Ophioplinthus abyssorum ( Lyman, 1883 )

Ophioplinthus abyssorum ( Lyman, 1883) View in CoL

Fig. 9a–k View FIGURE 9

Ophioglypha abyssorum Lyman, 1883: 238 View in CoL , pl. 3(25–27).

Homalophiura abyssorum View in CoL . — Clark, H.L., 1915: 326. — Clark, H.L., 1939: 112–113. — Clark, H.L., 1941: 92. — Litvinova, 1975: 199.

Ophiurolepis abyssorum View in CoL . — Hertz, 1927b: 93–94, pl. 7(13).

Homalophiura aff abyssorum View in CoL . — Vadon & Guille, 1984: 596, pl. 2(1–2).

Homophiura abyssorum View in CoL . — Paterson, 1985: 138, fig. 52. — Guille & Vadon, 1986: 169.

Ophioplinthus abyssorum View in CoL . — Martynov & Litvinova, 2008: 88, fig. 6b.

? Ophiurolepis mordax View in CoL . — Guille & Vadon, 1986: 184–185, pl. 1(1–2).

STUDY MATERIAL. — MD208 : stn CP4913, Walters shoal, Plaine Sud , 33° 48.82´S, 44° 5.9699´E to 33° 51.8´S, 44° 4.72´E, 1539–1615 m, 11/5/2017: 2 ( MNHN IE.2016.1378) ( DNA code= IE.2016.1378) GoogleMaps . — MD208 : stn CP4914, Walters shoal, Plaine Sud , 33° 51.1´S, 44° 4.8999´E to 33° 54.73´S, 44° 3.2199´E, 1598–1714 m, 11/5/2017: 1 ( MNHN IE.20234002); 21 ( MNHN IE.2016.1373) ( DNA code= IE.2016.1373) GoogleMaps .

COMPARATIVE MATERIAL EXAMINED. Ophioplinthus accomodata ( Koehler, 1922b) : IN2018_V06/169, Flat area south of Brians, 44° 14.364´S, 147° 17.58´E to 44° 13.938´S, 147° 18.12´E, 1443– 1422 m, 14/12/2018, MV F272358 ( DNA code=F272358). TAN0803/17, Seamount 1 Spastic Spider, Macquarie Ridge. New Zealand EEZ, 48° 32.93´S, 164° 57.75´E to 48° 32.66´S, 164° 58.04´E, 1318–1327 m, 30/3/2008, NIWA 39573 ( DNA code= NIWA 39573). Ophioplinthus cf. accomodata : IN2022_V09/071, Gascoyne Marine Park. Site:024, 20° 49.689´S, 111° 36.2038´E to 20° 51.295´S, 111° 35.522´E, 1990–2057 m, 4/12/2022, MV F310391 ( DNA code=F310391). Ophioplinthus inflata ( Koehler, 1897) : off Colombo, 6° 29´N, 79° 34´E, 1110 m, holotype, ZSI 5190/7. Ophioplinthus medusa Lyman, 1878 : PS61 ANT-XIX/4 (ANDEEP)/138-4, 62° 57.8´S, 27° 52.14´W to 62° 57.77´S, 27° 51.1´W, 4544–4545 m, 16/3/2002, CASIZ 161434 ( DNA code=CAS161434). Ophioplinthus sp. MoV.7271 IN2021_V04/28, Karma Seamount, 12° 49.551´S, 107° 2.7912´E to 12° 50.03´S, 107° 3.6´E, 2760– 2850 m, 11/7/2021, identified by O’Hara (2024b), MV F305601 ( DNA code=F305601).

Description. Disc 7 to 11 mm dd, arms 1.5 to 2 times dd, relatively longer on smallest specimens, pentagonal, rounded edges, dorsal side ( Fig. 9a View FIGURE 9 ) with prominent round centrodorsal and separated primary plates at mid-radius, some secondary larger plates radially and interradially, 2–3 rows of smaller scales between centrodorsal and primary plates, large marginal plate laterally, radial shields 1/6 dd, just touching or separate radially; hydroid tracks meander across the disc between plates; visible arm comb consists of a few low inconspicuous square papillae ( Fig. 9c View FIGURE 9 ). Oral shield pentagonal, with straight or slightly convex lateral and distal edges, opposed by a large ventral plate; can be fragmented by hydroid tracks ( Fig. 9b View FIGURE 9 ). Genital slit is half the disc radius in length, bordered by low square papillae, becoming more separate and plate or granule like near the margin. Oral tentacle pore with 4 scales on either side.

DAPs usually separate, except between 1st and 2nd plate, diamond shaped with rounded corners, hydroid tracks can obscure plate outlines ( Fig. 9a View FIGURE 9 ). LAPs squarish, overlapping neighbouring plates, penetrated by the tentacle pore ( Fig. 9e–f View FIGURE 9 ), arm spine articulations obscure. VAPs separate, enlarged proximally, becoming broadly triangular distally. Tentacle pores obvious for only the first 2–3 segments, thereafter they penetrate the LAP, opening near the lowest arm spine ( Fig. 9c View FIGURE 9 ). Three small simple arm spines, 1/10 segment in length, lowest longest. 1–2 tentacle scales on either side of the basal pores, hereafter one small pointed scale, next to the lowest arm spine. Colour (live, Fig. 6f View FIGURE 6 ): light grey dorsal disc, disc rim and arms pale.

Distribution. W Atlantic ( 1956–3442 m), E Atlantic ( 2100–3200 m), S Africa ( 1539–2700 m), W Indian Ocean ( 1880–3645 m).

Remarks. Following previous authors, these specimens have been named as O. abyssorum , whose type locality is in the West Indies at 1997 m. This species resembles O. tessellata ( Verrill, 1894) from the temperate bathyal North Atlantic, O. pseudotessellata Martynov & Litvinova, 2008 from off the Azores, and O. mordax ( Koehler, 1922b) from off eastern Antarctica. Distinguishing these species is difficult as they are often infested with a hydroid (named as Hydractinia ingolfi (Kramp) on O. tessellata , see Mortensen, 1933d: 92) that creates variation in the shape and distribution of dorsal and central disc plates, oral shields and dorsal arm plates. On the dorsal disc these hydroid tracks can have the appearance of heart-urchin fascioles ( Fig. 9a View FIGURE 9 ). However, O. tessellata is perhaps distinguished by its contiguous DAPs on the basal part of the arm, O. pseudotessellata has elongated radial shields, and O. mordax appears to lack a larger ventral plate distally to the oral shield (although this could be due to fragmentation from the hydroid infestation). Guille & Vadon (1986) identified a specimen from off northern Madagascar ( 2300–2500 m) as O. mordax based on a comparison with two Antarctic syntypes of O. mordax in the MNHN.

DNA data is required to satisfactorily separate these forms. Unfortunately, we have failed to obtain DNA sequence data from any of the North Atlantic Ophioplinthus species, nor an Antarctic O. mordax . The DNA data we do have indicates that the MD 208 specimens form a sister clade to an undescribed species ( Ophioplinthus sp. MoV.7271, see O’Hara 2024b) from the eastern Indian Ocean seamounts ( 1915–3172 m), which together are sister to O. accommodata ( Koehler, 1922b) from off southern Australia and New Zealand ( 1000–2902 m).

The two species of Ophioplinthus described from the western Indian Ocean are not closely related. Ophioplinthus inconveniens ( Hertz, 1927b) is known from the unique 4 mm holotype found off East Africa ( 1668 m) and resembles an Ophiomastus or Anthophiura . There are very few disc plates, with the primaries contiguous with each other and the radial shields. Ophioplinthus inflata from off Sri Lanka ( 1110 m) appears from the type figures to be more like an Ophiuroglypha with pronounced arm comb granules, large triangular oral shield, small ventral disc plates, radials that are contiguous over their distal half, and appearing to lack a hydroid infestation. The unique type of O. inflata in the ZSI has disintegrated (O’Hara pers. obs.).