Orbiniella sp., 1954

Georgieva, Magdalena N., Wiklund, Helena, Ramos, Dino A., Neal, Lenka, Glasby, Christopher J. & Gunton, Laetitia M., 2023, The Annelid Community of a Natural Deep-sea Whale Fall off Eastern Australia, Records of the Australian Museum (Rec. Aust. Mus.) 75 (3), pp. 167-213 : 192-195

Orbiniella sp.

Fig. 20

Material examined. NHMUKANEA 2022.431 , NHMUK ANEA 2022.421 – 430 (juveniles) , AMW.52198 , AM W.52200, IN2017_ V03 _100; 9 June 2017; off Byron Bay , NSW, Australia, beam trawl, start: 28.05°S 154.08°E, 999 m, end: 28.10°S 154.08°E, 1013 m. DNA vouchers: NHMUK ANEA 2022.431 ( COI, 16 S) , NHMUKANEA 2022.433 ( COI, 16 S) , AMW.52198 (16 S) , AMW.52200 (16 S) .

Description. Best preserved specimen NHMUKANEA 2022.431 complete, ~ 9 mm long and 0.7 mm wide for ~95 chaetigers. Specimen AM W.52198 anterior fragment with 15 chaetigers. Specimen AM W.52200 anterior end, with ~40 discernible chaetigers. Body somewhat dorsoventrally flattened throughout, not divided into distinct regions; individual chaetigers narrow, similar throughout; posterior parapodia not dorsally elevated ( Fig. 20A ). Live specimens not observed, ethanol-preserved specimenstanned ( Fig 20A ).

Prostomium bluntly conical ( Fig. 20B ), without appendages, eyes absent. Nuchal organs only detected as lateralpitson prostomium. Peristomium approximatelytwice as long as prostomium, weakly annulated, buttwo achaetous rings likely present.

Parapodia reduced to low mounds from which chaetae emerge; no neuropodialpostchaetal lobes. Branchiae present; absent in anterior-most segments and becoming apparent after approximately 30 segments in adult specimens ( Fig. 20A ). Branchiae initially small and conical, increasing greatly in lengthtowards posteriorwhere theybecome strap-like, slender and elongated ( Fig. 20C ); greatly reduced in size again in the few posteriormost chaetigers.

Chaetae include both crenulated capillaries ( Fig. 20D ) and short acicular spines ( Fig. 20E ) in both rami; furcate setae absent; no evidence of imbeddedaciculae. Notochaetae as bundles of crenulated capillaries of various lengths throughout; straight slightly serrated spines present from chaetiger 1 (up to 3 per ramus observed). Neurochaetae generally slenderer thannotochaetae composed of crenulated capillaries and up to 3 spines; neuropodial spines slenderer than in notopodial one, distally slightly curved into slender tip. Pygidium with two broad lobes, anal cirri not observed.

Variation. Juveniles ( Fig. 20F ) small specimens with length of 1.2–4 mm and width of 0.1–0.2 mm, for 20 to ~50 chaetigers, branchiae always present, appearing ~chaetiger 10 regardless of size, parapodial lobes never developed.

Distribution. IN2017_V03, Station 100. Pilot whale carcass, off Byron Bay, New South Wales, Australia in 999–1013 m.

Remarks. Specimens assigned here to Orbiniella sp. are morphologically similar to Orbiniella jamesi sp. nov., but differ in the following characters: neuropodial postchaetal lobes are absent, body is more robust, posterior branchiae are shorter and thicker than in O. jamesi sp. nov. The development and number of neuropodial postchaetal lobes have been suggested to differ among developmental stages in Orbiniidae , but here the differences were observed in specimens of similarlength (7–9 mm) andpossessing similar number of chaetigers (90–95). Anumber of very small juveniles ( Fig. 20F ) werealsofoundin the samples. However, the molecular results indicate that specimens assigned here to Orbiniella sp. and Orbiniella jamesi sp. nov., as well as juvenile specimens represent the same species ( Fig. 19 ). Currently, the understanding of developmental stages in Orbiniidae is limited despite some recent advances (see Blake, 2021) and we thus tentatively ascribe the specimens without neuropodial postchaetal lobes to Orbiniella sp. , rather than Orbiniella sp. nov. that has well-developed lobes. Gunton et al. (2021, fig. 18F) assigned a specimen fitting Orbiniellajamesi sp. as described here toan unknown genus of Protoariciinae , butthis placementis atoddswith the molecularphylogeny of thisstudy whichshowsprotoariciines in a more crown position compared to Orbiniella ( Fig. 19 ).

Figures

Figure 19. Phylogeny of the Orbiniidae family based on Bayesian analysis of a combined dataset of the genes COI, 16S and 18S. Numbers adjacent to nodes indicate posterior probabilities, and taxa for which sequences have been contributed by the present study are indicated in bold.

Figure 20. Orbiniella sp. specimen NHMUK.2022.431. (A) Preserved specimen in lateral view, scale bar is 1 mm; (B) branchiae from posterior segments, scale bar is 250 µm; (C) example of crenulated capillaries, scale bar is 25 µm; (D) example of spines, scale bar is 25 µm; (E) juveniles (NHMUK ANEA 2022.421–430), scale bar is 500 µm.