Kirkegaardia annulosa Hartman 1965, new combination

Kirkegaardia annulosa (Hartman, 1965), new combination

Figures 5–6

Tharyx annulosus Hartman, 1965: 167 –168, pl. 34 (In part); Hartman & Fauchald 1971: 111; Maciolek et al. 1987: Appendix D-2.

Tharyx annulosa: Laubier 1966: 633 –637.

Not Caulleriella annulosa Banse & Hobson, 1968: 31, fig. 7a (= Tharyx kirkegaardi Blake, 1991).

Monticellina dorsobranchialis: Blake 1991: 24 –26 (In part).

Material examined. Western North Atlantic, upper end of Atlantis Canyon, R /V Atlantis Sta. Slope 4, 39°56′30″N, 70°38′54″W, 400 m, coll. 28 Aug 1962, holotype and 75 paratypes (LACM-AHF Poly 0578–9).— Off Georges Bank, continental slope south Cape Cod, US North Atlantic ACSAR Program, Cruise 5, R/V Cape Hatteras, Sta. 12, replicate 2, 0 6 May 1986, 39°54.27′N, 70°55.17′W, 548 m, coll. G. Hampson, WHOI, Chief Scientist, 90 specimens (USNM 1407158); Cruise 6, R/V Cape Hatteras, Sta. 12, replicate 1, 30 July 1986, 39°54.24′N, 70°55.09′W, 563 m, coll. G. Hampson, WHOI, Chief Scientist, 110 specimens (USNM 1407159); Cruise 6, R/V Cape Hatteras, Sta. 12, replicate 3, 30 July 1986, 39°54.24′N, 70°55.09′W, 563 m, coll. G. Hampson, WHOI, Chief Scientist, 42 specimens (USNM 1407160).

Description. Holotype elongate, narrow, in two pieces. Anterior fragment about 16 mm long, 0.3 mm wide for about 55 setigers; thoracic region with about 12 narrow segments before changing to moniliform shape, segments dorsoventrally flattened, about 3.5x wider than long, not crowded (Fig. 5 D). Posterior fragment about 4.5 mm long for about 45 setigers, thus entire holotype 20.5 mm long, 0.7 mm wide, for about 100 setigers. Color in alcohol light tan. Specimens from ACSAR station 12 off Georges Bank similar in appearance and size to holotype. One typical complete specimen with 12 thoracic setigers, long narrow abdominal region with 65 setigers and a posterior region of 11–12 setigers for a total of about 90 setigers measuring 16.65 mm long and 0.5 mm wide across thorax. Specimens from ACSAR Sta. 11 larger, similar in size to paratypes.

Paratypes larger, more robust than holotype, none complete; longest incomplete paratypes with 20–25 thoracic setigers and 30–65 abdominal setigers; posterior fragments with 45–50 setigers. Thoracic region expanded laterally, flattened dorsally, rounded ventrally, with crowded segments each about 15x wider than long (Fig. 5 A– B). Thoracic region of two paratypes 0.75 mm wide and 0.45 mm dorsal to ventral (Fig. 5 A–B); subsequent segments becoming longer, oval to moniliform in shape continuing over a long abdominal region; far posterior segments producing an inflated region, ventrally flattened or grooved, with numerous narrow segments tapering to simple pygidium with terminal anus and conical ventral lobe (Fig. 5 C).

Pre-setigerous region twice as long as wide, as long as first six setigers of holotype and 15 setigers of largest paratypes (0.75 mm). Prostomium triangular, tapering to narrow, rounded anterior margin (Fig. 5 A–B, D), of same shape and proportions as shown by Hartman, without eyes or any detectable pigment; nuchal organs at posterolateral border of prostomium (Fig. 5 B), sometimes staining internally with Shirlastain A. Peristomium elongate, with one anterior lateral annular ring apparent on holotype and specimens from ACSAR Station 12; larger paratypes with at least three thin, weakly visible lateral grooves apparent when stained with Shirlastain A, producing four annular rings visible on lateral sides (Fig. 5 A–B); long dorsal crest extending from prostomium to end of pre-setigerous area on all specimens, well developed and obvious on paratypes and specimens from ACSAR samples (Fig. 5 A–B). Paired dorsal tentacles arise on posterior margin of peristomium; first pair of branchiae lateral to dorsal tentacles, also on peristomium (Fig. 5 A–B, D); second pair of branchiae on setiger 1.

Thoracic region with parapodia shifted dorsally over midline, producing a prominent mid-dorsal groove containing a low mid-dorsal ridge (Figs. 5 A, 6D). Opening of mid-dorsal groove narrowing and closing at end of thoracic region (Fig. 5 A). Branchiae dorsal to notopodia on edge of mid-dorsal groove. Parapodia shifted laterally in abdominal segments (Fig. 5 C).

Parapodia with only low setal tori or mounds from which setae project; notosetae typically longer than neurosetae, continuing to posterior segments; long natatory notosetae not observed. Notosetae of paratypes numbering 8–12 per fascicle in thoracic region, reduced to 5–8 in middle beaded segments; neurosetae numbering 10–12 per fascicle in thoracic segments, reduced to 5–8 in middle and posterior abdominal segments. Capillary setae of thoracic region to middle abdominal segments at about setiger 45 with smooth or only finely serrate edges; denticulate capillaries first evident with 400x from about setiger 50, continuing to far posterior segments. Denticles relatively inconspicuous, limited mainly to thickest part of shaft, not observed along tapering, narrow apex; denticles of neurosetae (Fig. 5 F–G) more prominent than those of notosetae (Fig. 5 H–I). Heavily serrated or denticulated setae as illustrated in original description not observed. Unusual spinous or spike-like setae observed in middle abdominal neuropodia of a few paratypes; each with finely serrated edge of fibrils along shaft tapering to sharp point; some with a subapical spur (Fig. 5 E); these possibly transitional between non-serrated capillaries and those with distinct denticles.

Posteriormost segments narrow, tapering to pygidium bearing short lobe ventral to anal opening (Fig. 5 C).

Methyl Green stain. Methyl Green imparts a distinct pattern. The entire pre-setigerous region stains only lightly or not at all. The posterior half of the thoracic region stains distinctly with several broad stripes forming a dark blue patch on the venter, most obvious on last 3–6 thoracic setigers (Figs. 6 A–B); these stripes extend dorsally up and over the parapodia becoming narrow and dorsolateral on individual segments. There are weak narrow transverse lines of stain also evident on the sunken dorsal surface of the thorax. Ventrally, a narrow line of stain extends posteriorly from the thoracic region along the narrow mid-ventral line; this is broken into separate spots on some specimens. Laterally, there are inter-segmental lateral spots or patches on most abdominal segments (Fig. 6 B–C); these are distinctive for this species.

Remarks. Kirkegaardia annulosa is an elongate thread-like species that belongs to the group of species in which the thoracic parapodia are shifted upwards so that the dorsum becomes restricted to a narrow area between the parapodia; a narrow dorsal groove is thus produced that in K. annulosa also carries a mid-dorsal crest.

Branchiae are inserted along the elevated sides and sometimes project into the dorsal groove. The branchiae remain in a mid-dorsal position in the abdominal region while the parapodia shift to a more lateral position. This arrangement also occurs to varying degrees in K. dorsobranchialis, K. heterochaeta, and several other species and is unique among cirratulids. K. annulosa was among the first species to be described from North America with this type of thoracic region (Hartman 1965); it has not been redescribed since the original report.

The types collected from Atlantis Canyon off New England and specimens from the ACSAR stations on the New England slope examined in this study occurred relatively close to one another both geographically and with depth. This large collection revealed that several aspects of the original description by Hartman (1965) differed from what is presented in this redescription, in part because the holotype is relatively small compared to the numerous paratypes, but also because the use of stains such as Shirlastain A and Methyl Green emphasize fine details not evident without these treatments.

The pre-setigerous area including the prostomium and peristomium was described and illustrated as long, cylindrical, and smooth (Hartman 1965). While this is generally true of the small holotype, one partial lateral annular groove was evident when stained with Shirlastain A, thus producing one narrow annular ring posterior to the prostomium and one longer one on the peristomium; this is the same arrangement found in the ACSAR specimens. However, in the larger paratypes 3–4 distinct annular rings produced by lateral grooves along the peristomium were evident when stained; further, a distinct dorsal crest that extends from the prostomium posteriorly to near the dorsal tentacles at the posterior margin was observed in all specimens. The ACSAR specimens were similar in size and appearance to the holotype. The crowded thoracic segments were said to transition to beaded abdominal segments by setiger 10 (Hartman 1965). However, the thoracic region of the holotype and the ACSAR specimens consists of only about 12 setigers; whereas, the larger paratypes typically have 20–25 crowded setigers before the more elongate abdominal segments begin.

Hartman (1965: Plate 34d–e) illustrated the denticulate capillaries as having prominent denticles along the expanded shaft. In this study, denticles were visible at 400x, but they were sparse and relatively inconspicuous on several specimens examined. The unusual spike-like spinous setae that occur in a few anterior abdominal neuropodia of some paratypes were not reported in the original description. Additionally, the first pair of branchiae were reported as occurring on setiger 1 when actually they occur lateral and slightly posterior to the dorsal tentacles on the peristomium; the second pair of branchiae occur on setiger 1. This is the same arrangement as in K. heterochaeta and K. dorsobranchialis and many other species reported in this paper (See Table 1).

Among species of Kirkegaardia reported in the present study, K. annulosa is closest morphologically to K. cristata n. sp. from shallow waters of the Puget Sound in the NE Pacific, K. kladara n. sp. from the continental slope off North Carolina, and K. hampsoni n. sp. from continental shelf depths along the U.S. Atlantic coast. All four species have a mid-dorsal ridge along the entire length of the peristomium and another ridge within the middorsal thoracic channel. In K. annulosa, however, the mid-thoracic ridge is formed by the entire surface of the middorsal channel being elevated, whereas the other three species have a separate narrow ridge arising from the middle of the channel. K. kladara n. sp. differs from K. annulosa and the others in this group by having the first pair of branchiae arising from setiger 1 instead of lateral to the dorsal tentacles on the peristomium. K. annulosa and K. hampsoni n. sp. both have 0–2 annular rings on the peristomium and a length-width ratio for the pre-setigerous area of 2:1, whereas K. cristata n. sp. has four annular rings on the peristomium and a length to width ratio of 1.6:1 on the pre-setigerous area.

The MG staining pattern can also be used to separate of K. annulosa and K. hampsoni n. sp. The broad ventral stripes that occur in the posterior segments of the thoracic region of K. annulosa give way to prominent lateral intersegmental spots that occur on at least 30–40 abdominal segments. These lateral abdominal spots alone can be used to identify the species among other cirratulids in benthic samples from the U.S. Atlantic slope. Similar spots occur on K. carinata n. sp. from deep continental slope sediments off northern California (see below). However, K. carinata n. sp. differs from K. annulosa in peristomial morphology and other MG staining reactions. K. hampsoni n. sp. has prominent dorsal and lateral stain retained on the peristomium, whereas peristomial staining on K. annulosa is weak and de-stains rapidly.

Kirkegaardia annulosa and K. hampsoni n. sp. are thus the two species of the genus most closely related to one another morphologically and locally appear to be a sibling species pair with one species, K. hampsoni n. sp., occurring in nearshore and outer continental shelf depths at about 30–150 m and the other species, K. annulosa, an upper continental shelf species occurring at about 250– 550 m. K. annulosa was also recorded by Hartman (1965) from deeper slope depths, but these records have not been confirmed.

The 75 paratypes of Kirkegaardia annulosa include numerous specimens of other cirratulids including Tharyx kirkegaardi Blake, 1991 and at least two species of Aphelochaeta. The record of Tharyx annulosus by Day (1973) from off Beaufort, NC, in 80–200 m is most likely K. baptisteae, which has been identified from shelf depths off Cape Lookout, NC, in the ACSAR program; K. annulosa was not collected in those surveys.

Biology. As part of the North Atlantic ACSAR program, K. annulosa (identified as Tharyx dorsobranchialis and T. annulosus) was the dominant benthic invertebrate at three 550 m stations sampled as part of the 2-year program. At station 12, the species was the single most abundant species out of 308 species identified as part of five surveys made between November 1984 and July 1986 (Maciolek et al. 1987). Examination of the station data indicates that K. annulosa was present but not among the dominant species at the shallower 250-m station and was absent or rare at deeper stations (> 1200 m) on the same transects. Thus, based on the type collection and the ACSAR results, the species appears to be an important component of the upper slope benthic community at depths of 250– 550 m.

Most specimens from North Atlantic ACSAR Sta. 12 were covered with closely adhering tube fragments that were encrusted with fine sand grains. Some specimens from the same station collected in May 1986 were gravid with tightly packed large yolky eggs measuring 146–195 µm in diameter (average = 172.6 µm); each egg had an obvious germinal vesicle.

Distribution. Western North Atlantic, upper continental slope depths 250–550 m; other reported collections in deeper slope and abyssal depths to 4540 m likely belong to other species.