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

Dysponetus bricklei n. sp.

Figure 4 A–D

Material examined. Cooper Bay, South Georgia, Sta. COOP 0 2 (54° 47.063′S, 035° 48.539′W), coralline encrusted bedrock/boulder with algal turf, 5–10 m, holotype (NMW.Z.2015.008.0001), 25.01.2010; Prion Island, South Georgia, Sta. PR002-6 (54° 1.862′S, 037° 15.032′W), coralline encrusted bedrock/boulder with algal turf, 15–18 m, 1 paratype (NMW.Z.2015.008.0002), 19.11.2010.

Description. Holotype posteriorly damaged, possibly incomplete, 1.47 mm long for 22 chaetigers. Paratype complete in 3 fragments, 2.04 mm for 14 chaetigers, possibly regenerating posteriorly; 1 segment removed for SEM, 1 segment removed for sequencing (failed). Maximum width measured both between segments, 0.57 mm (holotype) and 0.44 mm (paratype), and including chaetae, 1.2 mm (holotype) and 1.09 mm (paratype). Description based on both type specimens (head appendages more visible on paratype).

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

Prostomium (Fig. 4 A) sub-rectangular, only slightly wider anteriorly. Four eyes present, red-brown. Median antenna dorsally attached, same shape as but slightly shorter (68 µm) than lateral antennae (77µm). Lateral antennae bottle-shaped, arising immediately dorsal to palps (Fig. 4 B). Palps (90–100 µm long) directed posteriorly, more stout than antennae or cirri, approximately twice as long as wide. Nuchal organs not observed. Single mouth appendage present (Fig. 4 B). Single pair of jaws, visible with methyl green staining. Proboscis not observed.

First two segments slightly elevated dorsally with four pairs tentacular cirri (Fig. 4 A), dorsal pairs of similar shape and size as following dorsal cirri, ventral pairs slightly longer but same shape as later ventral cirri. First segment achaetous, second segment with notochaetae only, situated anterior to dorsal tentacular cirrus. Third segment biramous; dorsal cirri present, ventral cirri absent (Fig. 4 B). Following segments all biramous with both dorsal and ventral cirri. Single noto- and neuroacicula present in each parapodium.

Notopodial lobes reduced. Dorsal cirri long (170–380 µm), slender, similar in length to or just shorter than notochaetae, cirrophores present. Styles proximally swollen, distally tapering, tips blunt. Notochaetae long, inserted slightly dorsal and anterior to dorsal cirrus, directed posteriorly, not meeting over or crossing dorsum. Chaetae oval in cross-section, with alternating, offset sharp denticles, up to 17 (Fig. 4 C). Notochaetal count, midbody segments, 30–40.

Neuropodia well developed, with slightly longer dorsal lobe. Compound neurochaetae, with heterogomph shafts and bidentate falcigerous blades with long, fine teeth on the blade (Fig. 4 D); not longer than body width. Neurochaetal count, mid-body segments, 28–33. No accessory simple chaetae observed. Ventral cirri of similar shape to but smaller and shorter than dorsal cirri, bulbous base more pronounced (length 90–170 µm, longer posteriorly), inserted posteroventrally to neuropodial lobe, directed posteriorly.

Final segment (paratype) lacking chaetae, single lobe present each side, no cirri or cirrophores apparent; pygidium without projection, rounded. Posterior possibly regenerating.

Etymology. Dysponetus bricklei n. sp. is named after Dr Paul Brickle, co-founder of the Shallow Marine Surveys Group, in recognition of his large contribution to subtidal ecology throughout the South Atlantic as well as his continued support of this work.

Habitat. Coralline encrusted bedrock/boulder with algal turf in shallow (5–18 m) water.

Distribution. South Georgia (Prion Island, Cooper Bay).

Remarks. Dysponetus bricklei n. sp. is similar to D. bipapillatus, D. bidentatus, D. bulbosus, D. joeli, D. macroculatus, D. ovalisetosus n. sp. & D. populonectens in having the combination of the following characters: 4 large eyes, elongated palps and ventral cirri absent on segment 3. The remaining species all have either 0 or 2 eyes, except for D. antarcticus which has 4 eyes but has a ventral cirrus on segment 3. Using SEM, D. bricklei can be separated from all of the above, except for D. bidentatus and D. macroculatus (for which the character is undocumented), and D. ovalisetosus on the shape of the notochaetae which are oval in D. bricklei but D-shaped in all of the rest (e.g. Dahlgren 1996, Fig. 2 A for D. bipapillatus). This character, unfortunately, is not detectable with light microscopy, however there are several other characters which are. The palps of D. populonectens are elongated cylindrical whereas those of D. bricklei have a bulbous base, D. bipapillatus and D. macroculatus both have anterior insertion of the median antenna whereas D. bricklei has anterodorsal insertion, D. joeli has accessory neurochaetae and 20–26 neurochaetae as opposed to absent accessory chaetae and 28–33 neurochaetae, D. bidentatus has slender, tapering ventral cirri unlike the bulbous base and abruptly tapering tips of the ventral cirri on D. bricklei and D. ovalisetosus has neurochaetae with long, tapering hairs on the blade in contrast to the shorter spines on the neurochaetae of D. bricklei. Additionally, the ventral cirri of D. ovalisetosus have more bulbous bases than those of D. bricklei and the notochaetae and neurochaetae are more numerous.

Dysponetus bricklei is most similar to D. bulbosus in many characters. Both have the same arrangement of chaetae and cirri over the first 3 segments and similar-shaped dorsal and ventral cirri and head appendages.

Dysponetus bulbosus however has far fewer noto- and neurochaetae with only up to 16 notochaetae and 20 neurochaetae recorded compared to the average of 20–30 notochaetae and 28–33 neurochaetae counted on D. bricklei. Additionally, the final chaetiger of D. bulbosus has some chaetae and a pygidial projection compared to no chaetae or projection on D. bricklei (although the posterior of the latter may be regenerating).