Friday, August 9, 2019

[Mollusca • 2019] Worm-riding Clam: Montacutona sigalionidcola (Bivalvia: Heterodonta: Galeommatidae) from Japan and its Phylogenetic Position

Montacutona sigalionidcola 
Goto & Tanaka, 2019

A new galeommatid bivalve, Montacutona sigalionidcola sp. nov., is described from an intertidal flat in the southern end of the Kii Peninsula, Honshu Island, Japan. Unlike other members of the genus, this species is a commensal with the burrowing scale worm Pelogenia zeylanica (Willey) (Annelida: Sigalionidae) that lives in fine sand sediments. Specimens were always found attached to the dorsal surface of the anterior end of the host body. This species has a ligament lithodesma between diverging hinge teeth, which is characteristic of Montacutona Yamamoto & Habe. However, it is morphologically distinguished from the other members of this genus in having elongate-oval shells with small gape at the posteroventral margin and lacking an outer demibranch. Molecular phylogenetic analysis based on the four-gene combined dataset (18S + 28S + H3 + COI) indicated that this species is monophyletic with Montacutona, Nipponomontacuta Yamamoto & Habe and Koreamya Lützen, Hong & Yamashita, which are commensals with sea anemones or Lingula brachiopods. This result suggests that host shifting across different phyla occurred at least twice in this clade.

Keywords: Mollusca, ectocommensal, Galeommatoidea, host shift, polychaete, scale worm, Sigalionidae, symbiosis

FIGURE 3. Montacutona sigalionidcola sp. nov. (holotype, NSMT-Mo 79031, SL 3.9 mm) and its host Pelogenia zeylanica.
(A) A crawling individual of M. sigalionidcola. (B, C) Dorsal and ventral sides of M. sigalionidcola.
(D–F) Pelogenia zeylanica with M. sigalionidcola attached (arrowed).
Scale bar: 1 mm (A–C), 5 mm (D–F). 
Photo credits: R. Goto (A–F).

FIGURE 2. Kamiura Beach, Kushimoto, Wakayama Prefecture, Japan (type locality of Montacutona sigalionidcola sp. nov.)

Superfamily Galeommatoidea J.E. Gray, 1840
Family Galeommatidae sensu Ponder, 1998
Genus Montacutona Yamamoto & Habe, 1959

Diagnosis. Shell typically suborbicular in shape with submedian umbones. Two pronounced and spread laterals in left and right valves. A calcified, white portion (i.e., lithodesma) situated in-between the spread laterals. Outer demibranch significantly reduced or absent.

Montacutona sigalionidcola sp. nov. 
[Japanese name: Sunaurokomushi-yadori-gai]

Diagnosis. Shell ovate-elongate in shape with a small gape at posteroventral margin. Only inner demibranch present.

Distribution. Kushimoto (this study) and Tsubaki (Otani et al. 2005; this study), Kii Peninsula, Wakayama Prefecture, and Uchiura, Kamogawa, Boso Peninsula, Chiba Prefecture, Japan (Yanagi 2017).

Host. Pelogenia zeylanica (Annelida: Sigalionidae) (Fig. 3D–F). 

Host association. This species is tightly attached to the dorsal surface of the anterior end of the host by byssal threads (Fig. 3). The anterior end of the holotype was directed towards the anterior end of the host. The number of M. sigalionidcola per host ranged from one to four (Table 3). The infestation rate by M. sigalionidcola is high: roughly 80% of P. zeylanica harbored commensal bivalves (Tanaka, personal observations).

Etymology: The species name is derived from the family name [Sigalionidae] of the host and the suffix -cola (Latin), meaning a dweller or inhabitant.

Ryutaro Goto and Makoto Tanaka. 2019. Worm-riding Clam: Description of Montacutona sigalionidcola sp. nov. (Bivalvia: Heterodonta: Galeommatidae) from Japan and its Phylogenetic Position. Zootaxa. 4652(3)473–486. DOI: 10.11646/zootaxa.4652.3.4