Dysponetus antarcticus Darbyshire & Brewin, 2015, n. sp.

Dysponetus antarcticus n. sp.

Figure 3 A–D

1993: Dysponetus bulbosus Hartmann-Schröder: p 128–9.

Material examined. South of King George Island, Antarctica, Sta. near O’Higgins (63° 00.49′S, 057° 09.45′W), very fine sand with Bryozoa, gorgonians, much detritus, 97 m, holotype (ZMH P- 21914) paratype (ZMH P- 27740), 22.12.1991.

Description. Holotype posteriorly incomplete, 2.7 mm long for 15 chaetigers (15th chaetiger removed for SEM). Paratype whole, juvenile, 1.29 mm for 14 chaetigers. Maximum width measured both between segments, 0.7 mm (holotype) and 0.26 mm (paratype), and including chaetae, 3.0 mm (holotype) and 1.07 mm (paratype). Description based on holotype except where specified. Measurements and counts of chaetae and cirri from holotype only.

Body shape cylindrical, ventrally flattened, tapered over last 2–3 segments at posterior. Colour pink-brown in alcohol.

Prostomium sub-rectangular (Fig. 3 A), only slightly wider anteriorly. Eyes absent or degraded beyond visibility (see Remarks for comment on presence of eyes). Median antenna, present on paratype but not holotype, anterodorsally attached, same shape but slightly shorter than lateral antennae. Lateral antennae bottle-shaped (Fig. 3 B), 1 remaining on holotype (150 µm long), arising immediately dorsal to palps. Palps (190 µm long) directed posteriorly (Fig. 3 B), stouter than antennae or cirri, approximately 3 times as long as wide. Nuchal organs not observed. Single mouth appendage small, difficult to discern. Single pair of small jaws, barely visible through body even with methyl green staining. Proboscis not observed.

First two segments elevated dorsally with four pairs tentacular cirri, dorsal pairs lost, ventral pairs of similar shape and size as following ventral cirri. First segment achaetous, second segment with notochaetae only, situated anterior to dorsal tentacular cirrus, easily confused with first segment. Third and following segments all biramous, dorsal, ventral cirri, noto- and neurochaetae present. Cirrophores present.

Notopodial lobes reduced. Dorsal cirri long (307–373 µm, few remaining on holotype, only present mid-body and posterior), slender, slightly shorter than notochaetae. Cirrophores present, styles slightly proximally swollen, distally tapering, tips blunt. Notochaetae very long, greater than body width, inserted slightly dorsal and anterior to dorsal cirrus, directed posteriorly and laterally, crossing dorsum by 10th chaetiger. Notochaetae oval in crosssection (Fig. 3 C), with alternating, offset sharp denticles, up to 20. Notochaetal count, mid-body segments, up to 23.

Neuropodia well developed, with anterior ligule. Compound neurochaetae, with heterogomph shafts and bidentate falcigerous blades with short, robust teeth on the blade (Fig. 3 D); longer than body width. Neurochaetal count, mid-body segments, at least 20–30. At least 1 accessory simple chaeta present, similar to but smaller than notochaetae, inserted distally and anteriorly on neuropodial lobe. Ventral cirri of similar shape to but smaller and shorter than dorsal cirri (length 67–227 µm, longest on median chaetigers), arising ventral to neuropodial lobe, directed posteriorly.

Final segment (paratype) lacking chaetae. Pygidium with single large projection (35 µm), cylindrical, distally tapering, inserted posteroventrally, anus terminal, some signs of damage. Posterior possibly regenerating.

Etymology. This species is named after Antarctica, the locality from which it was collected.

Habitat. Very fine sand with bryozoan, gorgonians and detritus; 97 m.

Distribution. South of King George Island, Antarctica

Remarks. Dysponetus antarcticus n. sp. has 4 eyes (described as present in Hartmann-Schröder’s 1993 paper although now faded by time), ventral cirri on segment 3, elongated palps, single mouth appendage and an anterodorsally inserted median antenna.

The presence of ventral cirri on segment 3 places Dysponetus antarcticus initially closest to D. caecus, D. paleophorus and D. pygmaeus and, potentially, D. hebes Webster & Benedict, 1887 for which the character is undescribed. However, eyes are absent in all of these species except for D. hebes which has two. In addition, Dysponetus hebes, has sphaerical palps, an anteriorly inserted median antenna and a double mouth appendage, whereas Dysponetus antarcticus has elongated palps, an anterodorsally inserted median antenna and single mouth appendage.