Bathyalcyon robustum Versluys, 1906

Li, Yang & Xu, Kuidong, 2023, Anthomastus nanhaiensis, new species, and Bathyalcyon robustum Versluys, 1906, two mushroom soft corals (Octocorallia: Coralliidae) from Zhenbei Seamount in the South China Sea, Raffles Bulletin of Zoology 71, pp. 669-680 : 675-679

Bathyalcyon robustum Versluys, 1906

( Figs. 6‒8 )

Bathyalcyon robustum Versluys, 1906: 549‒553 ; Kinoshita, 1911: 121; Totton, 1933: 107; Bock, 1938: 4; Utinomi, 1960: 7‒9; Molodtsova, 2013: 492.

Anthomastus (Bathyalcyon) robustum – Bayer, 1993: 8‒10 (part), pl. 9.

Anthomastus (Bathyalcyon) robustum delta Bayer, 1993: 8‒10 , pls. 6‒8.

Material examined. MBM286427 , 1 specimen, collected on 21 July 2022 from the station FX-Dive 309 (15°03.51′N, 116°33.22′E) of Zhenbei Seamount with water depth of 537 m (temperature 8.04°C), attached on stone ; MBM286428 , 1 specimen, collected on 27 July 2022 from the station FXDive 315 (15°04.73′N, 116°33.87′E) of Zhenbei Seamount with water depth of 552 m (temperature 8.06°C), attached on stone .

Description

Colony form and size. In life, colonies attached to stones ( Fig. 6A, B ). Autozooid solitary, with anthocodia projecting from anthostele. Anthostele cylindrical, stiff, inner coenenchyme spongy, emerging from a membranous and spreading holdfast ( Fig. 6 ). In alcohol preservation, colony of the larger specimen (MBM286427) 90 mm long, with anthostele 68 mm long and 29 mm wide, and anthocodia 55 mm long in contraction; colony of the smaller specimen (MBM286428) 88 mm long, with anthostele 63 mm long and 26 mm wide, and anthocodia 24 mm long in contraction ( Fig. 6C, D ).

Polyps. Polyps dimorphic. Autozooid tentacles of the larger specimen up to 54 mm long, each side of rachis with 18–22 pinnules (19, 20, 22, 20, 22, 18, 19 respectively in 7 of 8 tentacles). Autozooid tentacles of the smaller specimen up to about 40 mm long, each side of rachis with 15–20 pinnules (18, 18, 18, 15, 19, 19/20, 19, 19 respectively). Pinnules arranged alternately rather than in pairs, with the middle ones larger than basal ones and upper ones. Siphonozooids fertile (oocytes approximately 0.40 mm in diameter), conspicuous, numerous, densely scattered among anthostele, measuring about 1 mm wide.

Sclerites. Tentacular pinnule sclerites mostly tuberculate rods usually with a twist, ranging 0.18–0.35 mm long ( Fig. 7A ). Tentacular rachis sclerites mostly spiny rods to multiradiates, ranging 0.94– 0.17 mm long; occasionally crosses ( Fig.

7B). Anthocodial wall sclerites mostly 8-radiates, ranging 0.09–0.16 mm long; occasionally irregular multiradiates, tuberculate rods and clubs ( Fig. 7C ). Pharyngeal sclerites predominantly tuberculate rodlets ranging 0.10–0.19 mm long ( Fig.7A ). Sclerites of outermost anthostelar coenenchyme mostly 8-radiates, ranging 0.09–0.14 mm long; occasionally tuberculate clubs and spindles ( Fig. 8B ). Sclerites of deeper layer of coenenchyme mostly tuberculate spindles, ranging 0.26–0.42 mm long; occasionally multiradiates and tuberculate clubs ( Fig. 8C ). Sclerites from holdfast mostly 6, 7, 8-radiates, ranging 0.11–0.18 mm long; fewer tuberculate spindles, ranging 0.18–0.24 mm long; occasionally crosses and tuberculate clubs ( Fig. 8D ).

Colour. Anthostele red and anthocodia l wall pinkish both in situ and in ethanol. Tentacles whitish in situ; aboral sides of tentacle rachis and pinnules reddish, and oral sides white in ethanol. Sclerites reddish under transmitted light.

Distribution. Zhenbei Seamount, South China Sea, 537– 552 m. It was previously known from the west Pacific including: Ceram Sea, Indonesia, 924 m ( Versluys, 1906); Malay Archipelago (732 m) and Philippines (274 m) ( Totton, 1933); Japan, 150–732 m ( Kinoshita, 1911; Bock, 1938; Utinomi, 1957, 1960). The subspecies Bathyalcyon robustum delta ( Bayer, 1993) was reported from the Gulf of Mexico at the water depth of 274 m ( Bayer, 1993).

Remarks. Bathyalcyon robustum Versluys, 1906 was originally described based on specimens from the Ceram Sea of Indonesia, and it was subsequently reported from sea areas in the western Pacific ( Versluys, 1906; Kinoshita, 1911; Totton, 1933; Bock, 1938; Utinomi, 1957, 1960). Bayer (1993) subsumed the monotypic genus Bathyalcyon Versluys, 1906 into Anthomastus Verrill, 1878 as a subgenus, and established the subspecies Anthomastus robustum delta Bayer, 1993 based on specimens from the Gulf of Mexico in the Altantic Ocean. Molodtsova (2013) reinstated Bathyalcyon Versluys, 1906 to genus status. We accept this taxonomic assignment, considering Bathyalcyon robustum has a single large autozooid, which is peculiar within the subfamily Anthomastinae Verrill, 1922 . Moreover, the form of pharyngeal and tentacular sclerites has been considered to be the two main characters distinguishing genera of Anthomastinae Verrill, 1922 ( Molodtsova, 2013). Our examination showed that the pharyngeal sclerites are predominantly rodlets and the tentacles contain multiradiates in Bathyalcyon Versluys, 1906 , like those in Pseudoanthomastus Tixier-Durivault & d’Hondt, 1974 , while the pharyngeal sclerites are predominantly platelets and tentacles are devoid of multiradiates in Anthomastus Verrill, 1878 .

As indicated by Bayer (1993), the morphology of B. robustum Versluys, 1906 from the western Pacific was almost the same as Bathyalcyon robustum delta ( Bayer, 1993) from the Atlantic except that the holdfast of the Pacific specimens has tuberculate spindles, while B. robustum delta ( Bayer, 1993) has tuberculate rods. This difference occurs also in the South China Sea specimens. In the future, molecular study is needed to clarify the phylogenetic position of Bathyalcyon Versluys, 1906 and the taxonomic status of B. robustum delta ( Bayer, 1993) .

Figures

Fig. 6. Morphology of Bathyalcyon robustum Versluys, 1906. A, MBM286427 in situ; B, clonies in aggregation with MBM286428 in the most right; C, MBM286427 in preservation; D, MBM286428 in preservation; E, siphonozooids; F, transverse section through middle anthostele of MBM286427, showing pharynx and mesenteries. Scale bars = 10 mm (C, D), 0.5 mm (E), and 5 mm (F).

Fig. 7. Sclerites of Bathyalcyon robustum Versluys, 1906. A, sclerites from tentacle pinnules of autozooids; B, sclerites from tentacle rachis of autozooids; C, sclerites from anthocodial wall. Scale bars = 0.10 mm.

Fig. 8. Sclerites of Bathyalcyon robustum Versluys, 1906. A, sclerites from pharynx; B, sclerites from outermost anthostelar coenenchyme; C, sclerites from deeper layer of coenenchyme; D, sclerites from holdfast. Scale bars = 0.10 mm.