Chaetozone australosetosa Blake 2018, new species

Chaetozone australosetosa new species

Figures 37–39

Chaetozone setosa: Hartman 1978: 166. Not Malmgren, 1867.

Chaetozone sp.: Hartman 1978: 166.

Tharyx spp. Hartman 1967: 118 (in part).

Material examined. East Antarctic Peninsula, Prince Gustav Channel, RVIB Nathaniel B. Palmer, coll. J.A. Blake, SM grab, Sta. 33, 24 May 2000, 64°11.959ʹS, 058°41.857ʹW, 587 m, RVIB Nathaniel B. Palmer, coll. J.A. Blake, holotype (LACM-AHF Poly 10219) and 72 paratypes (LACM-AHF Poly 10220); Sta. 01, 14 May 2000, 64°17.625ʹS, 058°34.678ʹW. 768 m, 151 paratypes (MCZ 149839); Sta. 27, 23 May 2000, 64°22.934ʹS, 058°36.976ʹW. 684 m, 34 paratypes, (MCZ 149840); Sta. 28, 23 May 2000, 64°22.018ʹS. 058°30.942ʹW. 794 m, 42 paratypes (MCZ 149841); Sta. 29, 24 May 2000, 64°21.361ʹS, 058°26.637ʹW, 690 m, 20 paratypes (LACM- AHF 10218); Sta. 30, 24 May 2000, 64°16.875ʹS, 058°26.985ʹW. 843 m, 9 paratypes (MCZ 149842); Sta. 34, 24 May 2000, 64°10.995ʹS, 058°34.140ʹW, 865m, 47 paratypes (LACM-AHF 10221); Sta. 35, 25 May 2000, 64°10.471ʹS, 058°28.505ʹW, 651 m, 27 paratypes (MCZ 149843); near Cape Longing, Sta. 02, 15 May 2000, 64°18.387ʹS, 058°37.911ʹW, 504 m, (31, MCZ 149844).— East Antarctic Peninsula, former Larsen Ice Shelf A area, Greenpeace Trough , RVIB Nathaniel B. Palmer, coll. J.A. Blake, SM grab, Sta. 05, 17 May 2000, 64°46.520ʹS, 060°10.720ʹW, 978 m (1, MCZ 149846); Sta. 16, 19 May 2000, 64°43.897ʹS, 059°55.745ʹW, 713 m, (8, LACM-AHF 10224); Sta. 17, 19 May 2000, 64°39.793ʹS, 060°07.662ʹW, 719 m, (1, JAB); Sta. 19, 20 May 2000, 64°42.778ʹS, 060°20.846ʹW, 879 m (5, JAB); Sta. 22, 20 May 2000, 64°46.632ʹS, 060°21.557ʹW, 868 m, (4, JAB); Sta. 23, 21 May 2000, 64°47.144ʹS, 060°21.566ʹW, 901 m, (10, JAB).— East Antarctic Peninsula, Larsen Ice Shelf area, border with Larsen B, RVIB Nathaniel B. Palmer, coll. J.A. Blake, SM grab, Sta. 10, 18 May 2000, 64°57.368ʹS, 060°13.392ʹW, 332 m, (2, MCZ 149847); Sta. 11, 18 May 2000, 64°56.669ʹS, 060°19.281ʹW, 350 m (33, MCZ 149848); Sta. 12, 19 May 2000, 64°55.101ʹS, 060°24.459ʹW, 317 m (74, MCZ 149848); Sta. 13, 19 May 2000, 64°53.517ʹS, 060°28.836ʹW, 323 m, (79, LACM-AHF 10222); Sta. 14, 19 May 2000, 64°51.818ʹS, 060°33.438ʹW, 419 m, (26, LACM-AHF 10223).— Weddell Sea, east of Larsen A area , RVIB Nathaniel B. Palmer, coll. J.A. Blake, SM grab, Sta. 25, 22 May 2000, 64°43.314ʹS, 059°38.459ʹW, 628 m, (15, USNM 1490762); Sta. 26, 23 May 2000, 64°39.564ʹS, 059°13.226ʹW, 564 m, (58, USNM 1490763); off Lindenberg Island, Sta. 03, 15 May 2000, 64°53.533ʹS, 059°30.694ʹW, 385 m, (39, MCZ 149845).— Weddell Sea, Coats Land, Halley Bay, NW of bay, USCG Glacier Sta. 7, 01 Mar 1969, 77°16ʹ S, 42°38ʹ W, 512 m (1, USNM 46814).— West Antarctic Peninsula, Marguerite Bay, USCG Eastwind, coll. D. Pawson & Squires, Sta. 4A, 24 Jan 1966, 67°53ʹS, 69°10.5ʹW, 300 m (6, USNM 1490761); Janus Island, outside of Arthur Harbor, R / V Hero Sta. 721-843, 26 Jan 1972, 64.792° S, 64.125°W, Blake Trawl, 107 m (1, USNM 1490759).— South Shetland Islands, Bransfield Strait, north of d’Urville Island, USNM Eltanin Cruise, 6, Sta. 418, 2 Jan 1963, 62.648°S, 56.170°W, Blake trawl, 311–426 m (1, USNM 56000).— West Antarctic Peninsula, Ross Sea, off Marie Byrd Land, west of Cape Colbeck, USNS Eltanin Cr. 51, Sta. 5769, 15 Feb 1972, 77.152°S, 158.993°W, Menzies trawl, 344–357 m (1 USNM 1490760).

Description. A moderate-sized species with elongate body, narrow anteriorly, gradually becoming thicker through anterior one-third (Figs. 37A, 38A, 39 A–B), then narrowing through posterior two-thirds (Fig. 39A, C). Holotype from Station 33 (LACM-AHF Poly 10219) complete with 55 setigers, 10.3 mm long, 0.3 mm wide across pre-setigerous region, 1.0 mm wide at setiger 22, 0.6 mm wide across posterior cinctured segments. Large paratypes similar to holotype in size and shape: paratype from Station 27 (MCZ 149840) with 59 setigers, 10.2 mm long and 1.0 mm wide across widest segments of anterior one-third. Other large complete paratypes with 55–63 setigerous segments and ranging from 10 to 10.3 mm long. A narrow mid-dorsal groove extends posteriorly as a line along body from about setiger 20; a mid-ventral groove extends from peristomium along entire body, not apparent in smaller specimens. Body weakly fusiform in shape due to some specimens having an enlarged stomach

prior to narrowing of body; stomach area usually swollen when filled with fine sediment (Fig. 39 A–C). Anterior segments crowded, about ten times as wide as long (Fig. 37A), in large specimens segments becoming longer, but still about three times as wide as long in posterior segments. Posterior segments with deep grooves between individual segments (Figs. 37C) and with parapodia modified, elevated, with thin membranes bearing spreading fascicles of spines and alternating capillaries producing a prominent cinctured armature (Fig. 38 B–C). Color in alcohol light tan; holotype and a few paratypes with patches of medial brown pigment on venter of a few segments from about setigers 18–22.

Pre-setiger region divided into prostomium, peristomium, and an achaetous segment fused with setiger 1. Prostomium small, triangular, narrowing to rounded tip (Fig. 37 A–B); nuchal organs reduced to thin slits at posterior margin of prostomium, difficult to observe with light microscope; eyespots absent. Peristomium distinctly separated from posterior asetigerous as a smooth triangular section with no annular rings, weakly separated from prostomium anteriorly and surrounding mouth ventrally. Achaetous segment dorsally bears a pair of dorsal tentacles followed by first pair of branchiae (Figs. 37A, 38A); this achaetous segment seamlessly merged both dorsally and ventrally with setiger 1, which bears second pair of branchiae; achaetous segment with a distinct dorsal groove separating it from anterior peristomial region, but no groove present ventrally (Fig. 37B). Branchiae continuing on subsequent segments for at least two-thirds of body. Branchiae, on setigerous segments, arising dorsal to notosetae (Figs. 37A, 38A).

Parapodia of anterior and middle segments reduced to low ridges or mounds from which setae arise; posterior setigers with swollen podia bearing raised membrane from which acicular spines arise forming prominent cinctures (Figs. 37C, 38B). Setae of anterior segments all capillaries numbering 8–10 per fascicle in noto- and neuropodia, with notosetae longer than neurosetae. Long, thin, natatory-like capillaries present in notopodia of largest sexually mature specimens, entirely absent in smaller immature specimens. Neuropodial blunt-tipped acicular spines first appear in posterior body segments from setigers 31–38 (37 in holotype); notopodial spines from setigers 34–44 (43 in holotype). Noto- and neuropodial spines initially numbering 1–3 in a fascicle with 5–6 capillaries, then increasing to 3–4 spines; posterior cinctures on last 6–8 setigers with up to 9–11 spines each in noto- and neuropodia, or 18–22 spines on a side (Fig. 38B), presence of alternating capillaries variable, with some cinctures having only 1–3 capillaries among spines; when present, capillaries thin and elongated. High membranes of left and right notopodia extending over dorsal midline forming shallow channel (Fig. 37C); ventral surface flattened. Individual spines weakly curved, with narrow blunted tip (Fig. 39E); each with internal core clearly visible in light microscopy (Figs. 38 C–D).

Body terminating in simple pygidium bearing 3–4 short papillae dorsal to anal opening and with broad cuplike structure ventral to anal opening (Figs. 37 C–D).

Variability. Due to the large number of specimens available for study, a full range of sizes is available from post-larvae with 16 setigers up to the adults as described. A 34-setiger juvenile is shown in Fig. 39 A–C. This specimen is 2.9 mm long and 0.22 mm wide and has most of the features of the large adults except that there is no dorsal groove separating the anterior peristomium from the achaetous segment that bears the dorsal tentacles and first pair of branchiae. These specimens also lack any of the long, natatory setae present in the adults that are apparently associated with sexual maturity in this species. The neuropodial acicular spines begin on setiger 12–13 and the notoacicular spines begin on setiger 15. The pygidium at this stage is only a simple rounded lobe rather than the cup-like structure of adults.

These specimens when stained with Shirlastain A and mounted in glycerin are more transparent than the larger adults and as such some internal anatomy is evident, including the brain or supraesophageal ganglion and the digestive track including the pharynx, esophagus, enlarged stomach filled with silt, and intestine (Fig. 39 A–C).

A 16-setiger post larva is shown in Fig. 39 D–F. At this early stage the prostomium and peristomium together are a smooth uninterrupted section continuing to setiger 1. The posterior setigers are moniliform prior to transitioning to the more typical cinctured segments of the adults; a few acicular spines are present in the last three setigers.

Methyl Green stain. The pre-setigerous region does not stain at all; the rest of the body stains uniformly green with no separate pattern; there are thin mid-dorsal and mid-ventral lines that stain darker and extend along most of the body.

Etymology. The epithet australosetosa is derived from the Latin austral for southern, and seta for bristle; in combination the name represents the “southern setosa” to contrast with Chaetozone setosa Malmgren, the typespecies from the Arctic.

Remarks. Chaetozone australosetosa n. sp. and a companion species, C. biannulata n. sp. (see below), are unique among species of Chaetozone in the manner in which the pre-setiger region posterior to the prostomium is divided into two parts, the second of which is interpreted as an achaetous segment that merges seamlessly with the anterior border of setiger 1. No separate segmental groove separates the achaetous segment from setiger 1. Both the dorsal tentacles and first pair of branchiae are located on the achaetous section anterior to setiger 1; the latter carries the second pair of branchiae. The first and second pair of branchiae are arranged in a line directly posterior to the dorsal tentacles. Chaetozone australosetosa n. sp. differs from C. biannulata n. sp. in having a short, more bulbous pre-setigerous area instead of an elongate, narrow, triangular-shaped form. Both species are easily distinguished from other Chaetozone species by the obvious separation of the peristomium from the achaetous segment and in turn, the merger of the achaetous segment with setiger 1.

Two other species with a similar peristomium and achaetous segment arrangement are C. reticulata n. sp. from deep-water in the Weddell Sea (see below) and C. hystricosa Doner & Blake, 2006 from off New England in 70– 100 m. However, in both of these species there is no dorsal groove separating the peristomium into two annular rings. The entire pre-setigerous region and setiger 1 combined is a single, elongate triangular-shaped area that is not interrupted by grooves. In addition, these two species have fewer spines in the posterior cinctures, i.e., about 11–13 spines on a side instead of the more than 20 in C. australosetosa n. sp. All three of the Antarctic species have a reduced number and irregular arrangement of capillaries accompanying the posterior acicular spines; C. hystricosa on the other hand, has a more typical pattern where an equal number of capillaries alternate with the spines. Chaetozone reticulata n. sp. has a distinctive reticulated pigment pattern over the dorsal surface; C. australosetosa n. sp., C. biannulata n. sp. and C. hystricosa have no consistent or distinctive pigment markings.

Distension of body segments with an enlarged “stomach” in some specimens of both C. australosetosa n. sp. and C. biannulata n. sp. is similar in some respects to another Antarctic species, C. shackletoni n. sp., and to C. brunnea Blake, 2006 from deep-water offshore California. In both of these latter species, the “stomach” is a permanent feature. In C. brunnea, the enlarged area consistently provides a distinct twist to the body; whereas, the enlarged stomach when present in C. australosetosa n. sp. and C. biannulata n. sp. causes a dorsal hump on the body and not a twist. In most specimens of C. australosetosa n. sp. examined, the stomach was not distended.

Habitat & biology. Among the nearly 300 species of benthic invertebrates identified from samples taken from the vicinity of the Larsen Ice Shelf A and Prince Gustav Channel in May 2000, Chaetozone australosetosa n. sp., with approximately 800 specimens in 33 samples, was the single most abundant species encountered. The surficial sediments at these locations consisted of 20–40% sand in the upper 0–5 cm (Gilbert & Domack 2003). The dominance of C. australosetosa n. sp. and other cirratulids in the benthic communities along the eastern side of the Antarctic Peninsula is striking and more typical of shallow-water assemblages we have sampled in North America. The benthic community ecology will be reported in another paper (Blake & Maciolek, in preparation). In addition, C. australosetosa is the most common and widespread species of Chaetozone encountered in shelf depths elsewhere in Antarctica.

Reproductive morphology included natatory setae in the largest specimens; eggs with diameters up to 165 µm were observed in specimens from the Larsen Ice shelf area collected in May 2000.

Distribution. Widespread in Antarctica; a dominant benthic species in shelf sediments of the East Antarctic Peninsula and the Weddell Sea, 317–978 m; West Antarctic Peninsula, Bransfield Straits, South Shetland Islands, McMurdo Sound.