Aphelochaeta spectabilis Blake 2018, new species

Aphelochaeta spectabilis new species

Figures 17–18

Aphelochaeta sp. 1: Hilbig 2001: 540 (in part); Hilbig et al. 2006: 715–719 (in part); Montiel et al. 2005: 197, 2016: Appendix 2 (in part).

Aphelochaeta sp. 3: Hilbig 2001: 540 (in part); Hilbig et al. 2006:717 (in part).

Aphelochaeta cincinnata: Hilbig 2001: 540 (in part); Hilbig et al. 2006: 715–719 (in part). Not Ehlers 1908.

Aphelochaeta cf. epitoca: Hilbig 2001: 540 (in part); Hilbig et al. 2006: 717 (in part). Not Monro 1930.

Material examined. West Antarctic Peninsula, Biscoe Islands, Grandidier Channel, Larrouy Island, R / V Hero Sta. 824-5-1, 65.94°S, 65.297°W, 16 Mar 1982, 246– 270 m, holotype (USNM 1490719) and 15 paratypes (USNM 1490720); Palmer Archipelago, Melchior Island, R / V Hero Sta. 16-2, 22 Mar 1982, 64.325°S, 62.993°W, 85 m, 2 paratypes (USNM 1490721); Sta. 16-3, 22 Mar 1982, 64.326°S, 62.993° W, 85 m, 3 specimens (USNM 1490722).— South Shetland Islands, off King George Island, R / V Polarstern, EASIZ II (ANT-XV/3), coll. B. Hilbig, Sta. 48-299, 14 Mar 1998, 62°15.8ʹS, 58°42.7ʹW, MG, 207 m, (~75, SMF 24897); Sta. 48-300, 14 Mar 1998, 62°16.8ʹS, 58°42.1ʹW, MG, 423 m (~80, SMF 24898); Sta. 48-325, 17 Mar 1998, 62°21.9ʹS, 58°42.6ʹW, MG, 805 m (18, SMF 24899); Sta. 48-326, 17 Mar 1998, 62°20.1ʹS, 58°38.8ʹW, MG, 606 m (11, SMF 24900).— Drake Passage, R / V Polarstern, EASIZ II (ANT-XV/3), coll. B. Hilbig, Sta. 48-334, 19 Mar 1998, 61°26.7ʹS, 58°06.6ʹW, MG, 1028 m (1, SMF 24901); Sta. 48-341, 19 Mar 1998, 61°34.5ʹS, 58°07.0ʹW, 429 m (3, SMF 24902); Sta. 48-345, 20 Mar 1998, 61°53.3ʹS, 59°06.9ʹW, MG, 218 m (23, SMF 24903); Sta. 48-356, 20 Mar 1998, 62°00.3ʹS, 59°14.9ʹW, MG, 130 m (24, SMF 24904).— East Antarctic Peninsula, Prince Gustav Channel,

RVIB Nathaniel B. Palmer Cr. 2000-03, coll. J.A. Blake, SM grab, Sta. 01, 14 May 2000, 64°17.625ʹS, 058°34.678ʹW, 768 m, (22, JAB); Sta.02, near Cape Longing, 15 May 2000, 64°18.387ʹS, 058°37.911ʹW, 504 m (1, MCZ 149834); Sta. 27, 23 May 2000, 64°22.934ʹS, 058°36.976ʹW, 684 m (3, MCZ 149835); Sta. 28, 23 May 2000, 64°22.018ʹS, 058°30.942ʹW, 794 m (1, LACM-AHF Poly 10210); Sta. 29, 24 May 2000, 64°21.361ʹS, 058°26.637ʹW, 690 m (2, LACM-AHF Poly 10211); Sta. 33, 24 May 2000, 64°11.959ʹS, 058°41.857ʹW, 587 m (3, LACM-AHF Poly 10212).— Weddell Sea, off Halley Bay, R / V Polarstern, EASIZ II (ANT-XV/3), coll. B. Hilbig, Sta. 48-137, 09 Feb 1998, 74°36.2ʹS, 27°12.7ʹW, MG, 1521 m (6, SMF 24906).

Description. Holotype an ovigerous female, complete but broken into two parts, 18 mm long, 0.9 mm wide across anterior setigers, with 105 setigers. Anterior setigers wide, expanded, crowded, about ten times wider than long (Figs. 17 A–B, 18A), setigers of middle body becoming longer, about as wide as long, some becoming rounded, but not moniliform; about 20–30 posterior segments forming expanded region with individual segments again becoming wider (Figs. 17D, 18 B–C). Dorsal surface without distinct grooves or ridges; anterior venter with narrow mid-ventral ridgeline formed of bulge where left and right sides meet, this absent in middle and posterior segments. Color in alcohol light tan with no separate pigment markings.

Prostomium triangular, narrowing anteriorly to rounded tip (Figs. 17 A–C, 18A–C, F–G); eyespots absent; nuchal organs narrow slits at lateral margins. Peristomium with three weakly developed annular rings, best seen laterally as lines separating annuli (Fig. 17 A–C); with broad dorsal crest extending posteriorly from prostomium to anterior margin of setiger 1 (Fig. 17 A–C); in a few specimens, dorsal crest extending mid-dorsally over setiger 1. Dorsal tentacles arising medially from posterior margin of peristomium (Figs. 17 A–C, 18 A). First pair of branchiae lateral to dorsal tentacles but on anterior margin of setiger 1 (Fig. 17 A–C); second pair of branchiae on posterior margin of setiger 1 dorsal to notosetae; branchiae on following segments in a similar position (Fig. 17 A–C). Branchiae continuing to about mid body.

Anterior setigers expanded with lateral parapodia and broadly rounded mid-dorsal and ventral surfaces. Individual anterior parapodia with distinct shoulders, best developed dorsally, with noto- and neuropodia close together (Fig. 17A). Parapodia of middle and posterior setigers reduced to low tori from which setae emerge directly from body wall. Setae all capillaries arranged in single rows of 8–10 in anterior expanded segments, reduced to 3–6 in middle and posterior segments. Long natatory notosetae present on holotype from middle of anterior expanded region (Fig. 17B) to near posterior end of body; natatory setae up to five times as long as normal capillaries.

Posterior end expanded, terminating in simple pygidium with rounded lobe ventral to anal opening (Figs. 17D, 18 B–C).

Variability. The most conspicuous variable is the degree to which MG stains the pre-setigerous region (see below). In most instances, the stain is sufficiently strong to highlight two clear oval areas between the prostomium and peristomium (Fig 18 D–F). In some specimens, however, the stained areas are weaker or the stain is broken up as a speckled pattern. The ventral side of the prostomium typically stains more consistently than the dorsal side, which does not stain at all or only weakly.

There are generally three weak grooves on the peristomium producing three annular rings that are only apparent in lateral view and after staining with Shirlastain A or MG (Fig. 18 E–F). When specimens are contracted, probably due to preservation, the three rings may be more apparent and extend dorsally. However, the typically weak nature of the annular rings shown in Fig. 17 A–B is the normal feature of this species. The anterior thoracic segments are not as swollen in smaller specimens. The dorsal and ventral thoracic surfaces are broadly elevated and rounded with no prominent grooves or ridges. However, a narrow, shallow line consisting of a bulge at the junction of each segment extends mid-ventrally along the anterior thoracic segments; this is best seen when stained with Shirlastain A or MG. The middle body segments may become somewhat rounded dorsally and ventrally but are not moniliform as in some other species. The enlarged posterior segments range from narrowly fusiform to greatly expanded and bulbous.

Methyl Green stain. Aphelochaeta spectabilis n. sp. has the most prominent and distinctive MG pattern of any species of Aphelochaeta encountered in this study. On the pre-setigerous region, the stain is heaviest on the dorsal surface where a type of mask develops that includes stain concentrated on the prostomium and peristomium with a pair of large, oval-shaped clear areas between them (Fig. 18A, D–F); the peristomial annuli are defined by unstained lines; the low dorsal crest is sometimes highlighted. The stain extends laterally to the ventral side leaving the lower lips of the mouth unstained; a prominent triangular-shaped darkly stained patch is present between the mouth and first setiger (Fig. 18D, F–G). Segmentally, each of the anterior noto- and neuropodia stain prominently (Fig. 18 D–G), with the stain continuing across the venter as a striped pattern. A similar stripe occurs on the dorsal surface of the anterior segments. The parapodia that are reduced in size in middle body segments are also stained, but the stain is less noticeable due to the smaller size.

Etymology. The epithet is from the Latin, s pectabilis, for showy, denoting the conspicuous Methyl Green staining patterns that characterize this species.

Remarks. Among the Antarctic species, the swollen anterior thoracic segments, weakly defined annular rings on the peristomium, and an expanded posterior region make Aphelochaeta spectabilis n. sp. readily recognizable. In addition, the distinctive and prominent MG staining pattern on the pre-setigerous region, parapodia, and transverse bands across the dorsal and ventral surfaces are distinctive for this species. The peristomial rings are only weakly developed with the annuli usually limited to thin lateral lines best seen when stained. The first pair of branchiae occurs on the anterior border of setiger 1 rather than the posterior margin of the peristomium as in related species and with the second pair on the posterior border of setiger 1, this species has two pairs of branchiae on the first setiger.

The MG staining pattern of the pre-setiger region and ventral transverse bands of A. spectabilis n. sp. are similar to those of A. brandtae n. sp. However, in A. brandtae n. sp. the ventral transverse bands are very broad and stain intensely across the entire segment in contrast to A. spectabilis n. sp. where the stained bands are narrow and generally limited to the middle of each segment; further the segmental bands extend up, on to and across the dorsal surface of A. spectabilis n. sp. instead of being limited to the venter and neuropodia as in A. brandtae n. sp. The abdominal segments of A. brandtae n. sp. are rounded, almost moniliform and with some mature females observed with large protruding eggs; similar segments of A. spectabilis n. sp. are narrow not rounded and eggs of females are not especially large and protruding.

The posterior end of A. spectabilis n. sp. is expanded as in most specimens of A. aubreyi n. sp. and A. palmeri n. sp., but the ventral surface is usually rounded, not flattened or grooved as in the other species.

Habitat & biology. Surficial sediments in the Prince Gustav Channel where Aphelochaeta spectabilis n. sp. occurred contained 20–40% sand at the time of the May 2000 survey (Gilbert & Domack 2003). The holotype is a mature female with eggs measuring 150–165 µm in the longest dimension.

Distribution. East and west Antarctic Peninsula, 85–1028 m; Weddell Sea, 1021–1521 m.