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

Dysponetus ovalisetosus n. sp.

Figure 5 A–G

Material examined. East Falkland: near Yorke Point, west of Cape Pembroke, Sta. 46f (51° 40.4′S, 057° 45.9′W), section of Phragmatopoma colony, 3–4 m, holotype (NMW.Z.2012.082.0067), 1 paratype (NMW.Z.2012.082.006 8) 20.01.2013; near Yorke Point, west of Cape Pembroke, Sta. 46e (51° 40.4′S, 057° 45.9′W), epifaunal scraping from rock, 3–4 m, 1 paratype (NMW.Z.2012.082.0069), 20.01.2013; west Cochon Island, Sta. 14 (51° 36.217′S, 057° 47.585′W), rock walls & gullies with epifaunal growth and pink encrusting algae, 10.4 m, 6 paratypes (NMW.Z.2011.039.0173), 25.11.2011; northeast Cochon Island, Sta. 16b (51° 36.366′S, 057° 47.082′W), epifaunal scraping, 12.5 m, 13 paratypes (2— NHMUK ANEA 2015.1116–7; 2— ZMH P- 27763; 9— NMW.Z.2011.039.0174, NMW.Z.2011.039.0176–7 (SEM)), 26.11.2011; Kelp Harbour, Sta. 30 (51° 47.021′S, 059° 19.848′W), rocks in silty sand, 9.3 m, 1 paratype (NMW.Z.2011.039.0175), 0 4.12.2011.

Description. Holotype complete, slight posterior damage, 2.3 mm long for 19 chaetigers. Complete paratypes 1.12–1.93 mm for 12–19 chaetigers; 14 incomplete specimens, either anteriorly or posteriorly incomplete. Maximum width measured (holotype) both between segments, 0.29 mm and including chaetae, 0.83 mm. Description and measurements based mainly on holotype unless otherwise specified.

Body shape cylindrical, ventrally flattened, widest mid-body, tapering over last few segments. Colour whiteyellow in alcohol.

Prostomium (Fig. 5 A) oblong, only slightly wider anteriorly. Four eyes present, red-brown. Median antenna, anterodorsally attached, same shape as but half as long (36 µm) as lateral antennae (73–77 µm). Lateral antennae bottle-shaped, arising immediately dorsal to palps (Fig. 5 A, C). Palps (68–77 µm long) directed posteriorly, stouter than antennae or cirri (Fig. 5 C). Nuchal organs not observed. Single mouth appendage present (Fig. 5 C). Single pair of jaws, visible with methyl green staining. Proboscis not observed.

First two segments slightly elevated dorsally with four pairs tentacular cirri, dorsal pairs of similar shape and size as following dorsal cirri, ventral pairs slightly longer but same shape as ventral cirri (Fig. 5 A, C). First segment achaetous, second segment with notochaetae only, situated anterior to dorsal tentacular cirrus (Fig. 5 A, C). Third segment biramous; dorsal cirri present, ventral cirri absent (Fig. 5 C). Following segments all biramous with both dorsal and ventral cirri and noto- and neurochaetae (Fig. 5 C). Cirrophores present, visible where any cirrus lost. Single noto- and neuroacicula present in each biramous parapodium.

Notopodial lobes reduced. Dorsal cirri long (160–230 µm), slender, slightly 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 (Fig. 5 D, E), approximately 14. Notochaetal count, mid-body segments, over 40.

Neuropodia well developed, more curved ventrally, tip of aciculum emergent. Compound neurochaetae, with heterogomph shafts and bidentate falcigerous blades with long, tapering hairs on the blade (Fig. 5 F); up to same length as notochaetae, not longer than body width. Neurochaetae inserted ventral to acicula, number over 30, midbody segments. 1–2 accessory simple chaetae present, similar to but much smaller than notochaetae, inserted distally and anteriorly on neuropodial lobe. Ventral cirri smaller and shorter than dorsal cirri (length 30–140 µm, longest on median chaetigers), bulbous base more pronounced with more abruptly tapering and finer tips, inserted posteroventrally to neuropodial lobe, directed posteriorly. Single pair of digitiform appendages (possible external genital organs—see Remarks) inserted anteroventrally to ventral cirri (Fig. 5 G) on segment 8 (length 100 µm).

Pygidium conical with single projection (0.27 µm), cylindrical, inserted posteroventrally (Fig. 5 B).

No eggs or sperm detected in any specimens.

Etymology. The specific name ovalisetosus is derived from the latin ‘ovali’ meaning ‘oval’ and ‘seta’ meaning ‘bristle’, referring to the shape of the notochaetae which are oval in cross-section in contrast to the D-shape reported for many other species of Dysponetus. Although the other species described herein also have oval notochaetae, the character was first detected in this species (see Remarks).

Habitat. Epifaunal turf on rocks, coarse sand; 3–13 m depth.

Distribution. Falkland Islands (East Falkland)

Remarks. Dysponetus ovalisetosus n. sp. belongs to the group of Dysponetus species with 4 eyes, a mouth appendage, elongated palps and ventral cirri absent on segment 3: D. bidentatus, D. bipapillatus, D. bricklei n. sp., D. bulbosus, D. joeli, D. macroculatus and D. populonectens. Of these species, the shape of the ventral cirri, with its bulbous base and abruptly tapered distal section, make it most similar in general appearance to D. bulbosus. However, it can be separated from most of these species by the shape of the notochaetae, which are oval in crosssection rather than D-shaped. The shape of the notochaetae has not yet been confirmed for either D. bidentatus or D. macroculatus. However, D. ovalisetosus has anterior insertion of the median antenna as opposed to dorsal insertion on D. bidentatus and the shape of the ventral cirri is much more abruptly tapering. In contrast to D. macroculatus, D. ovalisetosus has ventral cirri that taper far more abruptly and there are also far more noto- and neurochaetae present (for similar-sized animals) with fewer denticles present on the notochaetae. The remaining species, D. bricklei, also has oval notochaetae, however, as documented in the description for D. bricklei, D. ovalisetosus differs in the number of noto- and neurochaetae and the shape of the ventral cirri and neurochaetae. Dahlgren (1996; Fig. 2 A) was the first to specifically describe the cross-sectional shape of Dysponetus notochaetae with his description of D-shaped notochaetae in D. bipapillatus. In the same paper, however, he also described D. macroculatus but did not state whether the notochaetae for that species also had the same appearance. In 2009, Böggemann noted D-shaped notochaetae for D. caecus, D. hesionides and D. profundus and both Olivier et al. (2012) and Darbyshire (2012; Fig. 2 C) figured the same for D. joeli in 2012. No mention is made of shape in the description for D. populonectens however it can be determined from the published SEM images. Additional unpublished SEM’s of D. bulbosus, made during research for the same paper and figured earlier in this paper, confirm the D-shape for that species also. Cross-sectional shape is still unknown for D. bidentatus, D. hebes, D. macroculatus, D. paleophorus (which also has 1 or 2 flattened paleae in parapodia from segment 6) and D. pygmaeus. Dysponetus gracilis has notochaetae and paleae that are ‘broadly expanded instead of spinelike and curved’ (Hartman 1965). Dysponetus ovalisetosus was the first species on which oval notochaetae were observed although this character was later noted for both D. bricklei and D. antarcticus also. Further research is required to determine if these are the only Dysponetus species with this character or if any of the remaining species for which the character is unknown also exhibit it.

The additional pair of ventral digitiform appendages observed on chaetiger 8 of some specimens, were first described for D. bipapillatus (Dahlgren, 1996) but have not been reported from any other Dysponetus species until now. Dahlgren proposed that the appendages could be external genital organs, but no additional research has been done on their possible purpose. The appendages are not present on every specimen, being found on only 3 out of the 10 specimens of D. bipapillatus examined and 7 out of the 21 specimens of D. ovalisetosus, roughly the same ratio in each species. In neither case were gametes found at the same time that would indicate the sex of the specimen.