Orbiniella mimica Blake 2021, new species

Orbiniella mimica new species

Figures 53–54

urn:lsid:zoobank.org:act: AF091D23-7398-420E-B018-9D25E04A516F

Orbiniella sp. 2: Maciolek et al. 1987a: D-4; Hilbig 1994: 942 (in part).

Material examined. (34 specimens) Off New Jersey and Delaware, U.S. Mid-Atlantic ACSAR program, coll. Rosemarie Petrecca, Chief Scientist. Sta. 11: Cruise Mid-4, Rep. 1, 17 May 1985, 38°40.10′N, 72°56.43′W, 1519 m, holotype (USNM 1622511) and 3 paratypes (USNM 1622512); Cruise Mid-1, Rep. 1, 07 May 1984, 38°40.22′N, 72°56.27′W, 1520 m, 2 paratypes (USNM 1622513); Rep. 3, 07 May 1984, 38°40.22′N, 72°56.27′W, 1520 m, 1 paratype (USNM 1622514); Cruise Mid-2, Rep. 1, 04 Aug 1984, 38°40.20′N, 72°56.30′W, 1514 m (1, USNM 1622516); Cruise Mid-3, Rep. 1, 04 Dec 1984, 38°40.13′N, 72°56.27′W, 1540 m (3, USNM 1622517); Rep. 2, 04 Dec 1984, 38°40.14′N, 72°56.31′W, 1520 m (2, USNM 1622518); Cruise Mid-5, Rep. 1, 06 Aug 1985, 38°40.12′N, 72°56.45′W, 1505 m, 1 paratype (USNM 1622519); Rep. 2, 06 Aug 1985, 38°40.12′N, 72°56.47′W, 1502 m (4, USNM 1622520); Rep. 3, 06 Aug 1985, 38°40.14′N, 72°56.46′W, 1505 m, 2 paratypes (USNM 1622521). Sta. 2: Cruise Mid-3, Rep. 3, 02 Dec 1984, 38°35.68′N, 72°53.69′W, 2015 m (1, USNM 1622522). Sta. 9: Cruise Mid-5, Rep. 2, 06 Aug 1984, 37°17.21′N, 73°14.69′W, 2100 m (1, USNM 1622523). Sta. 13: Cruise Mid-1, Rep. 1, 02 April 1984, 37°53.33′N, 73°45.09′W, 1613 m (2, USNM 1622524); Rep. 2, 02 Apr 1984, 37°53.38′N, 73°45.10′W, 1613 m, 1 paratype (USNM 1622525); Cruise Mid-2, Rep. 1, 07 Aug 1984, 37°53.33′N, 73°45.01′W, 1614 m (1, USNM 1622526); Rep. 2, 07 Aug 1984, 37°53.28′N, 73°45.26′W, 1619 m (1, USNM 1622527); Rep. 3, 08 Aug 1984, 37°53.22′N, 73°45.17′W, 1619 m, 1 paratype (USNM 1622528); Cruise Mid-3, Rep. 1, 30 Nov 1984, 37°53.32′N, 73°45.10′W, 1615 m, 1 paratype (USNM 1622529); Rep. 2, 30 Nov 1984, 37°53.35′N, 73°45.00′W, 1615 m, 3 paratypes (USNM 1622530); Cruise Mid-4, Rep. 2, 19 May 1985, 37°53.29′N, 73°45.30′W, 1607 m (1, USNM 1622531); Cruise Mid-5, Rep. 1, 09 Aug 1985, 37°53.26′N, 73°45.21′W, 1607 m (1, USNM 1622532); Rep. 2, 09 Aug 1985, 37°53.27′N, 73°45.28′W, 1605 m (1, USNM 1622533); Rep. 3, 10 Aug 1985, 37°53.30′N, 73°45.27′W, 1608 m (1, USNM 1622534); Cruise Mid-6, Rep. 2, 16 Nov 1985, 37°53.27′N, 73°45.30′W, 1611 m, 1 paratype (USNM 1622535). Sta. 14: Cruise Mid-1, Rep. 3, 02 Apr 1984, 37°53.86′N, 73°44.68′W, 1503 m, 1 paratype (USNM 1622536); Cruise Mid-6, Rep. 1, 15 Nov 1985, 37°53.69′N, 73°44.69′W, 1515 m, 1 paratype (USNM 1622537).

Description. A small species, holotype (USNM 1622511) complete, with 36 setigers, 5.6 mm long and 0.8 mm wide; paratype (USNM 1622513) with 30 setigers, 5.76 mm long and 1 mm wide, another paratype (USNM 1622519) with 34 setigers, 5.45 mm long and 0.9 mm wide; and a larger paratype (USNM 1622525) with 38 setigers, 7.3 mm long and 0.9 mm wide. Body with similar segments throughout, lacking separate thorax and abdomen. Body dorsoventrally flattened with narrow mid-dorsal groove along most of body and narrow, mid-ventral ridge formed of bulges where segmental rings meet mid-ventrally. Segmentation including a prominent ring extending from each parapodium across dorsal and ventral surfaces; each segmental ring separated from next segment by one and then two intersegmental rings along most of body, producing a tri-annulate appearance (Figs. 53A, 54C); anterior and posterior-most segments with only a single intersegmental ring (Figs. 53 A–C, 54A). Each parapodium and segmental ring with numerous glands (Fig. 53A) that stain with Methyl Green (Fig. 54D); these glands not apparent in intersegmental rings. Parapodia all lateral (Fig. 53D), not shifted dorsally in posterior segments. Prostomium and parapodia of some anterior setigers with a band of short cilia extending across dorsal surface; these cilia not readily apparent except at 1000x magnification in light microscope, appearing as cells or groups of short cilia. Color in alcohol light tan to opaque white; a few scattered dark pigment spots apparent dorsally over peristomium and setiger 1 on a few specimens; a few anterior notopodia dusky brown on some specimens.

Pre-setiger region narrower than setiger 1 and following segments (Figs. 53 A–C, 54A), longer than wide, about as long as first 3–4 setigers. Prostomium wider than long, broadly rounded on anterior margin (Figs. 53 A–B, 54A); dorsal surface covered with numerous papillae (Fig. 53A), ventrally with vestibule leading to mouth opening on peristomium (Fig. 53B); eyespots absent; nuchal organs lateral ciliated patches on posterior margin (Fig. 53A). Peristomium formed of two separate rings entirely crossing dorsal (Figs. 53A, 54A) and ventral surfaces (Fig. 53B); anterior ring with anterior border sculptured with short rounded lobes on dorsum (Fig. 53A); posterior ring relatively smooth on anterior border. Ventrally, anterior peristomial ring forming posterior lip of mouth with 5–6 short medial lobes (Fig. 53B); a narrow medial groove continuing to anterior border of second peristomial ring, with 2–3 mounds on anterior edge (Fig. 53B). Mouth opening on peristomium enclosed by two lateral folds. Proboscis not fully everted on any specimens; one specimen (USNM 1622525) with partially everted rounded lobe.

Setiger 1 and those following with relatively simple noto- and neuropodia formed of short tori from which spines and capillaries emerge. A single short, digitate postsetal lobe present in notopodia; absent in neuropodia (Fig. 53D).

Noto- and neuropodia typically with a single smooth, pointed acicular spine and 6–12 long, serrated capillaries (Fig. 53 D–F); spines may increase up to two or three per noto- or neuropodium in some middle and posterior setigers but inconsistent between specimens. Capillaries not camerated, each with minute, sharply pointed barbs along length (Fig. 53F).

Pygidium narrow, terminating in four lobes around anal opening, each lobe with long, thin anal cirrus; dorsal pair shorter than ventral pair (Figs. 53C, 54B). Thin anal cirri often lost, broken off from the basal lobes, best observed entirely intact in smaller specimens.

Remarks. Among deep-water species of Orbiniella tabulated by Blake (2020) and having postsetal lamellae limited to the notopodia, O. mimica n. sp. is most similar to O. petersenae Parapar et al., 2015 from the northeast Atlantic in having triannulate segmentation along most of the body. Orbiniella mimica n. sp. differs, however, in having a papillated dorsal surface on the prostomium instead of a smooth surface, in having instead of lacking a mid-dorsal groove along the body, and having four long, thin pygidial cirri instead of short, thickened rounded lobes. SEMs of the pygidial lobes of O. petersenae in Parapar et al. (2015: Fig. 6A, D, F) do not show any indication that longer lobes had broken off. In addition, there are differences in the oral and other aspects of peristomial morphology. The transverse rows of minute ciliated cells observed on the dorsum of the prostomium and a few anterior setigers of some specimens of O. mimica n. sp. were not evident on other SEMs published for O. petersenae (Parapar et al. 2015).

Etymology. The epithet is from mimicus, Latin for an imitator, in reference to the fact that during the ACSAR programs O. mimica n. sp. and O. armata n. sp. were collectively identified as Orbiniella sp. 2 because differences were not recognized at the time. In addition, both species are also similar to O. petersenae recently described from the northeastern Atlantic. The differences between the two North American species, including the large noto- and neuropodial spines of O. armata n. sp. were not recognized until the more careful morphological comparisons in the present study.

Biology. Lower slope sediments at the Mid-Atlantic stations off New Jersey and Delaware where Orbiniella mimica n. sp. was most prevalent consisted of 96+ percent silt-plus-clay with only minimal amounts of sand-sized particles (Maciolek et al. 1987a).

Distribution. Continental slope off the U.S. Mid-Atlantic region, 1502–2144 m; most common in the 1500– 1620 m depth range.