Bonellia sp. aff. minor Marion & Rietsch, 1886

Goto, Ryutaro, Hirabayashi, Isao, Seike, Koji, Yamashita, Momo & Shimomura, Michitaka, 2024, Living together in dead coral rocks: macrosymbiotic communities associated with Bonellia echiuran worms (Annelida: Thalassematidae: Bonelliinae), involving new commensal bivalve and amphipod species, Zoological Journal of the Linnean Society 200 (4), pp. 915-939 : 922-925

Bonellia sp. aff. minor Marion & Rietsch, 1886

( Fig. 10 )

Bonellia viridis Goto et al. 2013: 1–6 , fig. 1J.

Bonellia sp. 3 Goto 2016: 251.

Bonellia minor Goto 2017: 529–530 , fig. 20.2q.

Bonellia sp. 3 Goto et al. 2020: 104.

Material examined: Twenty specimens collected from Sabiura , Kushimoto, Wakayama, Japan (33°28’43.61”N, 135°44’45.45”E) .

Description: Female. Colour of proboscis and trunk dark green in life ( Fig. 10A ). Trunk ovoid in shape, bearing papillae ( Fig. 10B, C ), up to c. 15 mm in length (fixed condition). Proboscis long, bifid at the tip ( Fig. 10A ). Paired small ventral chaetae present accompanied by developing ones ( Fig. 10D ). Interbasal muscle between chaetae present ( Fig. 10D ). A single, thin-walled, semitransparent gonoduct with a basally placed gonostome present on the right or left side of the nervous line. Gonoduct long, slender, containing numerous eggs ( Fig. 10E ). Gonostome small with flower-likegonostomallips ( Fig.10E ). Twoanalvesiclespresentand bearing numerous unbranched tubules ( Fig. 10F ). The end of tubules funnel-shaped. One to several males occurring in the pharynx. Mature males small (up to c. 1.5 mm in length, fixed condition), slender, without proboscis ( Fig. 10G ). Colour of trunk greenish in preserved condition. Paired ventral chaetae present. Juvenile males just after settlement possessing a sucker-like structure.

Habitat and burrow structure: This species usually inhabits a cavity or crevice inside the dead coral rocks. Bonellia sp. aff. minor modifies the inner space by bringing sandy sediments into it using a proboscis. The burrows possess multiple exits ( Figs 5 , 6 ).

Distribution: Kushimoto and Okinawa Island, Japan.

Burrow associate: The burrows of Bo. sp. aff. minor host the commensals such as the bivalve Ba. bonelliphila , the amphipod L. bonelliae , and the polychaete O. fauveli in Kushimoto (this study), and the scale worm ( Polynoidae species) in Okinawa Island, southern Japan ( Goto 2017). The shells of Ba. bonelliphila were attached by unidentified entoprocts and foraminiferans.

Remarks: Thegenus Bonellia containsaboutninespecies (WoRMS 2023). Our specimens are basically identical to Bo. minor mainly based on number of gonoducts, shape of anal vesicles, and morphology of dwarf males. Bonellia minor is very similar to Bo. viridis , but is distinguished from it in its anal vesicles which do not branch ( Stephen and Edmonds 1972), which was also observed in our specimens. The juvenile dwarf males attaching to the female’s proboscis has a sucker-shaped structure as in Bo. minor . ‘ Bonellia minor’ has been recorded from the warm Pacific coasts of south-western Japan ( Ikeda 1904, Satô 1939, Kawaguti 1971), which was also treated as Bonellia fuliginosa Rolando, 1822 in Ikeda (1924). Bonellia minor was described based on the specimen from the Gulf of Marseille in the Mediterranean Sea. Given the geographic distance from the type locality, it is more likely that ‘ Bo. minor ’ in Japanese waters represents a different species. According to Rietsch (1886), the end of the gonoduct sometimes forms an acorn-like shape in Bo. minor . However, such morphological modification was not observed in our specimens and thus this may be an interspecific difference. Goto (2016, 2017) suggested that ‘ Bo. minor’ in Okinawa, southern Japan, contains at least two species. The COI data suggested that our specimens from Kushimoto are identical to Bonellia sp. 3 recognized in Goto (2016, 2017; Fig. 3 ).

Figures

Figure 3. Haplotype network from COI data for Bonellia sp.aff. minor from Kushimoto and Okinawa, Japan. Each connection represents one inferred base-pair change.

Figure 5. Bonellia sp. aff.minor and its burrow associates (Basterotia bonelliphila, Leucothoe bonelliae, and Oxydromus fauveli) in dead coral rock. The inside of the burrows is partly occupied by sandy sediments collected by Bo. sp. aff. minor.

Figure 6. Micro-CT images of two burrows (A and B) of Bonellia sp. aff.minor. Burrow openings are indicated by white arrowheads.The blue and yellow portions indicate sediments occupied in burrows and the burrow cavity, respectively. Scale bars = 5 mm.

Figure 10. Bonellia sp.aff.minor.A,B, Whole body of living(A) and ethanol-fixed (B)specimens,ventral view.C, Anterior part ofthe trunk, ventral view.D, Base of the ventral chaetae connectedby interbasal muscles.E,A gonoduct containing numerous eggs.F, Anal vesicle sacs.G, Dwarf male (1.5 mm in length).Abbreviations:ac,anterior chaetae; av,anal vesicle sac; gd, gonoduct;gm, gonostomal lip;gp, genital pore; pr, proboscis; tr, trunk.Scale bars = 5 mm (A, B), 1 mm(C–G).Specimen ID (see Supporting Information, TableS2): BM16 (B, C, E, F), BM09(D, G).