Metaruncina antoni, Chernyshev, 2021

Chernyshev, Alexei V., 2021, A new species of Metaruncina (Gastropoda: Heterobranchia: Runcinida) from Vietnam with a discussion on the so-called rudimentary radula in runcinids, Zootaxa 4974 (1), pp. 197-200 : 197-199

Metaruncina antoni sp. nov.

( Figs 1 , 2A–E )

urn:lsid:zoobank.org:act:49C23EB5-1B4A-4F26-B066-B03F4EA724AA

Type material. Holotype No. 40037 ( MIMB), Tho Chu Island (09°19′ N, 103°30′ E), South China Sea, Vietnam, May 13, 2010, intertidal, dead corals and calcareous red algae, collected by A. V. Chernyshev during the expedition aboard the R / V “Akademik Oparin” . Paratype No. 40038 ( MIMB), collected with holotype . Paratypes (4 specimens) No. 40039 ( MIMB), Van Phong Bay (12°39′ N, 109°20′ E), South China Sea, Vietnam, May 21, 2010, intertidal, dead corals, and calcareous red algae, collected by A. V. Chernyshev .

Other material examined. 1 juv. specimen, Cu Lao Re Island , (15°23′ N, 109°05′ E), South China Sea, Vietnam, May 25, 2007, intertidal, among Halimeda sp. , collected by A. V. Chernyshev during the expedition aboard the R / V “Akademik Oparin” ; 3 specimens, No. 40039a ( MIMB), collected along with paratypes (all specimens were dissected) .

Etymology. The species is named in memory of Dr. Anton Chichvarkhin (1975–2020) for his contribution to the investigation of marine heterobranch mollusks.

Description. Living animals 2–4.5 mm long, with elongated oval notum and anteriorly truncated head. Larger specimens (3–4.5 mm long) have yellowish brown notum with two dark brown (to almost black) longitudinal, slightly irregular, and sometimes intermittent stripes ( Fig. 1A, B, F ). Dark brown margin of notum thickens at posterior part of notum, and laterally at the level of eyes visible through notum ( Fig. 1A, C, D, F ). Two large, rounded, dark spots located near anterior edge of notum ( Fig. 1 A–D ). Dark digestive gland shows through integument. Foot slightly lighter on dorsal surface, with small brown speckles against yellowish brown background; longitudinal medial aggregation of dark pigment expands towards notum; dark speckles fewer on ventral surface of foot ( Fig. 1E ); dark margin on foot absent. Gill large, protruding from under notum, with 4–5 pinnae on each side, dark coloured especially on pinnae edges ( Fig. 1B, F ). Specimens 2–2.5 mm long are lighter in colour, with irregular longitudinal stripes markedly intermittent. Shell translucent, barely visible through integument, elongated, tapering to posterior end ( Fig. 1B, F ), 240–260 µm long and 96–110 µm wide ( Fig. 2C ).

Jaws consist of numerous scale-like elements 5–6 µm wide, anterior elements with 6–7 denticles on anterior margin ( Fig. 2A ). Buccal apparatus with brown pigment. Radula of “juvenile” type, asymmetrical, 3 × 0.R.1 and one more row 1.R.1 (?0.R.2) ( Fig. 2A, B ); rachidian teeth bear slightly discernible serration on cutting edge ( Fig. 2B ). In mature specimens, gizzard plates 100–205 µm long, with 7–9 V-shaped ridges ( Fig. 2D ). Male copulatory apparatus without distinct penial papilla, slander seminal vesicle ca. 480 µm long, prostatic part ca. 600 µm long ( Fig. 2E ).

Remarks. The new species is placed in the genus Metaruncina by presenting rudimentary juvenile-type radula, single gill, internal posterior plate-like shell, and four gizzard plates. Metaruncina antoni sp. nov. is distinguished from the known two Metaruncina species in body colour: M. setoensis has uniformly black notum with white shell while M. nhatrangensis bears five dorsal black brown longitudinal stripes.

Geographical distribution and habitat. South China Sea, South Vietnam, Van Phong Bay, Tho Chu Island, and Cu Lao Re Island. In the intertidal zone, among calcareous red and green algae.

Discussion. The monotypic genus Metaruncina was established by Baba (1967) for Runcina setoensis Baba, 1954 . The main distinguishing feature of the new genus was a strongly reduced radula, different from the typical serial radula of runcinids. Such interpretation of Metaruncina radula is still accepted by many authors. It should be noted that the socalled “metamorphosis of radula” ( Thompson & Brodie 1988; Schmekel & Cappellato 2001, 2002), where the “juvenile” (provisional) radula is replaced by the “adult” (definitive) one, has been described from some of runcinids ( Thompson & Brodie 1988; Chernyshev 1998; Schmekel & Cappellato 2001, 2002). Some juvenile individuals may have both juvenile and definitive radula ( Thompson & Brodie 1988; Schmekel & Cappellato 2002; Chernyshev 1998).

A comparison of the provisional radula of Runcinida marisae Chernyshev, 1998 with the drawings of the radula of Metaruncina setoensis in the Baba’s work (1967) gave the author the idea that Metaruncina in the adult stage retains the “juvenile” radula, while the typical serial radula is not formed ( Chernyshev 1998). Further studies on the radula in Metaruncina nhatrangensis confirmed its similarity to the provisional radula of hatched juvenile specimens of Runcinida marisae ( Fig. 2F ) ( Chernyshev 2005). Thus, the radula in Metaruncina is non-homologous to the definitive radula in other runcinids and is rather a “juvenile” radula retained in the adult state, i.e., it represents an example of paedomorphosis. Besides species of the genus Metaruncina , a “juvenile” radula was described from Runcina avellana Schmekel & Cappellato, 2001 ( Schmekel & Cappellato 2001) and species of the genus Fofinha Moro & Ortea, 2015 ( Moro & Ortea 2015; Ortea & Moro 2020). However, it should be considered that Runcina avellana has a very small body size and was described without information about the copulative apparatus, and, therefore, it is possible that the studied specimens were juvenile individuals having no definitive radula ( Schmekel & Cappellato 2001). Nevertheless, Ortea (2013) transferred Runcina avellana to the genus Pseudoilbia M. C. Miller & Rudman, 1968 . Being described from a single specimen, Pseudoilbia lineata M. C. Miller & Rudman, 1968 has a serial asymmetric radula with the formula 35–39 × 2.0.2 ( Miller & Rudman 1968), and such radula is not provisional. For this reason, examination the Mediterranean Runcina species is required to clarify the generic placement of R. avellana .

Figures

FIGURE 1. Metaruncina antoni sp. nov., external view. (A) drawing of a living paratype. (B) compressed specimen under coverglass. (C, D) living holotype (No. 40037, MIMB), dorsal view. (E, F) fixed holotype (No. 40037, MIMB), (E) ventral view of the anterior body, (F) dorsal view of the posterior body. Abbreviations: e, eye; g, gill; s, shell. Scale bars: (A, C, D), 1 mm; (B, E, F), 0.5 mm.

FIGURE 2. Internal structures of Metaruncina antoni sp. nov. (A–E) (MIMB No. 40039a) and Runcinida marisae (F). (A) jaw and radula (arrows). (B) radula (arrows show rachidian teeth). (C) shell (SEM micrograph). (D) gizzard plates. (E) male copulative organ. (F) provisional radula of the hatched juvenile specimen. Abbreviations: p, prostatic part; s, seminal vesicle. Scale bars: (A, B), 20 µm; (D, C), 50 µm; (E), 200 µm; (F), 10 µm.