Microrbinia linea Hartman, 1965

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 : 94-99

Microrbinia linea Hartman, 1965

Figures 47–48

Microrbinia linea Hartman, 1965: 129–130 , pl. 24; Hartman & Fauchald 1971: 92; Maciolek et al. 1987a: D-4; Maciolek et al. 1987b: D-3; Blake et al. 1987: C-4; Blake 1993: 127–128, figs. 2–3; 1994: 921–925, Fig. 2 ; Blake & Grassle 1994: 850, 853, 855–861, 864–865; Hilbig 1994: 942.

Material examined (3,371 specimens). Off New England, U.S. North Atlantic ACSAR program, coll. G.W. Hampson, Chief Scientist. Sta. 3: Cruise NA-4, Rep. 2, 22 Nov 1984, 40°01.42′N, 66°20.32′W, 1334 m (2, USNM 1622339 ). Sta. 5: Cruise NA-1, Rep. 1, 05 Nov 1984, 40°05.16′N, 67°30.03′W, 2065 m (17, USNM 1622340 ); Rep. 2, 05 Nov 1984, 40°05.16′N, 67°30.01′W, 2070 m (23, USNM 1622341 ); Rep. 3, 05 Nov 1984, 40°05.29′N, 67°29.96′W, 2045 m (41, USNM 1622342 ); Cruise NA-2, Rep. 1, 29 Apr 1985, 40°05.07′N, 67°29.78′W, 2060 m (27, USNM 1622343 ); Rep. 2, 29 Apr 1985, 40°05.07′N, 67°29.78′W, 2065 m (21, USNM 1622344 ); Rep. 3, 29 Apr 1985, 40°05.07′N, 67°29.88′W, 2065 m (22, USNM 1622345 ); Cruise NA-3, Rep. 1, 04 Jul 1985, 40°05.11′N, 67°29.84′W, 2058 m (13, USNM 1622346 ); Rep. 2, 04 Jul 1985, 40°05.08′N, 67°29.85′W, 2060 m (10, USNM 1622347 ); Rep. 3, 04 Jul 1985, 40°05.03′N, 67°29.84′W, 2065 m (40, USNM 1622348 ); Cruise NA-4, Rep. 3, 25 Nov 1985, 40°05.07′N, 67°29.81′W, 2071 m (21, USNM 1622349 ); Cruise NA-5, Rep. 1, 29 Apr 1986, 40°05.06′N, 67°29.94′W, 2052 m (14, USNM 1622350 ); Rep. 2, 29 Apr 1986, 40°05.07′N, 67°29.87′W, 2072 m (21, USNM 1622351 ); Rep. 3, 30 Apr 1986, 40°05.01′N, 67°29.90′W, 2085 m (11, USNM1622352 ); Cruise NA-6, Rep. 1, 26 Jul 1986, 40°05.08′N, 67°29.79′W, 2063 m (22, USNM 1622353 ); Rep. 2, 26 Jul 1986, 40°05.03′N, 67°29.95′W, 2078 m (30, USNM 1622354 ); Rep. 3, 26 Jul 1986, 40°05.09′N, 67°29.67′W, 2055 m (33, USNM 16223455 ). Sta. 6: Cruise NA-2, Rep. 2, 29 Apr 1985, 40°05.03′N, 67°29.13′W, 2108 m (3, USNM 1622356 ); Cruise NA-4, Rep. 3, 25 Nov 1985, 40°05.09′N, 67°29.24′W, 2114 m (1, USNM 16223557 ). Sta. 8: Cruise NA-4, Rep. 1, 25 Nov 1985, 40°10.21′N, 67°37.24′W, 2184 m (2, USNM 1622358 ); Cruise NA-6, Rep. 3, 26 Jul 1986, 40°10.21′N, 67°37.28′W, 2188 m (1, USNM 1622359 ). Sta. 9: Cruise NA-1, Rep. 3, 12 Nov 1984, 39°50.53′N, 70°01.68′W, 1225 m (1, USNM 1622360 ); Cruise NA-6, Rep. 2, 28 Jul 1986, 39°50.41′N, 70°01.62′W, 1230 m (1, USNM 1622361 ); Rep. 3, 28 Jul 1986, 39°50.43′N, 70°01.69′W, 1221 m (1, USNM 1622362 ). Sta. 10: Cruise NA-1, Rep. 1, 12 Nov 1984, 39°48.22′N, 70°05.33′W, 1200 m (1, USNM 1622363 ); Rep. 2, 13 Nov 1984, 39°48.10′N, 70°05.29′W, 1234 m (4, USNM 1622364 ); Rep. 3, 15 Nov 1984, 39°48.09′N, 70°05.17′W, 1235 m (2, USNM 1622365 ); Cruise NA-2, Rep. 1, 03 May 1985, 39°48.11′N, 70°05.26′W, 1210 m (5, USNM 1622366 ); Rep. 2, 03 May 1985, 39°48.10′N, 70°05.21′W, 1212 m (2, USNM 1622367 ); Rep. 3, 03 May 1985, 39°48.12′N, 70°05.24′W, 1210 m (2, USNM 1622368 ); Cruise NA-4, Rep. 1, 27 Nov 1985, 39°48.09′N, 70°05.29′W, 1222 m (3, USNM 1622369 ); Cruise NA-5, Rep. 1, 03 May 1986, 39°48.06′N, 70°05.25′W, 1240 m (3, USNM 1622370 ); Rep. 2, 03 May 1986, 39°48.05′N, 70°05.52′W, 1270 m (1, USNM 1622371 ); Rep. 3, 03 May 1986, 39°48.09′N, 70°05.42′W, 1237 m (10, USNM 1622372 ); Cruise NA-6, Rep. 1, 28 Jul 1986, 39°48.06′N, 70°05.26′W, 1226 m (9, USNM 1622373 ); Rep. 3, 28 Jul 1986, 39°48.09′N, 70°05.25′W, 1228 m (8, USNM 1622374 ). Sta. 12: Cruise NA-5, Rep. 2, 06 May 1986, 39°54.27′N, 70°55.17′W, 548 m (1, USNM 1622375 ). Sta. 13: Cruise NA-1, Rep. 1, 09 Dec 1984, 39°48.45′N, 70°54.86′W, 1245 m (1, USNM 1622376 ); Cruise NA-6, Rep. 3, 30 Jul 1986, 39°48.25′N, 70°54.95′W, 1273 m (1, USNM 1622377 ). Sta. 14: Cruise NA-2, Rep. 1, 05 May 1985, 39°40.91′N, 70°54.17′W, 2095 m (3, USNM 1622378 ).— Off New Jersey and Delaware , U.S. Mid-Atlantic ACSAR program, coll. Rosemarie Petrecca, Chief Scientist. Sta. 6: Cruise Mid-5, Rep. 2, 02 Aug 1985, 39°05.64′N, 72°03.24′W, 2080 m (1, USNM 1622379 ). Sta. 12: Cruise Mid-5, Rep. 3, 07 Aug 1985, 38°29.23′N, 72°42.22′W, 2495 m (1, USNM 1622380 ).— Southeastern USA, U.S. South Atlantic ACSAR program, coll. J.A. Blake, Chief Scientist. Off Cape Lookout, North Carolina, Sta. 3: Cruise SA-1, Rep. 2, 15 Nov 1983, 34°13.42′N, 75°39.45′W, 1690 m (9, USNM 1622381 ). Sta. 4: Cruise SA-1, Rep. 1, 16 Nov 1983, 34°11.68′N, 75°39.54′W, 1960 m (76, USNM 1622382 ); Rep. 2, 17 Nov 1983, 34°12.54′N, 75°39.03′W, 1940 m (52, USNM 1622383 ); Rep. 3, 17 Nov 1983, 34°11.77′N, 75°38.97′W, 1910 m (81, USNM 1622384 ); Cruise SA-2, Rep. 1, 20 May 1984, 34°11.70′N, 75°38.60′W, 2064 m (70, USNM 1622385 ); Rep. 2, 20 May 1984, 34°11.90′N, 75°38.70′W, 2029 m (95, USNM 1622386 ); Rep. 3, 20 May 1984, 34°11.50′N, 75°39.00′W, 1969 m (91, USNM 1622387 ); Cruise SA-3, Rep. 1, 13 Jul 1984, 34°12.10′N, 75°38.60′W, 2066 m (111, USNM 1622388 ); Rep. 2, 14 Jul 1984, 34°10.41′N, 75°39.10′W, 2005 m (62, USNM 1622389 ); Rep. 3, 14 Jul 1984, 34°11.17′N, 75°38.98′W, 2006 m (65, USNM 1622390 ); Cruise SA-4, Rep. 1, 23 May 1985, 34°11.20′N, 75°38.44′W, 2093 m (51, USNM 1622391 ); Rep. 2, 23 May 1985, 34°11.22′N, 75°38.44′W, 2051 m (53, USNM 1622392 ); Rep. 3, 23 May 1985, 34°11.29′N, 75°38.67′W, 2015 m (103, USNM 1622393 ); Cruise SA-5, Rep. 1, 24 Sep 1985, 34°11.27′N, 75°38.63′W, 2032 m (86, USNM 1622394 ); Rep. 2, 24 Sep 1985, 34°11.35′N, 75°38.74′W, 2000 m (50, USNM 1622395 ); Rep. 3, 24 Sep 1985, 34°11.22′N, 75°38.56′W, 2051 m (58, USNM 1622396 ); Cruise SA-6, Rep. 1, 24 Nov 1985, 34°11.17′N, 75°38.64′W, 2054 m (41, USNM 1622397 ); Rep. 2, 24 Nov 1985, 34°11.21′N, 75°38.61′W, 2049 m (35, JAB); Rep. 3, 24 Nov 1985, 34°11.23′N, 75°38.53′W, 2057 m (37, JAB). Off Cape Fear, North Carolina, Sta. 11: Cruise SA-4, Rep. 1, 22 May 1985, 33°04.86′N, 76°25.13′W, 800 m (99, USNM 1622398 ); Rep. 2, 22 May 1985, 34°04.92′N, 76°24.97′W, 802 m (116, USNM 1622399 ); Rep. 3, 22 May 1985, 33°04.87′N, 76°25.14′W, 799 m (61, USNM 1622400 ); Cruise SA-5, Rep. 1, 23 Sep 1985, 33°04.83′N, 76°25.19′W, 796 m (55, USNM 1622401 ); Rep. 2, 23 Sept 1985, 34°04.83′N, 76°25.17′W, 800 m (58, USNM 1622402 ); Rep. 3, 23 Sep 1985, 33°04.86′N, 76°25.12′W, 797 m (82, USNM 1622403 ); Cruise SA-6, Rep. 1, 22 Nov 1985, 33°04.95′N, 76°25.15′W, 804 m (79, USNM 1622404 ); Rep. 2, 22 Nov 1985, 33°04.94′N, 76°25.17′W, 804 m (113, USNM 1622405 ); Rep. 3, 22 Nov 1985, 33°04.84′N, 76°25.06′W, 807 m (76, USNM 1622406 ). Sta. 12: Cruise SA-4, Rep. 1, 22 May 1985, 33°00.31′N, 76°07.39′W, 1996 m (5, USNM 1622407 ); Rep. 3, 22 May 1985, 33°00.20′N, 76°07.25′W, 2000 m (2, USNM 1622408 ); Cruise SA-5, Rep. 1, 22 Sep 1985, 33°00.36′N, 76°07.27′W, 2004 m (1, USNM 1622409 ); Cruise SA-6, Rep. 1, 21 Nov 1985, 33°00.55′N, 76°07.45′W, 1992 m (4, USNM 1622410 ); Rep. 3, 21 Nov 1985, 33°00.38′N, 76°07.46′W, 1994 m (20, USNM 1622411 ). Sta. 13: Cruise SA-4, Rep. 1, 21 May 1985, 32°55.19′N, 76°49.78′W, 3015 m (7, USNM 1622412 ); Rep. 2, 21 May 1985, 32°55.78′N, 76°49.80′W, 3013 m (14, USNM 1622413 ); Rep. 3, 21 May 1985, 32°55.02′N, 76°49.47′W, 3015 m (17, USNM 1622414 ); Cruise SA-6, Rep. 1, 20 Nov 1985, 32°55.16′N, 75°50.25′W, 2999 m (59, USNM 1622415 ); Rep. 2, 20 Nov 1985, 32°55.22′N, 75°50.20′W, 3002 m (18, USNM 1622416 ); Rep. 3, 21 Nov 1985, 32°55.25′N, 75°50.08′W, 3006 m (14, USNM 1622417 ). Off Charleston, South Carolina, Sta. 14A: Cruise SA-5, Rep. 1, 20 Sep. 1985, 32°32.25′N, 77°15.24′W, 600 m (1, USNM 1622418 ). Sta. 14: Cruise SA-4, Rep. 1, 20 May 1985, 32°23.64′N, 77°01.13′W, 805 m (83, USNM 1622419 ); Rep. 2, 20 May 1985, 32°23.64′N, 77°01.19′W, 802 m (105, USNM 1622420 ); Cruise SA-5, Rep. 1, 19 Sep 1985, 32°23.67′N, 77°01.18′W, 796 m (36, USNM 1622421 ); Rep. 2, 19 Sep 1985, 32°23.72′N, 77°01.24′W, 799 m (64, USNM 1622422 ); Rep.3, 19 Sep 1985, 32°23.63′N, 77°01.11′W, 799 m (85, USNM 1622423 ); Cruise SA-6, Rep. 1, 18 Nov 1985, 32°23.73′N, 77°01.10′W, 799 m (90, USNM 1622424 ); Rep. 2, 18 Nov 1985, 32°23.67′N, 77°01.09′W, 799 m (63, USNM 1622425 ); Rep. 3, 18 Nov 1985, 32°23.70′N, 77°01.06′W, 799 m (85, USNM 1622426 ). Sta. 15: Cruise SA-4, Rep. 3, 16 May 1985, 32°10.74′N, 76°42.93W, 2003 m (25, USNM 1622427 ); Rep. 2, 16 May 1985, 32°12.05′N, 76°42.18′W, 1991 m (38, USNM 1622428 ); Cruise SA-5, Rep. 1, 18 Sep 1985, 32°12.002′N, 76°42.23′W, 1988 m (18, USNM 1622429 ); Rep. 2, 18 Sep 1985, 32°11.99′N, 76°42.23′W, 1991 m (13, USNM 1622430 ); Rep. 3, 18 Sep 1985, 32°11.97′N, 76°42.19′W, 1991 m (23, USNM 1622431 ). Sta. 16: Cruise SA-4, Rep. 1, 15 May 1985, 31°35.23′N, 75°10.62′W, 3008 m (17, USNM 16224323 ); Rep. 3, 15 May 1985, 31°35.10′N, 75°10.36′W, 3008 m (23, USNM 1622433 ); Cruise SA-5, Rep. 1, 14 Sep 1985, 31°35.19′N, 75°10.39′W, 3009 m (10, USNM 1622434 ); Rep. 2, 16 Sep 1985, 31°35.14′N, 75°10.28′W, 3011 m (9, USNM 1622435 ); Rep. 3, 16 Sep 1985, 31°35.00′N, 75°10.46′W, 2999 m (10, USNM 1622436 ); Cruise SA-6, Rep. 1, 20 Nov 1985, 31°35.12′N, 75°10.34′W, 3029 m (25, USNM 1622437 ); Rep. 2, 20 Nov 1985, 31°35.10′N, 75°10.34′W, 3009 m (12, USNM 1622438 ); Rep. 3, 20 Nov 1985, 31°35.16′N, 75°10.22′W, 3012 m (15, USNM 1622439 ) .

Description. A small, threadlike species; largest complete specimen (USNM 1622397) with 39 setigers, 8.4 mm long and 0.21 mm wide across thorax; another specimen (USNM 1622393) with 35 setigers, 6.5 mm long and 0.2 mm wide. Hartman (1965) reported largest specimens with about 35 setigers, 0.4 mm wide and 7.5–8.5 mm long. Over 3,100 specimens available in ACSAR collections representing a full range of sizes and reproductive modes. Body thin, cylindrical along entire length ( Fig. 48E ), with 6–7 short, uniannulate anterior thoracic setigers ( Figs. 47 A–B, 48A–B) followed by elongate, biannulate abdominal setigers with parapodia located at posterior margins ( Figs. 47 C–D, 48D, F). Dorsal and ventral longitudinal grooves or ridges absent. A few anterior abdominal setigers of presumed males with conical gland-like dorsal organs on individual segments ( Figs. 47A, C , 48A , C–D). Females with one, rarely two, swollen middle abdominal segments bearing two long ova ( Figs. 47D, F ; 48 F ) measuring between 233 and 321 µm in longest dimension ( Blake 1993). Color in alcohol: opaque white.

Pre-setiger region long, narrow, about as long as first three setigers, merging with setiger 1 both dorsally and ventrally ( Figs. 47 A–C, 48A–B, E). Prostomium triangular, tapering to narrow conical tip; nuchal organs narrow slits on posterior lateral margins, difficult to observe; eyespots absent. Peristomium a single large smooth ring dorsally ( Figs. 47A , 48A ); ventrally forming anterior and posterior lips of mouth; each lip with 5–6 short lobes ( Fig. 47B ); everted proboscis not observed on any specimen.

Thoracic segments numbering 6–7, each relatively short, about three times wider than long and with narrow transverse intersegmental groove both dorsally and ventrally ( Figs. 47 A–B, 48A–B). Transition to abdominal segments denoted by a narrowing and elongation of individual segments and thickening and elongation of neuropodia ( Figs. 47A , 48A ). Abdominal segments with parapodia located at posterior margin preceded by intersegmental annular ring ( Figs. 47A , 48A ); some abdominal segments 5–6 times longer than wide ( Figs. 47D , 48E ), sometimes moniliform. Branchiae entirely absent.

Notopodia with either a short papilla or no visible postsetal lobe on setigers 1–2; thereafter prominent digitate notopodial postsetal lobe present, longest in posterior thoracic and anterior abdominal setigers ( Figs. 47A , 48A ). Neuropodia with a short digitate postsetal lobe in thoracic setigers, becoming long and thickened in abdominal setigers ( Fig. 47 B–C). Neuropodia of abdominal segments lateral, not shifted dorsally as in most orbiniids; prominently visible as thickened lateral lobes along body ( Fig. 47C ). Notopodia located in dorsal location along body ( Fig. 47A ).

Notopodia with a spreading fascicle of 6–10 camerated capillaries in thoracic setigers and anterior abdominal setigers; capillaries gradually replaced in middle and posterior abdominal setigers with 4–6 serrated spines; these with distinct serrations along one margin, tapering to a curved, pointed tip ( Figs. 47E , 48H ). Neurosetae of thoracic segments 4–5 camerated capillaries; these continuing over anterior abdominal setigers, then reduced to 1–3 smooth capillaries in middle and posterior abdominal setigers.

Pygidium a simple lobe bearing four thin anal cirri ( Fig. 48E, G ).

Remarks. Microrbinia linea is a small meiofaunal polychaete that is a dominant species in soft sediments on the U.S. Atlantic continental slope, especially off North and South Carolina. The species is unusual among orbiniids in having conical gland-like dorsal organs on the surface of some anterior abdominal segments. These structures appear to be associated with males; females have one or rarely two elongate swollen segments containing large eggs, but none of the dorsal gland-like structures.

Morphologically, M. linea can be recognized by the thin, threadlike body, in which six or seven short anterior setigers constitute the thorax; these transition to abdominal segments that are longer, biannulate, and sometimes moniliform, with the actual parapodia located on the posterior margin of individual segments. The neuropodia arise laterally in abdominal segments rather than being dorsally elevated as in larger orbiniids. The abdominal notopodia occur on the dorsal lateral surface. There are no branchiae. The unusual serrated spinous notosetae that occur in middle and posterior abdominal segments have not been reported in other orbiniids.

Biology. Microrbinia linea occurs along the entire U.S. Atlantic continental slope from off New England to South Carolina. The species was the top ranked dominant benthic invertebrate species at several sites off the Carolinas: Sta. 4 (2000 m) off Cape Lookout; Sta. 11 (800 m), Sta. 12 (2000 m), and Sta. 13 (3000 m) off Cape Fear; Sta. 14 (800 m) and Sta. 15 (2000 m) off Charleston ( Blake & Grassle 1994). The species was also abundant at a few sites off New England but was not a dominant species (Maciolek et al. 1987b).

Blake (1994) provided data on vertical distribution in 10 x 10 x 10 cm cores that were cut to 0–2, 2–5, and 5–10 cm depth intervals from which the individual organisms were retained, counted, and identified. Raw numbers and percentages for these depth intervals for Microrbinia linea at Station 4 (2000 m) were: 0–2 cm: 65 (25%), 2–5 cm: 186 (71.5%), and 5–10 cm: 9 (3.5%). Thus 96.5% of all specimens of M. linea occurred in the upper 5 cm with the majority in the 2–5 cm depth interval. These results suggest that M. linea is a subsurface deposit feeder, but due to its small size is more or less limited to the upper 5 cm.

Due to the large numbers of specimens of M. linea available from seasonal collections on the North Carolina slope, Blake (1993) was able to assess some aspects of reproduction and size frequency of specimens from Sta. 4 off Cape Lookout at 2000 m. As part of this study, the unusual conical, gland-like dorsal organs on anterior abdominal segments of some specimens were observed. A histological examination revealed them to be glandular in nature, but no distinct pore or opening through either the structure or the integument was observed. This structure was prevalent in males and likely plays some role in copulation. Females lacked these structures.

Out of 577 individuals examined, 253 were found either with gametes or some evidence of sexuality. Of these, 201 were males (79.4%) and 52 were females (20.6%). Mature females bear two large eggs in a single segment ( Figs. 47E , 48F ) or rarely a second segment. The eggs are elongate (200–364 x 75–152 µm) and with a germinal vesicle in a finely granulated cytoplasm ( Fig. 47F ). The largest eggs were found in July 1984 (mean = 369 x 119 µm) and May 1985 (mean = 321 x 136 µm). Males were observed with gametes at all stages of spermiogenesis. Mature sperm are 8–10 µm long and long-headed. The highest percentages of mature males were observed in July and September, while females were most prevalent in May, September, and November ( Blake 1993).

Size classes of specimens from Sta. 4 were based on thoracic width: four size classes were present: 5–10 µm, 10–15 µm, 15–20 µm, 20–25 µm. The 10–15 µm size class was most abundant in November 1983, and May and July 1984, while the 15–20 µm size class was most abundant in May, September, and November 1985. No seasonal trends are apparent from these data. The presence of females with large eggs in each seasonal set of samples and the relatively uniform size class data suggested that reproduction and recruitment is year round ( Blake 1993).

Distribution. U.S. Atlantic continental slope, New England to South Carolina; 600–3015 m; off mouth of the Amazon River, 770–1500 m.

Figures

FIGURE 2. Leitoscoloplos acutus (Verrill, 1873). A, anterior half of large specimen, dorsolateral view; B, middle thoracic setiger, anterior view; C, middle abdominal setiger, posterior view; D, fascicle of abdominal notosetae with capillaries and furcate seta; E, posterior setigers, left lateral view; F, posterior end, dorsolateral view. A, E (MCZ 161445); B (USNM 1620847); C–D (USNM 1620841); F (MCZ 161419). All stained with Shirlastain A.

FIGURE 47. Microrbinia linea Hartman, 1965. A, anterior end, dorsal view; B, anterior end, ventral view; C, anterior end, right lateral view; D, middle abdominal setigers with eggs; E, abdominal notopodial serrated spine; F, egg.A–B, E (USNM 1622397); C–D, F (USNM 1622388; after Blake 1993).

FIGURE 48. Microrbinia linea Hartman, 1965. (USNM 1622392): A, anterior end, dorsal view; B, anterior end, ventral view; C, anterior abdominal setiger with dorsal organs, lateral view; D, anterior abdominal setiger with dorsal organs, dorsal view; E, entire worm showing threadlike nature of body; F, abdominal ovigerous segment, dorsal view; G, posterior end with anal cirri; H, abdominal notopodial serrated spine. A–G stained with Shirlastain A.