Basterotia bonelliphila Goto & Hirabayashi, 2024

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 : 925-927

Basterotia bonelliphila Goto & Hirabayashi , sp. nov.

(New Japanese Name: Yokaze-gai)

( Figs 4A, B , 5 , 11–13 )

Zoobank registration: urn:lsid:zoobank.org:act:9D61F68C-2F66-492C-97F7-B650CB870025 .

Material examined: Holotype ( Fig. 11 ): NSMT-Mo 79470 [SL (shell length) 4.0 mm, SH (shell height) 1.6 mm, SW (shell width) 1.7 mm, Sabiura, Kushimoto, Wakayama, Japan, obtained from the burrow of Bo. sp. aff. minor , collected by I. Hirabayashi, 3 June 2020] . Paratype ( Fig. 12 ): NSMT-Mo 79471 (SL 2.6 mm, SH 1.3 mm, Sabiura, Kushimoto, Wakayama, Japan, obtained from the burrow of Bo. sp. aff. minor , collected by I. Hirabayashi, 3 June 2020) . Other material ( Fig. 13 ): 1 specimen (SL 4.3 mm, Sabiura, Kushimoto, Wakayama, Japan, obtained from the burrow of Bo. sp. aff. minor , collected by I. Hirabayashi, 4 July 2020) .

Type locality: Sabiura , Kushimoto, Wakayama, Japan (33°28’43.61”N, 135°44’45.45”E; Fig. 1 ) .

Description: Shell length up to 4.3 mm, small for genus; shell thin, fragile, equivalve, strongly inequilateral, length/height ratio about 2.3, compressed (width/length ratio 0.43, holotype). Shell gaping from anteroventral to anterodorsal margins. Shell outline cuneate; anterodorsal margin short, sloping steeply; anterior end rounded; ventral margin straight, nearly parallel with the dorsal margin; posteroventral margin obliquely rounded-subtruncate to a point near base; ventral margin long, nearly straight, slightly curved towards anterior and posterior ends. Beaks low, small, inflexed, prosogyrous, situated at about 17–19% from anterior end. A weak, oblique ridge running from the umbo to the posteroventral corner. Shell externally white with light brown tint; shell sculpture with irregular commarginal growth lines. Hinge ( Figs 11G, H , 12E, F ): A projecting prominent cardinal tooth present both in right and left valves. Cardinal tooth in right valve shorter than that in left valve. Ligament external, short, posterior to umbo (opisthodetic), moderate in size. Prodissoconch II 246 to 248 µm long (paratype). Soft tissue ( Fig. 11E, F ): Ctenidia narrow long, composed of outer and inner demibranches. Labial palps small, narrow, present anterior to foot. Mantle without tentacles, thin except for the posterior siphonal regions. Inhalant and exhalant siphons short, located at posterior end. Inhalant siphon muscular, tubular, longer than exhalant siphon ( Fig. 13 ). Evenly separated small papillae present on the base of the inhalant siphon ( Fig. 13D ). Pedal gape situated at the anteroventral position ( Fig. 13C ). Foot white, slender with the tip rounded ( Fig. 13C ). Digestive gland brownish, occupying nearly half of visceral mass ( Fig. 11E, F ). Anterior and posterior adductor muscles ovate in shape, with anterior smaller than posterior ( Fig. 11E, F ).

Host: Bonellia sp. aff. minor ( Annelida: Thalassematidae : Bonelliinae ).

Host association and biology: This bivalve species lives attached by byssal threads onto the burrow wall of Bo. sp. aff. minor in the dead coral rocks ( Figs 4B , 5 ). The live animals actively crawl away using their foot and extend their inhalant and exhalant siphons from the posterodorsal margin by slightly opening the shells.In the aquaria, the bivalve extends its short inhalant siphon against the host body trunk.

Distribution: Only known from the type locality (Sabiura, Wakayama, Japan).

Epifauna: Dorsal marginal surface of the shells was often attached to by entoprocts ( Figs 4B , 11A–D ), whereas the lateral surface of the shells was often attached to by foraminiferans ( Figs 4B , 11D ).

Etymology: The new species is named after its commensal association with the spoon worm Bonellia .

Remarks: Species of Basterotia are known to inhabit the burrows of the thalassematin echiurans in sediment bottoms ( Anker et al. 2005, Goto et al. 2011, 2017, Goto and Kato 2012, Goto 2017). Basterotia bonelliphila is distinct among the genus with its small, elongate shells and in inhabiting the burrows of Bonellia (Bonelliinae) in the dead coral rocks ( Figs 4A , 5 ). Probably, its small shell size and elongate shell shape are adaptations to the Bonellia ’s narrow burrow. Some congeneric species [e.g. Basterotia sp. from Rodrigues Island in Oliver and Holmes (2004)] and some species belonging to other basterotiin genera [e.g. Basterotina rectangularis Coan, 1999 and Ensitellops hertleini W.K. Emerson & Puffer, 1957 in Coan and Valentich-Scott (2012)] have relatively similar elongate, small shells. Although the biology of these species remains unknown, they may also inhabit the narrow burrows of Bonellia or other echiurans in the hard substrata. The shells of Basterotina are different from those of Basterotia in having low, non-projecting hinge teeth ( Coan 1999), whereas the shells of Ensitellops are more elongate and possess a usual internal ligament ( Coan and Valentich-Scott 2012, Huber 2015).

The outline of this species changes from subtrapezoidal to subelliptical as it grows. Young individuals of Ba. bonelliphila (e.g. paratype, SL ~ 2.6 mm, Fig. 12 ) are relatively similar to the type of Basterotia subalata (Gatliff & Gabriel, 1910) (SL ~ 7.5 mm) from southern Australia, the type of Basterotia corbuloidea (Dall, 1899) (SL ~ 6.5 mm) from North Carolina, or Basterotia oblonga E.A. Smith, 1890 (SL ~ 8.5 mm) from St. Helena in shell shape. However, the large individuals of Ba. bonelliphila are not similar to those species.

The siphons of Basterotia are often surrounded by numerous tentacles ( Goto et al. 2011, 2017). However, Ba. bonelliphila lack such tentacles except for very short papillae ( Fig. 13D ).

In Basterotia bivalves, small juveniles often attach to the ventral margin of larger individuals (e.g. Goto et al. 2011). However, we did not find such small individuals in this study. Our sampling was limited to early summer (June and July). Thus, the settlement of juveniles of this species may occur in different seasons.

Figures

Figure 1. Geographic location of the study site (Kushimoto, Wakayama Prefecture, Japan).

Figure 4. Macrosymbiotic fauna associated with Bonellia sp. aff.minor.Macrosymbionts with hosts in the burrows (A, C, E). White arrowheads indicate macrosymbionts on the host’s burrow wall (A, E) or trunk (C). Close-up view of the living specimens of each macrosymbiont (B, D, F). A, B, Basterotia bonelliphila (holotype:NSMT-Mo 79470). C, D, Leucothoe bonelliae (SMBL-V0666). E, F, Oxydromus fauveli.Scale bar = 1 mm (B, D, F).

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 11. Basterotia bonelliphila (holotype NSMT-Mo 79470). A, B, Dorsal and ventral views. Anterior is right. C, D, External view of right and left valves. E, F, Internal view of right and left valves. The tip of the foot was removed for DNA analysis.G, H, Hinge structure of left and right valves. Abbreviations: aa, anterior adductor muscle; ac, anterior cardinal tooth; apa, anterior-venral pedal aperture; d, demibranches; dg, digestive gland; exl, external ligament; f, foot; lp, labial palps; pa, posterior adductor muscle; p2, prodissoconch II; s, exhalant and inhalant siphons. Scale bars = 1 mm (A–F), 100 µm (G, H).

Figure 12. Basterotia bonelliphila (paratype NSMT-Mo 79471). A, B, External view of right and left valves. C, D, Internal view of right and left valves. E, F, Hinge structure of right and left valves.G, H, Prodissoconch II of right and left valves. Abbreviations: ac, anterior cardinal tooth; exl, external ligament; p2, prodissoconch II. Scale bars = 1 mm (A–D), 100 µm (E–H).

Figure 13. A living individual of Basterotia bonelliphila. A, Dorsal view of an individual opening its valves and extending its inhalant and exhalant siphons from the postero-dorsal margin. Posterior is left.B, Ventral view of an individual extending its inhalant siphon.Posterior is left. C, Ventral view of an individual extending its inhalant siphon and foot. Posterior is right.D, Close-up of the inhalant siphon. Arrowheads indicate the papillae on the base of the siphon. Scale bars = 1 mm.Abbreviations:exs, exhalant siphon; ins, inhalant siphon.