Leucothoe limidicola, Hanaoka & Goto & Nakajima & Kodama, 2025

Leucothoe limidicola View in CoL sp. nov.

[New Japanese name: Yukimino-no-maruhasami-yokoebi]

Figs. 5–12 View FIGURE 5 View FIGURE 6 View FIGURE 7 View FIGURE 8 View FIGURE 9 View FIGURE 10 View FIGURE 11 View FIGURE 12

Material examined. Specimens were collected at 34°28'43.5"N, 136°52'57.9"E, subtidal rocky bottom, about 10 m deep, found from Li. hirasei, Kamekobana, Sugashima Island, Mie Prefecture, Japan, 1 June 2023, by H. Nakajima using SCUBA. Holotype: NSMT-Cr 32987, female, 10.7 mm. Paratypes: NSMT-Cr 32988, 1 male, 8.9 mm; NSMTCr 32989, 1 female, 5.2 mm; NSMT-Cr 32990, 1 female, 5.0 mm GoogleMaps .

Diagnosis. Head anterior margin (lateral cephalic lobes) rounded. Eyes small. Antenna 1, accessory flagellum consisting of 2 articles. Antenna 2 subequal in length with antenna 1. Epistome wider than long. Mandible, palp article 2 covered with dense slender setae respectively; molar-like-structure (sensu White 2019) present, located near (distal side) base of palp. Maxilla 1 palp consisting of 1 article. Maxilla 2 inner plate broader than outer plate. Maxilliped inner plates dorsodistal margin rounded, ventrodistal margin with v-shaped indentation. Gnathopod 1 dactylus reaching more than 0.2 × propodus length. Gnathopod 2 (male) carpus lateral surface distally dentate; medial surface distally tapered, apex without robust setae. Gnathopod 2 (female) carpus lateral surface distally rounded, medial surface distally tapered, apex without robust setae; propodus palmar margin convex with 4 major tubercles distally and many small or faint dentation proximally. Pereopod 5–7 bases expanded; posterodistal margin in pereopod 6 and 7 bases straight or weakly concave, oblique. Epimeral plate 1 lateral surface with ridge from anterodorsal to posteroventral corner, with tuft of setae, posterior margin weakly concave. Epimeral plate 2 ventral margin with 6 robust setae, posteroventral angle acutely produced posteriorly. Epimeral plate 3 anteroventral angle rounded, with 2 short robust setae, posteroventral angle acutely produced posteriorly. Telson triangulate, about 2.8 times as long as basal width, apex roundly pointed with 2 setae.

Description. Based on holotype (NSMT-Cr 32987, female, 10.7 mm). Body ( Fig. 5A View FIGURE 5 ) typical of the genus, smooth, rounded.

HEAD ( Fig. 5B View FIGURE 5 ) as long as pereonite 1, anterior margin (lateral cephalic lobes) rounded; rostrum apically pointed, slightly curved downwards, apex beyond anterior margin of lateral cephalic lobes; eyes oval, about 0.4 times as long as head, not overlapped by anterior margin of coxa 1, with more than 30 ommatidia. Antenna 1 ( Fig. 5C View FIGURE 5 ), about 0.2 times as long as body length, length ratio of peduncular articles 1–3 1.0: 0.96: 0.23; peduncular article 1, anterior margin with several fine setae, posterior margin with several fine setae, anterodistal and posterodistal corners with group of setae, peduncular article 2, anterior and posterior margins with row of fine and short slender setae, anterodistal and posterodistal corners with group of setae, article 3, anterior margin with several short setae, posterior margin bare, anterodistal and posterodistal corners with 1 or 2 seta(e); accessory flagellum present ( Fig. 5D View FIGURE 5 ), shorter than half length of primary flagellar article 1, consisting of 2 articles, article 1 with 2 slender setae, article 2 tiny, bare; flagellum about 0.3 times as long as peduncle, consisting of 9 articles, all of flagellar articles with distal setae, distalmost article small. Antenna 2 ( Fig. 5E View FIGURE 5 ) subequal in length with antenna 1; length ratio of peduncular articles 3–5 1.0: 3.0: 2.0, article 3, anterodistal and posterodistal corners with 1 or 2 seta(e), article 4, anterior and posterior margins with fine setae, setae on margins denser in proximal 0.4, anterior and posterior corners with setae, article 5, anterior and posterior margins with sparse short setae, anterodistal and posterodistal corners with setae; flagellum consisting of 6 articles, some of flagellar articles and distalmost article with fine setae.

Mouthparts. Upper lip ( Fig. 6A View FIGURE 6 ) asymmetrically lobate, anterior margin setose, epistome wider than long, bare. Mandible ( Fig. 6B, C View FIGURE 6 ); palp with 3 articles, length ratio of article 1–3 about 1.0: 3.3: 1.3, article 1 without setae, article 2 covered with dense facial slender setae; article 3 covered with several facial setae and 2 distal setae; left and right incisor with 7 and 9 teeth, respectively, both left and right lacinia mobilis with single set of teeth, left lacinia mobilis with 10 teeth, right lacinia mobilis smaller than left lacinia mobilis, with many (> 14) micro-teeth, 17 and 17 raker spines present on left and right mandibles, respectively, molar-like-structure ( sensu White 2019) present, located near (distal side) base of palp. Lower lip ( Fig. 6D View FIGURE 6 ); inner plate fused, outer plate anterior margin with gape, anterior margin setose. Maxilla 1 ( Fig. 7A View FIGURE 7 ); palp consisting of 1 article, palp margins without constriction, with 4 distal setae; outer plate distal margin oblique, with 7 robust and 8 slender setae, slender setae located more proximal area than robust setae; inner plate small, rounded, with 1 slender seta. Maxilla 2 ( Fig. 7B View FIGURE 7 ); inner plate broader than outer plate, with 12 setae on medial to distal margin; outer plate with 16 setae on lateral to distal margin. Maxilliped ( Fig. 7C View FIGURE 7 ); inner plates dorsodistal margin rounded, ventrodistal margin with v-shaped indentation, distal margins with large and small robust setae; outer plate, mediodorsal surface with row of slender setae, ventromedial margin smooth with sparse simple slender setae, weakly and expanded distomedially into lobe, reaching 0.2 of palp article 1, distomedial corner with robust seta, distolateral corner with simple and plumose seta(e); palp consisting of 4 articles, length ratio of article 1–4 about 1.0: 1.3: 1.1: 0.9, article 1, lateral margin weakly convex and many small or faint dentation with plumose setae, medial margin weakly convex with dense simple slender setae, submarginal to facial margin with rows of simple setae on dorsal to ventral surface, distal margin oblique, distolateral corner weakly extended with group of simple and plumose setae, article 2, lateral margin convex and many small or faint dentation, bare, medial margin weakly convex or rather straight and many small or faint dentation with row of dense slender marginal setae on dorsal to ventral surface, submarginal to facial margin with rows of slender setae on dorsal to ventral surface, distolateral corner pointed, bare, article 3, lateral margin weakly convex with short and fine setae, medial margin weakly concave in about proximal 0.4, weakly convex in distal 0.6, with dense fine and short setae on distal half, article 4 tapering, curved medially, with fine setae on distal half of medial margin, distally acute.

PEREON. Gnathopod 1 ( Fig. 8A View FIGURE 8 ); coxa 1.2 times as deep as wide, anterior margin weakly concave, anteroventral corner rounded with marginal fine setae without notch, ventral margin convex, posteroventral corner with fine short setae, posterior margin concave, medial surface with long seta and short setae on anteroventral corner; length ratio of basis: ischium: merus: carpus: propodus: dactylus about 1.0: 0.3: 0.2: 1.5: 0.8: 0.3; basis long, anterior margin weakly convex to straight with sparse fine setae, posterior margin, broadened in proximal 0.2, straight in distal 0.8, with posterolateral area of sparse fine setae on proximal 0.4–0.8, posterodistal corner with short seta, anterodistal corner bare; ischium, anterior margin concave, bare, posterior margin straight to weakly convex, bare, posterodistal corner produced distally into small projection with small seta, distal margin rounded; merus, dorsal surface concave, distally tapering, blunted, with 2 setae apically; carpus ( Fig. 8B View FIGURE 8 ), palmar margin micro-dentate with sparse setae, posterior margin smooth with sparse setae, distally tapering apically rounded, weakly curved upwards; propodus straight, anterior margin bare except for anterodistal corner with seta, palmar margin micro-dentate with several sparse robust setae and shorter fine setae in distal 0.6; dactylus smooth, about 0.4 times as long as propodus, anterior and posterior margin bare, distally tapering, curved posteriorly, apex pointed.

Gnathopod 2 ( Fig. 8C View FIGURE 8 ); coxa slightly wider than deep, subquadrate, anterodorsal corner rounded with fine seta, anterior margin convex with fine setae, anteroventral corner rounded, ventral margin straight, bare, posterodorsal corner posteriorly produced into sharp, posterior margin weakly excavate with posteroventral fine seta, facial setae absent, coxal gill slender; basis long, anterodistal corner without tubercle, anterior margin straight with several short setae, posterior margin broadened from proximal to middle, straight in distal 0.6, bare, posterodistal corner rounded, bare; ischium bare, anterior margin concave in proximal 0.4, convex in distal 0.6, posterior margin straight, posterodistal corner rounded; merus subtriangular, anterodistal corner with 2 medial short setae, distal margin with dense medial long setae; carpus weakly curved, anterolateral margin concave, lateral surface ( Fig. 8D View FIGURE 8 ) distally rounded without setae, apex of carpus reaching about proximal 0.3 of propodus; medial surface of carpus ( Fig. 8E View FIGURE 8 ) distally tapered, covered with dense short and long setae, distally apex without robust setae; propodus, medial surface with dense setae row displaced to midline, palmar margin convex with 4 major tubercles distally and many small or faint dentation proximally, lacking tiny setae; dactylus curved, bare, mediofacial area without setae, distally acute, almost reaching distal end of carpus.

Pereopod 3 ( Fig. 9A View FIGURE 9 ) simple; coxa 1.5 times as long as wide, with facial setae on medial surface, anterior to ventral margin rounded, bare, posterior margin straight with several fine setae, coxal gill slender, 2.5 times as long as coxal depth; length ratio of basis: ischium: merus: carpus: propodus: dactylus about 1.0: 0.2: 0.5: 0.4: 0.4: 0.1; basis slender, elongate, curved anteriorly, anterior margin with row of short setae in proximal 0.7, posterior margin with several short setae on distal 0.3, anterodistal and posterodistal corners with seta(e); ischium short, anterior margin bare, posterodistal corner with small acute projection and short seta; merus anterior margin straight with short setae, anterodistal corner slightly extended with robust seta, posterior margin bare, posterodistal corner with short seta; carpus anterior margin straight to faintly convex, bare, anterodistal corner with short robust seta, posterior margin straight to faintly convex, with several short slender and robust seta(e); propodus anterior margin straight, bare, posterior margin with several short robust setae; dactylus curved, falcate, bare. Pereopod 4 ( Fig. 9B View FIGURE 9 ) simple, coxa 1.2 times as long as wide similar to pereopod 3; coxa enlarged, deeper and wider than coxa 3, with facial setae on medial surface, anterior margin straight, ventral margin weakly rounded, posterodistal margin straight, posteroproximal margin slightly concave, coxal gill stout, 1.9 times as long as coxal depth, 1.1 times as wide as coxal gill on pereopod 3; length ratio of basis: ischium: merus: carpus: propodus: dactylus about 1.0: 0.1: 0.6: 0.4: 0.4: 0.2; basis slender, elongate, curved anteriorly, anterior margin with row of short setae and posterior margin with short setae on proximal 0.4 and distal 0.2; ischium posterodistal corner with short seta; merus anterior margin with fine seta proximally, posterior margin with fine setae proximally and subdistally, anterodistal corner extended with 2 short robust setae, posterodistal corner with 2 setae; carpus anterior margin weakly convex with fine several setae, posterior margin straight with several fine setae, posterodistal corner with robust and slender seta; propodus anterior margin straight with 2 fine setae, posterior margin with row of robust setae, posterodistal corner with robust seta; dactylus curved, falcate, bare.

Pereopod 5 ( Fig. 10A View FIGURE 10 ); coxa bilobate without facial setae, anterior lobe wider than posterior lobe, posterior lobe deeper than anterior lobe with 7 marginal short setae on ventral margin; length ratio of basis: ischium: merus: carpus: propodus: dactylus about 1.0: 0.3: 0.7: 0.5: 0.6: 0.2; basis about 1.0 times as long as width, anterior margin straight with 18 robust setae, posterior margin roundly convex on proximal half, weakly tapered (rather than rounded) on distal half, with 14 fine setae on entire length; ischium anterior margin bare except for anterodistal corner with short seta, posterior margin bare; merus anterior margin with middle robust seta, anterodistal corner with short robust seta, posterior margin convex, bare, posterodistal corner extended with 2 short robust setae; carpus anterior margin with 6 robust setae, posterior margin with robust seta on middle area and 1 robust seta on posterodistal corner; propodus anterior margin with 10 robust setae on anterodistal corner, posterior margin with 4 simple setae; dactylus curved, falcate, bare.

Pereopod 6 ( Fig. 10B View FIGURE 10 ) generally similar to pereopod 5 but slightly larger than pereopod 5; coxa bilobate without facial setae, smaller than coxa 5, anterior lobe 0.9 times as deep as that of coxa 5, posterior lobe 0.9 times as deep as that of coxa 5, anterior lobe with simple seta on anteroventrally, posterior lobe slightly deeper than anterior lobe, ventral margin lacking robust setae but with simple setae on ventral to posteroventral margin, coxal gill smaller than coxal gill on pereopod 5; length ratio of basis—dactylus about 1.0: 0.2: 0.7: 0.4: 0.6: 0.2; basis 1.1 times as long as pereopod 5 basis, 1.2 times as wide as pereopod 5 basis anterior margin straight with 14 robust setae, posterior margin roundly convex with sparse tiny setae in proximal 0.6, straight to slightly concave with tiny seta in distal 0.4, medial surface with facial setae; ischium bare except for anterodistal corner with short seta; merus anterior margin with 1 middle robust seta, anterodistal corner with 2 robust setae, posterior margin weakly convex with 3 robust setae, posterodistal corner extended with 3 short setae; carpus anterior margin with 8 short robust setae, posterior margin bare except for posterodistal corner with robust seta; propodus anterior margin with 11 robust setae, and short seta on anterodistal corner, posterior margin with 5 simple setae; dactylus curved, falcate, bare.

Pereopod7 ( Fig.10C View FIGURE 10 )generally similar to pereopods5 and 6, but slightly larger than pereopod5;coxa semicircular lacking robust setae, posteroventral corner with 2 simple setae, coxal gill absent; length ratio of basis—dactylus about 1.0: 0.2: 0.6: 0.4: 0.5: 0.2; basis anterior margin straight to weakly convex with 13 robust setae, posterior margin weakly convex in proximal 0.6, oblique and slightly concave in distal 0.4, with 14 tiny setae in entire length, medial surface with facial setae; ischium anterior margin with 2 short setae, posterior margin bare; merus anterior margin with 3 short robust setae, posterior margin weakly convex with 3 short robust setae, posterodistal corner extended with 3 short robust setae; carpus anterior margin straight with 5 robust setae, anterodistal corner with 2 short and 2 long robust setae, posterior margin with 3 simple setae, posterodistal corner weakly extended with short seta; propodus anterior margin straight with 12 short robust setae, posterior margin with 5 simple setae; dactylus curved, falcate, bare.

PLEON. Epimeral plates ( Fig. 11A View FIGURE 11 ), epimeral plate 1 anterior margin slightly concave anteroventral angle with tuft of setae, ventral margin rather straight, posteroventral corner angular, blunt, posterior margin weakly concave, lateral surface with ridge from anterodorsal to posteroventral corner; epimeral plate 2 anteroventral corner rounded, ventral margin weakly convex or rather straight with 6 short robust setae, posteroventral angle acutely produced posteriorly, posterior margin weakly concave, lateral surface, with ridge; epimeral plate 3 anteroventral angle rounded, with 2 short robust setae, ventral weakly convex, posteroventral angle acutely produced posteriorly but shorter than that of epimeral plate 2, posterior margin concave.

Uropod 1 ( Fig. 11B View FIGURE 11 ) length ratio of peduncle, inner ramus, outer ramus 1.0: 0.7: 0.9; peduncle lateral margin with row of 9 robust setae in proximal 0.6, medial margin with row of 5 robust setae from proximal 0.3 to 0.8, both distolateral and distomedial corner with robust seta; both rami tapering, outer ramus bare, inner ramus with 2 robust setae on lateral margin. Uropod 2 ( Fig. 11C View FIGURE 11 ) short, about 0.8 times as long as uropod 1, length ratio of peduncle, inner ramus, outer ramus, about 1.0: 1.2: 0.8; peduncle lateral margin with row of 4 robust setae on distal 0.6, medial margin bare, distolateral and distomedial corner with robust seta; both rami tapering distally, outer ramus with 6 robust setae laterally; inner ramus lateral and medial margin with 5 and 2 robust setae, respectively. Uropod 3 ( Fig. 11D View FIGURE 11 ) subequal in length with uropod 1, length ratio of peduncle, inner ramus, outer ramus, 1.0: 0.8: 0.8; peduncle lateral margin with row of 9 robust setae, medial margin with 2 robust setae in distal 0.3; both rami tapering distally, inner ramus with 3 robust setae, outer ramus with 8 robust setae. Telson ( Fig. 11E View FIGURE 11 ) triangulate, about 2.8 times as long as basal width, apex ( Fig. 11F View FIGURE 11 ) roundly pointed with two short setae, not trident, right margin with several short setae, left margin without setae (asymmetric in holotype), facial setae absent.

Male characters. Generally similar to females except for gnathopod 2 carpus ( Fig. 12A, B View FIGURE 12 ); lateral surface ( Fig. 12C View FIGURE 12 ) distally dentate with 5 teeth, without setae; medial surface ( Fig. 12D View FIGURE 12 ) distally tapered, covered with dense short and long setae, apex without robust setae.

Coloration in life. Body generally white with a vermilion red pattern ( Fig. 13A, B View FIGURE 13 ). Pereonites 1–3 whitish dorsally to anteriorly, vermilion red ventrally to posteriorly; pereonites 4–7 and urosomites vermilion forming a kind of ‘saddleback’ color pattern (see White & Reimer, 2012a –d); pleonites 1–3 generally white, each pleonite with a red point near anterior margin. Pereonites and coxae sometimes with white ‘snowflake pattern’ (see Kodama et al. 2022). Eyes white to pale vermilion. Appendages are generally pale pink to colorless. Antenna 1 peduncle whitish, flagellum vermilion red.

Distribution. Known only from the type locality, Sugashima Island, Mie Prefecture, central Japan.

0.2

Ecological notes. All specimens were collected from the inside of the mantle or surface of tentacles of a single host bivalve, Li. hirasei ( Fig. 14A, B View FIGURE 14 ). The host (the host specimen was not retained after the collection) was found from under a rock on subtidal rocky bottom about 10 m depth around Kamekobana, Sugashima Island, Mie Prefecture, central Japan. Even when Le. limidicola sp. nov. was removed and placed away from the host bivalve, the amphipods returned to the inside of the shell via the tentacles of the host.

Genetic analysis. There was no genetic variation among the 18S sequences of four individuals of Le. limidicola sp. nov. The COI gene sequences were obtained only from two of these individuals. There were two base-pair differences in the 658 bp of the COI sequences between them (0.3%). According to the molecular phylogenetic analyses based on the 18S gene, Leucothoe comprised two subclades (A and B, Fig. 15 View FIGURE 15 ) as in Goto et al. (2024). Leucothoe limidicola sp. nov. belongs to clade A, forming a monophyletic group with Le. amamiensis, Le. akaisen , and Leucothoe vulgaris , and was sister to Le. akaisen . The uncorrected p -distances in the 18S and COI genes between Le. limidicola sp. nov. and its sister Le. akaisen were 1.5–1.6% and 9.3–9.4%, respectively. The 18S alignments of Le. limidicola sp. nov. and Le. akaisen contain seven species-specific indels.

Etymology. The epithet of the new species ‘ limidicola ’ consists of the family name of its host, Limidae Rafinesque, 1815 and the Latin adjective suffix ‘-colus’ meaning ‘inhabiting’, derived from the commensal association of this new species with limid bivalves.

Remarks. Leucothoe limidicola sp. nov. is the closest to Le. akaisen described from Ryukyus, Japan. They share (1) the rounded anterior margin of the head; (2) the long dactylus of gnathopod 1 (more than 0.2 times longer than propodus); (3) the maxilla 1 palp consisting of only 1 article; and (4) the roundly pointed (not trident) telson. However, Le. limidicola sp. nov. can be distinguished from Le. akaisen by the following characters: (1) mandibular palp articles 2 and 3 have dense facial slender setae ( Fig. 6B View FIGURE 6 ) but those of Le. akaisen have less setation ( Fig. 2C View FIGURE 2 ); (2) gnathopod 1 dactylus has no setae on posterodistal corner in this species ( Fig. 8A View FIGURE 8 , 12A View FIGURE 12 ), but in Le. akaisen two fine setae are present there ( Figs. 3A View FIGURE 3 , 4A View FIGURE 4 ); (3) gnathopod 2 carpus has no robust setae in this species ( Figs. 8D, E View FIGURE 8 , 12C, D View FIGURE 12 ), but that of Le. akaisen has a robust seta distally ( Figs. 3C, D View FIGURE 3 , 4C, D View FIGURE 4 ); (4) male gnathopod 2 carpus is distally dentate on lateral surface in this species ( Fig. 12C View FIGURE 12 ), but that of Le. akaisen is distally tapered ( Fig. 3C View FIGURE 3 ); (5) female gnathopod 2 dactylus has no setae on mediofacial area in this species ( Fig. 8C View FIGURE 8 ), but that of Le. akaisen has two fine setae ( Fig. 4B View FIGURE 4 ); (6) posterior margin of pereopods 5–7 bases are rather straight in this species ( Fig. 10A–C View FIGURE 10 ) but that of Le. akaisen is rounded ( Fig. 2D, E View FIGURE 2 ); (7) lateral surface of epimeral plate 1 has a ridge in this species ( Fig. 11A View FIGURE 11 ) but that of Le. akaisen is absent; (8) anteroventral angle of epimeral plate 3 has 2 short robust setae in this species ( Fig. 11A View FIGURE 11 ), but that of Le. akaisen is absent; and (9) telson is triangulate in this species ( Fig. 11E View FIGURE 11 ) but that of Le. akaisen is more rounded ( Fig. 2B View FIGURE 2 ).

The ML phylogenetic tree based on the 18S genes suggests that this new species forms a monophyletic group with Le. amamiensis White & Reimer, 2012, Le. vulgaris White & Reimer, 2012 , Le. akaisen, Le. vermicola Kodama, White, Hosoki & Yoshida, 2022, and Le. bonelliae Shimomura, Hirabayashi & Goto, 2023 (bootstrap support value = 76). Furthermore, the new species was genetically close to its sister Le. akaisen (18S: 1.5–1.6%, COI: 9.3–9.4%). These genetic differences of 18S are similar to the interspecific differences in Leucothoe , such as Le. akaoni White & Reimer, 2012 and Le. daisukei White & Reimer, 2012 (18S: 0.2–0.8%). In addition, the alignments of Le. limidicola sp. nov. and Le. akaisen contain species-specific seven indels, which also support that they are different species. The interspecific genetic distances of COI between sister species reported in Leucothoe usually exceed 20% ( Goto et al., 2024). However, this threshold may be overestimated, as COI sequences have been examined for only a limited number of species in this group. In amphipods, interspecific thresholds of COI divergence vary among families, but a recent study suggested that a threshold of around 7% may be appropriate for most amphipod species ( Tempestini et al., 2018). From this perspective, the observed COI divergence between Le. limidicola and Le. akaisen falls within the expected range of interspecific variation.

Discussion. Leucothoe limidicola sp. nov. and Le. akaisen were collected from different habitats despite their close phylogenetic relationship. The new species was found in association with Li. hirasei , which inhabited a rocky bottom, whereas Le. akaisen was collected from coral rubble. Although the host of Le. akaisen was presumed to be a coral reef sponge ( White & Reimer, 2012c), it remains possible that it was associated with Li. hirasei in the coral rubble, as the specimen was collected through coral rubble wash.

There are three possible explanations regarding their habitat preference. (1) Leucothoe limidicola sp. nov. and Le. akaisen may both be commensals that prefer to associate with Li. hirasei , though this remains to be confirmed for Le. akaisen . (2) Both Leucothoe species inhabit a wide range of environments around gravel beds and opportunistically use benthic invertebrates such as Limaria as hosts. (3) Leucothoe akaisen and Le. limidicola sp. nov. may have distinct habitat preference, with only Le. limidicola sp. nov. using Li. hirasei as a host. Further field observations are necessary to evaluate each hypothesis. Notably, closely related sister species of Leucothoe , such as Le. vermicola and Le. bonelliae, are known to use different hosts ( Kodama et al. 2022; Goto et al. 2024). This suggests that Le. akaisen and Le. limidicola sp. nov. have developed different microhabitat preferences.

The degree of host specificity may vary depending on the host taxon. Herein, we summarize the cases of bivalve-commensalism in Leucothoe . Walker (1895a) reported Le. spinicarpa (Abildgaard, 1789) from Pecten sp. though later Vader (2013) treated Walker’s material as Le. spinicarpa auct. [reference mistake: Walker (1895a) is correct, but Vader & Tandberg (2013) quote Walker (1895b) incorrectly]. Ledoyer (1979) reported Le. furina (Savigny, 1816) from Pinna sp. Linnaeus, 1758. Morton (1980) and Ortiz (1975) reported Le. spinicarpa from Ctenoides scabra (Born, 1778) (as Lima scabra ) and Atrina rigida ([Lightfoot], 1786), but Thomas & Klebba (2007) accepted their specimens as Le. flammosa Thomas & Klebba 2007 . Leucothoe flammosa has been collected from Ctenoides scabra (as Lima scabra ), Anadara notabilis (Roding, 1798) , Americardia media Linnaeus, 1758 , Dendostrea frons (Linnaeus, 1758) , Lithophaga antillarum Orbigny, 1846 , Lucina pennsylvanica Linnaeus, 1758 , and Mytilopsis leucopheata Conrad, 1831 ( Thomas & Klebba 2007, Vader & Tandberg 2013). Thomas & Klebba (2007) also reported unidentified Leucothoe sp. B from Laevicardium laevigatum Linnaeus, 1758 . Recently, Le. tumida Myers, 2013 and Le. eltoni Thomas, 2015 were reported from Pteria penguin (Röding, 1798) ( Myers 2013, Thomas 2015).

In summary, bivalve-commensalism is currently found in at least six Leucothoe species including Le. limidicola sp. nov. In contrast, the host specificity varies among species. Leucothoe flammosa was collected from several genera of bivalves, whereas Le. limidicola sp. nov. was only collected from Li. hirasei . A further investigation is needed to reveal the relationship between Leucothoe and their hosts.