5 subpodial lobes may be present. However, a fully developed subpodial fringe with about 22 papillae on a side is not apparent until 30–31 thoracic setigers are present. The difference in numbers and arrangement of these lobes and papillae are so different between juveniles and larger adults of this species that smaller specimens have most likely been mis-identified. In our collections some of the smaller specimens were originally referred to the genus Leodamas (Maciolek et al. 1985) . Day (1973: 89) described a separate species from North Carolina with 21 thoracic setigers, O. americana , that was stated to differ from O. swani in having “… fewer thoracic segments, fewer foot-papillae and fewer stomach papillae.”

The distribution of barbed and smooth thoracic neuropodial uncini or spines also appears to change with growth. All specimens appear to have narrow, elongate uncini with blunt tips and transverse rows of ribs on the shaft in setigers 1–3. From setiger 4, heavier recurved spines with expanded tips and rows of transverse barbs are the dominant spines, with the narrower uncini reduced to the lowermost locations in posterior rows. Heavier spines with expanded tips and transverse ribs on the convex side occur from about setiger 16 in smaller specimens and from setigers 10–31 in the largest specimens. In larger specimens, the transverse ribs disappear and the spines are more or less smooth on the convex surface; most of these smooth spines of posterior thoracic neuropodia have two subterminal grooves near the tip. In some views the lines of these grooves may appear to be flanges but are actually grooves on either side of a ridge. Thus, there are three different kinds of thoracic neuropodial uncini: (1) a thin elongate blunt-tipped spine with transverse ribs across the shaft; these are limited to setigers 1–3 and posterior or ventral sections of a few subsequent setigers; (2) larger recurved spines from setiger 4 that have blunt tips and prominent transverse ribs on the concave side of the shaft, and (3) large spines, sharply curved apically with ribs on the convex side of the curved tip or with the convex side lacking transverse ribs and divided by two longitudinal grooves.

Biology. Several specimens were observed with large eggs in, or associated with, the subpodial flanges. The largest eggs measured were 225–335 µm in diameter from a specimen at Sta. 5-22 (USNM 1622293). Large eggs of this diameter suggest direct development, likely with brood protection ( Blake 1980).

Sediments on Georges Bank where Orbinia swani was collected mainly consisted of>95% quartz sand with minor amounts of gravel, shell fragments, and echinoderm test fragments and minor amounts of silt and clay-size particles (Maciolek-Blake et al. 1985). Most specimens of O. swani from Georges Bank were observed to have large sand grains in their guts. Grain size at one sample from off Long Island having O. swani consisted of 99.6% sand.

Remarks. Pettibone (1957) described a large specimen from Maine with 31 thoracic setigers that served as the holotype and basis for the original description of Orbinia swani . The present collection from Georges Bank includes 21 specimens representing a range of sizes with 16–31 thoracic setigers. These collections demonstrate important differences between smaller specimens and the larger adults.

As noted in the Variability section above, several key morphological indicators change with growth. Since most species of Orbinia and Phylo have a conspicuous and well-developed fringe of subpodial or stomach papillae, smaller specimens that lack this morphology are difficult to correctly identify. However, O. swani differs from other species in having abdominal neuropodia with a single subpodial lobe that surmounts a narrow subpodial flange and continues along most of the abdominal segments. This feature is apparent in all specimens examined regardless of size. The smaller specimens, however, owing to the reduced number of postsetal lamellae and subpodial papillae, cannot be accurately identified as O. swani , unless a growth sequence is available. It is likely that other species have similar issues and some species have likely been described as separate species, when in effect they are juveniles of another species. Smaller specimens where the subpodial lamellae are absent or few in number might even be assigned to another genus such as Leodamas or Scoloplos .

Pettibone (1957), in her description of O. swani , did describe some differences with the thoracic neuropodial uncini but did not document their distribution along the body. She did not report the presence of flail setae in the abdominal neuropodia. However, flail setae are difficult to recognize as such and she likely assumed they were small capillaries rather than setae with the thin aristate tips described here.

Orbinia americana Day, 1973 , described from off Beaufort, North Carolina, was based on a single specimen with 21 thoracic setigers. Day (1973) noted the similarity to O. swani but that his specimen had fewer thoracic neuropodial postsetal lobes and fewer subpodial lobes. This is a juvenile pattern and suggests that O. americana is likely a synonym of O. swani , but due to the COVID-19 pandemic and closure of museums, it was not possible to obtain the holotype for confirmation of the synonymy at this time.

Distribution. Maine intertidal; off Long Island, 17 m; Georges Bank, 55– 86 m.

Figures

FIGURE 38. Orbinia swani (Pettibone, 1957). A, anterior end, dorsal view; B, anterior end, left lateral view; C, anterior end, ventral view; D, setiger 10, posterior view; E, setiger 25, posterior view; F, anterior abdominal setiger, posterior view; G, middle abdominal setiger, posterior view. A–C (USNM 1622287); E–G (USNM 1622278). All stained with Shirlastain A.

FIGURE 39. Orbinia swani (Pettibone, 1957). A, thoracic neuropodial uncinus, setiger 2, row 1; B–C, thoracic neuropodial uncini, setiger 2, rows 4–6; D, neuropodial uncini from posterior thoracic setiger; E, abdominal neuropodial flail setae; F, abdominal notopodial furcate seta; G, anterior half of specimen with 21 thoracic setigers, left lateral view; H, same, close-up view of pre-setiger and anterior 8 thoracic setigers, left lateral view; I, posterior segments and pygidium, left lateral view. A–F (USNM 1622278); G–I (USNM 1622294). G–I stained with Shirlastain A.

Tables

TABLE 2. Distribution and numbers of postsetal and subpodial lobes on specimens of Orbinia swani Pettibone, 1957 of different sizes and O. americana Day, 1973.

Specimens of O. swani	No th Set	No. set w/1 pst lobe	No. set w/2 pst lobes	No. set w/3 pst lobes	No. set w/4 pst lobes	No. set w/5 pst lobes
M-6, Sta. 5-12 (1st)	16	1–10	11–13	14–15	16–17	—
M-6, Sta. 5-12 (2nd)	17	1–10	11–15	16	17	—
M-4, Sta. 5-4	18	1–10	11–13	14–15	16–18	—
M-11, Sta. 5-28	21	1–10	11–14	15–16	17–18	19–21
M-10, Sta. 10	31	1	2	3	4	5–9
Holotype of O. americana	21	?	?	3	4	?