Parexogone minuscula Hartman 1953

Parexogone minuscula (Hartman, 1953)

Figure 7

Exogone minuscula Hartman, 1953: 26 –27, Fig. 5 A–F.

Exogone obtusa Hartmann-Schröder & Rosenfeldt, 1988: 44 –45, Figs 24–27.

Parexogone cf. gambiae. Fukuda 2010: 163 –165, Fig. 48 (not P. gambiae Lanera, Sordino & San Martín, 1994).

Material examined. Project ' Oceanprof '. 22°04’32.8”S, 39°54’11.4”W, 0–2 cm, 722 m deep: 2 specs (MNRJP 1172), 30 Jun 2003; 21°57’11.8”S, 39°56’04.2”W, 0–2 cm, 698 m deep: 3 specs (MNRJP 1173), 29 Jun 2003; 22°36’01.3”S, 40°21’43.7”W, 0–2 cm, 754 m deep: 4 specs (MNRJP 1174), 11 Jun 2003; 22°31’11.8”S, 40°15’12.1”W, 0–2 cm, 743 m deep: 1 spec. (MNRJP 1175), 18 Jun 2003; 22°27’31.1”S, 40°09’23.5”W, 0–2 cm, 749 m deep: 1 spec. (MNRJP 1176), 18 Jun 2003.

Additional material examined. Exogone minuscula — Antarctica, South Shetland Islands, Livingston Island, off False Bay, 31 m deep: 1 spec. (USNM 46549), coll. W.L. Schmitt, Feb 1963, det. O. Hartman; Antarctica, Antarctica Peninsula, Palmer Archipelago, Anvers Island, Arthur Harbor, 40 m deep: 20 specs (USNM 46849), coll. J.S. Rankin, 17 Mar 1968, det. O. Hartman. Exogone obtusa — Antarctica, Antarctica Peninsula, near Joinville (63°30'S, 54°15'W), mud with gravel and stones, 220 m deep: 1 spec. (ZMH P-18965, holotype), coll. R/V ' Polarstern' st. 120, 21 Nov 1984, det. G. Hartmann-Schröder, 1987; Antarctica, Antarctica Peninsula, near Joinville (62°51'S, 56°00'W), 133 m deep: 4 specs (ZMH P-19165, paratypes), coll. R/V ' Polarstern' st. 158, 26 Nov 1984, det. G. Hartmann-Schröder & P. Rosenfeldt.

Description. Body filiform, longest specimen examined 2.6 mm long, 0.2 mm wide, with 33 chaetigers. Palps triangular, totally fused, distally blunt (Fig. 7 A). Prostomium rectangular, with two pairs of eyes in trapezoidal arrangement; antennae inserted close to each other, on middle of prostomium or slightly posteriorly; lateral antennae digitiform, reaching chaetiger 2, if directed posteriorly; median antenna elongate, reaching chaetiger 4, if directed posteriorly (Fig. 7 A). Peristomium shorter than subsequent segments with small, papilliform peristomial cirri. Dorsal cirri ovate, slightly larger than peristomial cirri, absent on chaetiger 2; dorsal cirri progressively longer towards posterior body (Fig. 7 A). Ventral cirri shorter than dorsal cirri, larger than peristomial cirri. Anterior parapodia with 10–16 compound chaetae each, midbody with 5–6, posterior parapodia with 2–3 compound chaetae each; compound chaetae with subdistally spinulated shafts and spinulated and bidentate blades, with distal tooth rounded and larger; blades with tips progressively more strongly curved towards posterior body, especially in ventralmost chaetae; blades 50–15 µm long on anterior body, 23–15 µm long on midbody and 20–10 µm long on posterior body (Fig. 7 B–C, F, I–K). Dorsal simple chaetae present from anterior body, slightly sigmoid, bidentate and subdistally spinulated, distal tooth slightly larger and rounded on anterior body, with tip progressively more acute towards posterior body (Fig. 7 D–E); ventral simple chaetae present from midbody, more conspicuously sigmoid than dorsal simple chaetae, bidentate and subdistally spinulated (Fig. 7 G). Parapodia throughout with single acicula each, with inflate, rounded and apparently hollow tip (Fig. 7 H). Pygidium with long anal cirri, with length equivalent to length of 3 posteriormost chaetigers together. Pharynx through 4 chaetigers, with conical tooth slightly away from anterior border; proventricle through 4–5 chaetigers, with ca. 17–19 rows of muscle cells (Fig. 7 A).

Remarks. This species is readily recognizable by the morphology of the dorsal simple chaetae and compound chaetae, with the characteristic falcate, bidentate blades, more conspicuously in ventralmost falcigers from midbody onwards. We examined the type material of P. obtusa and could not find significant differences to P. minuscula, therefore considering the former as a junior-synonym of the latter species.

Geographic distribution and bathymetric range. Atlantic Ocean—South Georgia, Moraine Fjord, 148 m deep (type locality); Falkland Islands, 12–40 m deep (Hartman 1953). Brazilian specimens were collected from 698–754 m deep.