Oligobregma renuncula, Blake, 2023

Blake, James A., 2023, New Species of Scalibregmatidae (Annelida) from Slope and Abyssal Depths off Eastern Australia, Records of the Australian Museum (Rec. Aust. Mus.) 75 (3), pp. 271-298 : 290-293

Oligobregma renuncula sp. nov.

urn:lsid:zoobank.org:act:F62F250F-7F71-4473-94DB-3A69C1D3D037

Figs 11–12

Holotype: Abyssal plain, off Queensland, eastern Australia, SW of Fraser Island, Coral Sea , RV Investigator, Sta. 110, coll. 11 June 2017, BS, distance 6.1 km, 25.221°S 154.160°E to 25.261°S 154.200°E, 4005–4010 m ( AM W.52702) . Paratypes (20): same data as holotype (18, W.53994); —abyssal plain off Moreton Bay, Queensland, RV Investigator, Sta. 103, coll. 10 June 2017, BS, distance 6.8 km, 27.000°S 154.223°E, 27.061°S 154.223°E, 4260–4280 m (1, W.53995) ; —abyssal plain off New South Wales, eastern Australia, Hunter Marine Park , RV Investigator, Sta. 079, coll. 04 June 2017, Brenke sledge , distance 3.6 km, 32.527°S 153.898°E to 30.163°S 153.524°E, 4031 m (1, W.53996). All in Australian Museum.

Description. A small species, holotype complete with 29 setigers, 8.8 mm long, 0.87 mm wide across setiger 4, expanded middle segments 1.4 mm wide; most paratypes smaller, mostly juveniles, largest complete with 23 setigers, 3.45 mm long and 0.45 mm wide across setigers 1–4. Holotype with setigers 1–4 short, 4–5 times wider than long, following segments becoming thicker, about three times wider than long ( Fig. 11A ), producing sausage-shaped middle section, then narrowing again in posterior half of body. Smaller paratypes and juveniles enlarged and sausage-shaped from about setiger 5–6 ( Fig. 12A ), then narrowing again in far posterior setigers.

Setigers 1–4 narrow, with reduced parapodia and with setal fascicles appearing to arise from body wall ( Figs 11A , 12A ); setigers 5–12 expanded, stretched, with short podial lobes ( Fig. 12A ); developing dorsal and ventral cirri by setiger 13–14; setigers 13–29 narrow with prominent parapodia and cirri ( Figs 11C , 12B ). Body with transverse rows of raised pads: setigers 1–4 with two rows, setiger 4–13 with three rows, but these mostly obscured in expanded segments; setigers 14–29 with three transverse rows. Venter with setigers 1–3 relatively smooth, with transverse rows of pads first evident from setiger 5, with four rows weakly developed ( Fig. 11B ). Venter with mid-ventral ridge evident from mid body to posterior end. Branchiae absent. Colour in alcohol, light tan. Body pigmentation not present, but segmental pairs of conspicuous pigmented nephridia observed ventrally from setiger 3 to mid-body ( Figs 11B , 12A ). These nephridia best observed in smaller paratypes but a few present in holotype.

Prostomium V-shaped or triangular, with anterior margin broadly curved, dorsally tapering to narrow posterior margin, extending over peristomium and merging with setiger 1 ( Fig. 11A ); ventrally merging with anterior lip of mouth ( Fig. 11B ); two short frontal lobes or horns arising subterminal from anterior margin of holotype ( Fig. 11A ); these not developed or present as developing anlage on juveniles or smaller specimens ( Fig. 11B ); eyes absent; nuchal organs as grooves between posterior lateral margin of prostomium and peristomium, cilia not observed. Peristomium with two narrow rings dorsally and laterally, interrupted mid-dorsally by posterior end of prostomium ( Fig. 11A ); ventrally dominated by expanded oral area consisting of anterior lip with about five large lobes and posteriorly by numerous narrow lobes forming large posterior lip of mouth ( Fig. 11B ). Many specimens with short proboscis emergent.

Parapodia from about setiger 13 becoming longer, developing prominent dorsal and ventral cirri ( Fig. 12B ); dorsal cirri asymmetrical, triangular-shaped, directed dorsolateral, terminating in narrow rounded tip ( Fig. 12C ); ventral cirri asymmetrical with broad basal attachment, directed ventrolateral ( Fig. 12D ); both dorsal and ventral cirri with numerous golden-coloured elongate glands directed toward elongated surfaces; each gland internally striated and pigmented ( Fig. 12B–D ); interramal papilla not observed.

Heavy curved acicular spines present in notopodia of setigers 1–3 and neuropodia of setigers 1–2. Notopodia of setigers 1–2 with 5–6 spines in first and second rows and 2–3 long capillaries in second row, setiger 3 with 5–7 spines in first row and all capillaries in second row; setiger 4 with all capillaries in both rows. Neuropodia of setigers 1–2 with 5‒7 spines in anterior and posterior rows accompanied by a few capillaries; setigers 3–4 with all capillaries. Notopodial spines ( Fig. 11D ) thicker than neuropodial spines ( Fig. 11E ). Individual spines curved, narrowing to pointed tip, not aristate; not hirsute, but with internal striae within shaft. Short spinous setae anterior to heavy spines absent. Furcate setae first present from setiger 5 anterior to long capillaries; furcate setae with unequal tynes bearing short bristles between tynes ( Fig. 12E ).

Pygidium of holotype with terminal anus surrounded by about 4–5 large lobes; four short anal cirri present ( Fig. 11C ). Smaller specimens lacking anal cirri or those present as short emergent anlage of cirri.

Remarks. Oligobregma renuncula sp. nov. is unusual among known species of Scalibregmatidae in the presence of prominent darkly pigmented pairs of nephridia clearly visible ventrally of all specimens examined. Details of nephridial morphology are known for Scalibregma inflatum through the studies of Danielssen (1859) and Ashworth (1901) and for Sclerocheilus minutus Grube, 1863 through work by DeHorne and DeHorne (1913), all summarized by Goodrich (1945). Although pairs of nephridia have been well-studied in these scalibregmatids and are visible in prepared optical sections or by histology, they have not been recorded as being pigmented and conspicuous in general observations.

Ashworth (1901) characterized the nephridia of Scalibregma inflatum and presumably all scalibregmatids as consisting of four parts: (1) a small funnel that opens segmentally on the anterior septa and leads to (2) a short tube that connects to (3) a larger and elongated U-shaped tube consisting of two linked parallel ciliated tubes that lead to (4) the nephridiopore.According to Ashworth (1901) the ciliated U-shaped tube is the only part of the nephridial morphology that is visible without dissection. In O. renuncula sp. nov. the visible portion includes the two adhering parts of the U-shaped tube, one part of which is pigmented and the other clear and unpigmented ( Fig. 12F ). The pigment is contained in numerous minute spherical bodies ( Fig. 12G ) the nature of which requires histology or electron microscopy (but see below).

In addition to the pigmented nephridia, O. renuncula sp. nov. is easily distinguished from the three other deep sea species of Oligobregma from off eastern Australia identified in this study by the numerous elongated and internally striated glands present in the dorsal and ventral cirri of posterior parapodia.

Biology. One specimen ( AM W.53994) had eggs 130–135 µm in diameter. A detailed examination of the pigmented nephridia from one paratype ( AM W.53994) using Phase Contrast optics magnified to 3000× revealed that the pigmented spherical bodies might be sperm nuclei; each sphere is approximately 2.5 µm in diameter; some appear to have an acrosome on one side, but no tail was observed.These observations suggest that the pigmented nephridia might serve as sperm storage organs or possibly spermatophores. Any further study of these unusual structures will require electron microscopy .

Etymology. The epithet is from the Latin, renis, for kidney, in reference to the conspicuous pigmented nephridia that characterize this species.

Distribution. Abyssal depths off eastern Australia from New South Wales to Queensland, Coral Sea, 4005–4280 m.

Figures

Figure 11. Oligobregma renuncula sp. nov. (A) anterior end, dorsal view; (B) anterior end, ventral view; (C) posterior end, dorsal view; (D) notopodial acicular spine from setiger 3; (E) neuropodial spine from setiger 3. A, C, holotype (AM W.52702); B, D, E, paratype (AM W.53994).

Figure 12. Oligobregma renuncula sp. nov. (A) anterior end, dorsal view; (B) setiger 23, anterior view; (C) dorsal cirrus, setiger 23; (D) ventral cirrus, setiger 23; (E) furcate setae from setiger 23; (F) pigmented nephridial loop; (G) detail of nephridial loop showing pigmented granules.A, F, G, paratypes (AM W.53994); B–E, holotype (AM W.52702). Arrows on C and D indicate furcate setae.