Poraniomorpha tartarus, Mah, 2023

Mah, Christopher L., 2023, New occurrence of Poraniidae (Valvatacea, Asteroidea) in Australia with a new genus and species from deep-sea settings, Memoirs of Museum Victoria 82, pp. 119-131 : 125-129

Poraniomorpha tartarus n. sp.

Figure 3a–e .

Etymology. The species epithet is named for Tartarus, the mythical Greek underworld, alluding to this species’ occurrence at great depth. Noun held in apposition.

Diagnosis. Body strongly stellate ( Fig. 3a ). Actinolateral fringe rounded with rounded edge ( Fig. 3c, d ). Abactinal surface hard, very resistant to the touch. Surface covered by minute, pointed granules embedded in dermal integument ( Fig. 3b, d ). Drying of specimen suggests flattened marginals are polygonal plates, approximately 48 per interradius. Actinal region narrow, surface is flat, with approximately 50 shallow segments corresponding with adambulacral and marginal plates but also bisecting the actinal intermediate region extending to the oral plate. Actinal surface also covered by small granules, covered by dermis, 5–30 per actinal segment, each with a hyaline tip invested in the dermal integument. Dark brown colour adjacent to the adambulacral spination on the disk and along the arms ( Fig. 3c, d, e ). One or two furrow spines, with 1–2 enlarged subambulacral spines, each approximately twice the thickness of and more elongate than the furrow spines ( Fig. 3d, e ).

Comments. Although P. tartarus n. sp. possesses a rounded actinolateral edge, unlike other Poraniomorpha species, several other characters – including irregular imbricate plates, the pointed granules invested in the dermal tissue, and the distinctively enlarged subambulacral and furrow spination on the adambulacral plates ( Fig. 3d, e ) – support placement within Poraniomorpha .

Poraniomorpha tartarus n. sp. invites comparison with Atlantic species such as P. abyssicola ( Verrill, 1895) and P. tumida ( Stuxberg, 1878) , which it resembles very closely. They share similar abactinal plate morphology, displaying closely articulated irregular-shaped, imbricate, mound-like plates bearing a cover of spinose granules covering the surface such that plate boundaries are obscured. The disk in these species is strongly arched and arms are similarly elongate and tapering. Furrow spines are relatively few in both species (1–2 in P. tartarus n. sp. versus 2–3 in P. abyssicola but 3–5 in P. tumida ). Poraniomorpha tartarus n. sp. is distinguished by the enlarged subambulacral spine, the absence of most papulae from the abactinal surface, and the difference in actinal plate texture, which possesses distinct transverse segments and lacks the numerous pointed spinelets seen in P. abyssicola . Neither P. abyssicola nor P. tumida are known to display the distinct colouration of P. tartarus n. sp.

Poraniomorpha tartarus n. sp. is the first known occurrence of this genus in the Southern Hemisphere and is also the deepest known species of the genus (3850–3853 m). Poraniomorpha abyssicola occurs at comparable depth in the Atlantic (2976–3740 m).

Although very little is known about the biology of Poraniomorpha spp. , the NOAA vessel Okeanos Explorer observed P. abyssicola feeding on a sponge at 3403 m ( Mah, 2020). Poraniomorpha tartarus n. sp. may feed on similar prey.

Occurrence. Known only off East Gippsland, Victoria, Tasman Sea , 3850–3853 m .

Description. Body strongly stellate (R/r=3.7–3.9) ( Fig. 3a, c ) with elongate arms, round in cross-section. Disk and arms confluent, disk strongly convex, rms thick, tips strongly upturned. Interradial arcs acute. Actinolateral fringe rounded with no distinct edge.

Disk plates not evident, thick integument covers all of body surface ( Fig. 3a, b ). Body texture hard to touch. General surface topology rough, presenting a wrinkled appearance, covered by minute, pointed granules invested in dermal integument ( Fig. 3b ). Individual plates irregular in shape, surface texture mound-like. Granular cover is complete, evenly distributed, approximately 5–6 along a 1.0 mm line. Papulae mostly absent from abactinal surface with some occurring interradially (5–8 observed between arms in each interradius), with few present proximally on the lateral sides of each arm. Anus at center of disk, flanked by 8–10 angular granules around edge. Dermal granules around anus slightly larger than those elsewhere. Madreporite convex, outline circular, large, approximately 1.5 (smaller specimen) to 3.0 (larger specimen) cm in diameter ( Fig. 3a, b ). On the holotype, madreporite adjacent to contact with superomarginal plates. Basal portion of madreporite covered by small dermal granules. No pedicellariae observed. Interradii each with a discrete fold each bearing 10–15 slender grooves that synchronise with those on marginal to actinal plates. Shallow transverse fasciolar channels extend from abactinal surface along lateral surface aligned with those on actinal surface and adambulacral plates.

Marginal plates completely obscured by dermal integument. Drying shows marginal plates, flattened, polygonal, approximately 48 per interradius (armtip to armtip), boundaries are obscured, exact count uncertain. Single row of approximately 5–8 papulae, in mostly single pores, along lateral surface of arm.

Actinal surface flat with approximately 50 shallow transverse grooves tracking from marginal to adambulacral plates forming segments. These grooves bisecting the actinal intermediate region tracking from the oral plate ( Fig. 3d, e ). Actinal surface covered by small granules, 5–30 per actinal segment, covered by dermis, each with a hyaline tip invested in the dermal integument. A dark colour pattern, especially evident on NMV F241811 ( Fig. 3c ), present adjacent to the adambulacral spine series and around the mouth extending interradially on to the disk.

Furrow spines (1–2) large and prominent, covered by dermal integument, conical tip, blunt spines widely spaced interlacing with furrow spines on opposing side ( Fig. 3b, d, e ). Subambulacral spine mostly single, but two are present on approximately 40% of adambulacrals, especially on NMV F241811, approximately twice the thickness of each furrow spine arranged transversely relative to the furrow spine. Remaining adambulacral plates with single, short subambulacral spine, variably blunt and smooth, a minority of spines with notched or roughened tip. At least one of the larger subambulacral spines comparable in size with one of the furrow spines. Those secondary subambulacrals spines smaller, less than half the height and thickness of the furrow spine. Dermal integument covers the adambulacral plate; no other accessories are present.

Oral plates with four furrow spines and one spine from each oral plate projecting into the mouth (two total). Oral plate surface, with a total of 4–6 suboral spines (two or three per half).

Colour in life white to dark brown on disk, arms mottled, dark brown to white, interradii dark brown. Underside is white with dark brown in each interradius around mouth ( Fig. 3c, d, e ). Dark colouration present in patches along adambulacral series.

Material examined. Holotype. NMV F 241811 East Gippsland , Victoria, Tasman Sea, Australia, - 38.479° S, 150.185° E, 3850–3853 m, Coll. O’Hara et al. aboard RV Investigator 24 May 2017, 1 wet spec. R =5.4, r=1.3.

Paratype. NMV F 241807 East Gippsland , Victoria, Tasman Sea, Australia, - 38.479° S, 150.185° E, 3850–3853 m, Coll. O’Hara et al. aboard RV Investigator 24 May 2017, 1 wet spec . R=5.7, r=1.3.

Figures

Figure 3. Poraniomorpha tartarus n. sp., holotype. NMV F 241811: a, abactinal; b, abactinal surface showing madreporite, surface texture; c, actinal view; d, actinal intermediate region and inferomarginals; e, actinal view furrow spines, oral region. Scale bars a=9.0 mm, b=5.0 mm, c=9.0 mm, d, e=2.0 mm.