Molecular and Morphological Assessment of Juvenile and Adult Forms in the Giant Worm Eunice Cf. Aphroditois (Annelida: Eunicidae) and Its Phylogenetic Position in the Family.

Eunice aphroditois (Pallas, 1788) is a large polychaete worm (up to 3 m in length) and the type species of the genus. In Japan, a similar but potentially different species, Eunice cf. aphroditois, is distributed mainly in the rocky shores of the temperate and warm Pacific coasts. Juveniles and adults were suggested to be distinguished by their body color. The juvenile form was previously regarded as distinct species, Eunice flavopicta Izuka, 1912 and Eunice ovalifera Fauvel, 1936, although they are now considered synonymous with E. aphroditois. In this study, we revisited the validity of the present taxonomy based on morphological observations including SEM and microCT, and three molecular markers (cytochrome c oxidase subunit I [COI], 16S rRNA, and histone H3 genes) and investigated the phylogenetic position of E. cf. aphroditois in the family Eunicidae using the combined dataset of three genes (COI + 16S rRNA + 18S rRNA). The adult and juvenile forms were different in body size, color, the distribution of the branchiae and subacicular hooks, and maxillae shape, but not in other characteristics. One individual showed an intermediate body color between the two forms. The adult and juvenile forms shared major haplotypes and the maximum K2P genetic distance of COI was 1.7%, which can be considered within intraspecific variation. In the phylogenetic tree based on the combined gene dataset, E. cf. aphroditois was closely related to Eunice roussaei Quatrefages, 1866 and Eunice cf. violaceomaculata Ehlers, 1887, which are large species from the Mediterranean Sea and the Caribbean Sea, respectively.

Evolutionary patterns of host switching, lifestyle mode, and the diversification history in symbiotic zoantharians.

Symbioses play important roles in forming the structural and distributional patterns of marine diversity. Understanding how interspecies interactions through symbioses contribute to biodiversity is an essential topic. Host switching has been considered as one of the main drivers of diversification in symbiotic systems. However, its process and patterns remain poorly investigated in the marine realm. Hexacoral species of the order Zoantharia (=zoantharians) are often epizoic on other marine invertebrates and generally use specific taxa as hosts. The present study investigates the patterns of host switching and the diversification history of zoantharians based on the most comprehensive molecular phylogenetic analyses to date, using sequences from three mitochondrial and three nuclear markers from representatives of 27 of 29 genera. Our results indicate that symbiotic zoantharians, in particular those within suborder Macrocnemina, diversified through repeated host switching. In addition, colonization of new host taxa appears to have driven morphological and ecological specialization in zoantharians. These findings have important implications for understanding the role of symbioses in the morphological and ecological evolution of marine invertebrates.

Stasis and diversity in living fossils: Species delimitation and evolution of lingulid brachiopods.

The Lingulidae are often considered living fossils, because they have shown little morphological change since the Paleozoic. Limited morphological variation has also made the taxonomic study of living lingulids challenging. We investigated species diversity and phylogenetic relationships of extant lingulids and show that they are substantially more diverse than realized, demonstrating that morphological stasis was commonly accompanied by speciation. Species delimitation based on cytochrome c oxidase subunit I (COI) gene sequences from 194 specimens sampled from East Asia, Australia, Oceania, and the Americas suggested 14-22 species in the lingulids (9-17 species in Lingula and 4-5 species in Glottidia), in contrast to the 11-12 species currently recognized globally in the family. Four-gene phylogenetic analyses supported the sister relationship between Lingula and Glottidia. Within Lingula, L. adamsi, which possesses large, brownish shells, was recovered as sister to all remaining Lingula species, which have more or less greenish shells. Within the greenish Lingula clade, the 'L. anatina' complex was sister to the clade that includes the 'L. reevei' complex. The 'L. anatina' complex was further separated into two major clades with partly separate ranges centered on (i) temperate East Asia, and (ii) the tropical west-central Pacific. Within Glottidia, Pacific species were nested within Atlantic species. Time-calibrated phylogenetic analyses suggested that Lingula likely originated in the early Cretaceous contrary to a previously proposed hypothesis advocating a Cenozoic origin. The separation of Lingula and Glottidia appears to date from the Mesozoic, not from the Carboniferous, contrary to a previous hypothesis. Overall, our results uncovered substantial cryptic diversity in lingulids, which will form the basis for conservation and further taxonomic revision.

Molecular phylogenetic assessment of Spirobranchus kraussii-complex (Annelida: Serpulidae) from the Japanese Archipelago.

Spirobranchus kraussii (Annelida: Serpulidae) was recognized as being widely distributed both in the Pacific and Atlantic Oceans. However, the sampling records far from its type locality (South Africa) have been questioned. Actually, recent molecular phylogenetic studies showed that S. kraussii contains genetically distinct species. In this study, we performed molecular phylogenetic analyses of S. cf. kraussii collected from Japan using the nucleotide sequences of a mitochondrial gene and two nuclear genes. Three lineages were recovered within Spirobranchus kraussii-complex in Japan, and one (Spirobranchus sp. 6) showed moderate genetic difference (approximately 4%) in the mitochondrial cytb gene sequence from Spirobranchus sp. 1, an undescribed sequenced species from Honshu Island, Japan. However, the nucleotide sequences of the 18S rRNA gene and ITS2 region were nearly indistinguishable. The other lineage was clearly distinct from the other previously sequenced species and is thus considered to be another distinct species of this species complex (Spirobranchus sp. 5). Although detailed morphological assessment of these lineages is necessary to define their taxonomic status, the present study provided further implications for the species diversity within the S. kraussii-complex.

Snails riding mantis shrimps: Ectoparasites evolved from ancestors living as commensals on the host's burrow wall.

The molluscan class Gastropoda includes over 5,000 parasitic species whose evolutionary origins remain poorly understood. Marine snails of the genus Caledoniella (Caledoniellidae) are obligate parasites that live on the abdominal surface of the gonodactylid mantis shrimps. They have highly modified morphological characteristics specialized to the ectoparasitic lifestyle that make it difficult to infer their close relatives, thereby posing a question about their current systematic position in the superfamily Vanikoroidea. In the present study, we performed molecular phylogenetic analyses using three nuclear and three mitochondrial gene sequences to unveil the phylogenetic position of these enigmatic snails. The resulting trees recovered Caledoniella in the superfamily Truncatelloidea and within a subclade of commensal species that live on the burrow wall of marine benthic invertebrates. More specifically, Caledoniella formed the sister clade to a commensal snail species living in mantis-shrimp burrows and they collectively were sister to Sigaretornus planus (formerly in the family Tornidae or Vitrinellidae), a commensal living in echiuran burrows. This topology suggests that the species of Caledoniella achieved their ectoparasitic mode of life through the following evolutionary pathway: (1) invasion into the burrows of benthic invertebrates, (2) specialization to mantis shrimps, and (3) colonization of the host body surface from the host burrow wall with the evolution of the parasitic nature. The final step is likely to have been accompanied by the acquisition of a sucker on the metapodium, the loss of the radula and operculum, and the formation of monogamous pair bonds. The present molecular phylogeny also suggested parallel evolution of planispiral shells in a subclade of Truncatelloidea and enabled us to newly redefine the families Caledoniellidae, Elachisinidae, Teinostomatidae, Tornidae and Vitrinellidae.

Phylogeography of Mytilisepta virgata (Mytilidae: Bivalvia) in the northwestern Pacific: Cryptic mitochondrial lineages and mito-nuclear discordance.

The purplish bifurcate mussel Mytilisepta virgata is widely distributed and represents one of the major components of the intertidal community in the northwestern Pacific (NWP). Here, we characterized population genetic structure of NWP populations throughout nearly their whole distribution range using both mitochondrial (mtDNA cox1) and nuclear (ITS1) markers. Population genetic analyses for mtDNA cox 1 sequences revealed two monophyletic lineages (i.e., southern and northern lineages) geographically distributed according to the two different surface water temperature zones in the NWP. The timing of the lineage split is estimated at the Pliocene- mid-Pleistocene (5.49-1.61 Mya), which is consistent with the timing of the historical isolation of the East Sea/Sea of Japan from the South and East China Seas due to sea level decline during glacial cycles. Historical sea level fluctuation during the Pliocene-Pleistocene and subsequent adaptation of mussels to different surface water temperature zones may have contributed to shaping the contemporary genetic diversity and deep divergence of the two mitochondrial lineages. In contrast to mtDNA sequences, a clear lineage split between the two mitochondrial lineages was not found in ITS1 sequences, which showed a star-like structure composed of a mixture of southern and northern mitochondrial lineages. Possible reasons for this type of mito-nuclear discordance include stochastic divergence in the coalescent processes of the two molecular markers, or balancing selection under different marine environments. Cryptic speciation cannot be ruled out from these results, and future work using genomic analyses is required to address whether the thermal physiology of these mussels corresponds to the deep divergence of their mitochondrial genes and to test for the existence of morphologically indistinguishable but genetically separate cryptic species.

Worm-riding clam: description of Montacutona sigalionidcola sp. nov. (Bivalvia: Heterodonta: Galeommatidae) from Japan and its phylogenetic position.

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.

Remarkably loud snaps during mouth-fighting by a sponge-dwelling worm.

Many aquatic animals, including mammals, fishes, crustaceans and insects, produce loud sounds underwater [1-6]. Soft-bodied worms would seem unlikely to produce a loud snap or pop because such brief, intense sounds normally require extreme movements and sophisticated energy storage and release mechanisms [5]. Surprisingly, we discovered a segmented marine worm that makes loud popping sounds during a highly stereotyped intraspecific agonistic behavior we call 'mouth fighting'. These sounds - sound pressures up to 157 dB re 1 µPa at 1 m, with frequencies in the 1-100 kHz range and a strong signal at ∼6.9 kHz - are comparable to those made by snapping shrimps, which are among the most intense biological sounds that have been measured in the sea [6]. We suggest a novel mechanism for generating ultrafast movements and loud sounds in a soft-bodied animal: thick, muscular pharyngeal walls appear to allow energy storage and cocking; this permits extremely rapid expansion of the pharynx within the worm's body during the strike, which yields an intense popping sound (likely via cavitation) and a rapid influx of water. Clearly, even soft-bodied marine invertebrates can produce remarkably loud sounds underwater. How they do so remains an intriguing biomechanical puzzle that hints at a new type of extreme biology.

The Usefulness of Basic Movement Scale in Hip Fracture Patients: Construct Validity from a Cross-Sectional Study.

OBJECTIVE: The aim of the study was to investigate the validity of using total score and to examine the constitution and characteristics of the Basic Movement Scale in postsurgery patients with hip fracture. DESIGN: The dimensionality and the threshold difficulty intervals between each score and item difficulty hierarchy of the Basic Movement Scale were examined using factor analysis and Rasch analysis in 37 patients admitted to our hospital between April and November 2015. RESULTS: For factor analysis, the contribution ratio of the first factor was 78.9%, that of the second factor was 6.5%, and there were no items that fit the Rasch analysis. The threshold was reversed at 6 of the 48 locations. The difficulty of the 12 Basic Movement Scale items was distributed roughly evenly among all 9 lots, with some deviation. There was one very easy item, and there were some items almost overlapping in difficulty. CONCLUSIONS: The results showed a unidimensional association between the items and evaluation index. The difficulty threshold of each score was approximated to the interval scale. Therefore, the Basic Movement Scale has evident construct validity and enables quantitative evaluation of physical ability, assessment of the effects of daily training, and general predictions of the feasibility of patients' clinical goals.

Morphology and Habitats of the Hermit-Crab-Associated Calyptraeid Gastropod Ergaea walshi.

Ergaea walshi, a gastropod with a markedly flat shell, often lives inside empty snail shells occupied by hermit crabs. We investigated its lifestyle, shell growth pattern, and habitat preference for host hermit crabs and host snail shells. Four hundred sixteen snail shells, including 363 shells with hermit crabs and 53 empty shells, were collected from intertidal zones of sandy and muddy flats around Kii Peninsula, Japan. The specimens comprised seven hermit crab species occupying 24 shell species; E. walshi was harbored in 13.2% of snail shells with hermit crabs and 17.0% of those without hermit crabs. Although no preference was detected for particular species of hermit crab or snail shell, E. walshi preferred to live inside of snail shells with wider apertures used by comparatively bigger hermit crabs. This suggests that the occurrence of E. walshi was influenced by host size rather than host species. When looking at growth patterns, we found that the attached shell portion of E. walshi continued to be enlarged horizontally, while growth in shell height slowed at approximately 5.0 mm. The conspicuously flattened shell of E. walshi is considered as a growth pattern for adapting to the narrow space within the snail shell occupied by hermit crabs. Consistent with this idea, our comparison of shell growth patterns in 23 calyptraeid species showed that shell of E. walshi is the flattest in this family.

Molecular phylogeny of Maldanidae (Annelida): Multiple losses of tube-capping plates and evolutionary shifts in habitat depth.

Inter-familial relationships of the phylum Annelida have been widely studied using molecular phylogenetic/genomic approaches; however, intra-familial relationships remain scarcely investigated in most annelid families. The Maldanidae (bamboo worms) comprise more than 280 species of 40 genera and six subfamilies that occur in various environments from intertidal to hadal zones. Within this family, the taxon Maldanoplaca, which consists of four subfamilies (Maldaninae, Notoproctinae, Nicomachinae, and Euclymeninae), was proposed based on the presence of cephalic and anal plates. Phylogenetic relationships within the family remain largely undetermined based on molecular data. In this study, we reconstructed a molecular phylogeny using 52 maldanid species from six subfamilies based on two nuclear genes (18S rDNA and 28S rDNA) and two mitochondrial genes (16S rDNA and COI). Our analysis confirmed the monophyly of the subfamilies Rhodininae, Maldaninae, Lumbriclymeninae, and Nicomachinae, but neither Maldanoplaca nor the subfamily Euclymeninae were recovered as monophyletic. Nicomachinae was clustered within Euclymeninae. Ancestral state reconstruction suggested that cephalic plates were lost at least three times, despite the functional importance of capping tubes, and that anal plates were lost once. Mapping habitat depth on the phylogenetic tree suggested that habitat shifts among depth zones frequently occurred in distinct maldanid lineages.

A new large tellinid species of the genus Pharaonella from the Ryukyu Archipelago, Japan (Mollusca, Bivalvia).

A new tellinid species, Pharaonella amanyusp. n., is described from sand banks around Amami Islands, the Ryukyu Archipelago, in southern Japan. A molecular phylogenetic analysis suggests that this new species is closely related to P. sieboldii. This species has long siphons and lives buried deep in well-sorted white sand syntopically with Tonganaella tongana. These rare, large tellinid species are indicators of unspoiled tidal/subtidal sand flats, which should receive the highest priority conservation in the Ryukyu Archipelago.

Borniopsis mortoni sp. n. (Heterodonta, Galeommatoidea, Galeommatidaesensu lato), a new bivalve commensal with a synaptid sea cucumber from Japan.

The Galeommatoidea is a bivalve superfamily that exhibits high species diversity in shallow waters. Many members of this superfamily are associated commensally with burrowing marine invertebrates in benthic sediments. The genus Borniopsis is known only from eastern Asia and exhibits high host diversity (e.g., mantis shrimps, crabs, holothurians, sipunculans and echiurans). A new species, Borniopsis mortoni sp. n., is described from mud flats at the mouth of the Souzu River, southwestern Shikoku Island, Japan. This species has elongate-ovate shells covered by a tan to dark brown periostracum, and lives attached by both its foot and byssal threads to the body surface of the synaptid sea cucumber Patinapta ooplax. Several individuals of Borniopsis mortoni are often found on the same host, but sometimes more than 10 individuals can occur together. Borniopsis mortoni is one of the smallest species in this genus. Probably, its small body size is an adaptation to the mode of life in a narrow host burrow. Until now, only two other Borniopsis species were known to have commensal associations with synaptids. Thus, this is the third example of a synaptid-associated species from this genus. In addition, we briefly review the galeommatoideans commensal with apodid sea cucumbers.

Morphology, Biology, and Phylogenetic Position of the Bivalve Platomysia rugata (Heterodonta: Galeommatoidea), a Commensal with the Sipunculan Worm Sipunculus nudus.

The bivalve superfamily Galeommatoidea is characterized by its symbiotic associations with other marine invertebrates. However, for many galeommatoideans, the host species remains unknown. Platomysia (Galeommatoidea) is a monotypic genus including a single species P. rugata, which is distinguished from other galeommatoideans in having distinct and evenly spaced commarginal ribs on its shell surface. This species was described based on a single right valve shell collected in Nanao Bay, Japan Sea, by Habe in 1951 and has been known only from Japanese waters. However, the biology of living animals has never been reported. We found that this species lives in the burrows of the sipunculan worm Sipunculus nudus in mud flats in the Seto Inland Sea, Japan. We investigated its host association and described its shell morphology and anatomy. In addition, we performed a phylogenetic analysis using two nuclear (18S and 28S ribosomal RNA) genes to determine its phylogenetic position in Galeommatoidea. The result suggests that this species belongs to the clade of commensal bivalves together with Pseudopythina, Byssobornia, and Pergrinamor. Platomysia rugata and other two groups of sipunculan-associated galeommatoideans were not monophyletic, suggesting that association with sipunculans occurred at least three times in the galeommatoid evolution.

Symbiotic Association of the Bivalve Tellimya fujitaniana (Galeommatoidea) with the Heart Urchin Echinocardium cordatum (Spatangoida) in the Northwestern Pacific.

The bivalve Tellimya fujitaniana ( Yokoyama, 1927 ) (Galeommatoidea, Heterodonta) was described based on a fossil shell. Until now, the biology of living animals has not been reported. In this study, we found T. fujitaniana in a commensal relationship with the heart urchin Echinocardium cordatum (Pennant, 1777) (Spatangoida, Echinoidea) on the intertidal mud flats of the Seto Inland Sea, Japan. We investigated the morphology, host associations, and reproductive biology of this bivalve species. The elongate-ovate shell is covered by a reddish-brown ferruginous deposit. The mantle is exposed anteriorly to form a temporal siphon, while posteriorly one pair of short tentacles is exposed. Small individuals (shell length, SL, ≤ 2.1 mm) were attached to the host's body surface; middle-sized individuals (SL 3.0-3.2 mm) were attached to or stayed close to larger T. fujitanianathat were living freely in the host burrow. Nearly all the large individuals (SL ≥ 4.8 mm) lived freely in the host burrow, behind the urchin. This suggests that the host utilization pattern of T. fujitanianachanges with development. Specimens with SL ≥ 4.8 mm had mature gonads, mostly occupied by ova, and some individuals were brooding eggs or veliger larvae in the gills. This species was previously assigned to Fronsella. However, the morphology and ecology of this bivalve are very similar to those of Tellimya ferruginosa (the type species of the genus Tellimya) in the northeastern Atlantic Ocean. Thus, we have reassigned this species to genus Tellimya. We also confirmed that T. fujitaniana and T. ferruginosa can be genetically distinguished using the mitochondrial COI gene.

A comprehensive molecular phylogeny of spoon worms (Echiura, Annelida): Implications for morphological evolution, the origin of dwarf males, and habitat shifts.

Echiurans (spoon worms) are derived annelids that have secondarily lost segmentation. Recently, two molecular phylogenetic studies were performed to resolve the interfamily relationship of echiurans. However, the tree topologies were incongruent and taxon sampling was limited in both the studies. Thus, the phylogenetic relationships within echiurans remain contentious. In this study, I reevaluated the molecular phylogeny of echiurans, using three nuclear (18S, 28S, and H3) and two mitochondrial (16S and COI) genes of 49 echiuran species belonging to 17-19 genera and five families. Results showed that echiurans form the following two major clades: a sexually monomorphic group (Echiuridae, Urechidae, and Thalassematidae) and a sexually dimorphic group (Bonelliidae and Ikedidae). The sister group relationships between Urechidae and Echiuridae, as well as between Ikedidae and Bonelliidae, were supported. The analysis also supported the following relationships among genera within Thalassematidae: {Arhynchite [(Thalassema, Lissomyema) (Ochetostoma, Listriolobus, Ikedosoma, Anelassorhynchus)]}. Furthermore, I evaluated the evolutionary patterns of important taxonomic characteristics (body-wall longitudinal musculature, proboscis shape, gonostmal lip shape, and body color) and habitat shifts (water depth and substrate type), using ancestral state reconstruction analyses. The analyses showed that sexually dimorphic echiurans originated in the shallow waters and secondarily invaded the deep sea, although deep-to-shallow habitat reversal was also detected. In contrary to the previous hypothesis, sexual dimorphism with dwarf males in echiurans may have been a preadaptation to the deep-sea environment. The analyses also showed that habitat shifts from soft sediments to hard substrates occurred in Thalassematidae and Bonelliidae, respectively. A new classification of echiurans, in which Echiura comprises two superfamilies, namely Echiuroidea (with Echiuridae, Urechidae, and Thalassematidae) and Bonellioidea (with Bonelliidae and Ikedidae), is proposed.

Active pollination favours sexual dimorphism in floral scent.

Zoophilous flowers often transmit olfactory signals to attract pollinators. In plants with unisexual flowers, such signals are usually similar between the sexes because attraction of the same animal to both male and female flowers is essential for conspecific pollen transfer. Here, we present a remarkable example of sexual dimorphism in floral signal observed in reproductively highly specialized clades of the tribe Phyllantheae (Phyllanthaceae). These plants are pollinated by species-specific, seed-parasitic Epicephala moths (Gracillariidae) that actively collect pollen from male flowers and pollinate the female flowers in which they oviposit; by doing so, they ensure seeds for their offspring. We found that Epicephala-pollinated Phyllanthaceae plants consistently exhibit major qualitative differences in scent between male and female flowers, often involving compounds derived from different biosynthetic pathways. In a choice test, mated female Epicephala moths preferred the scent of male flowers over that of female flowers, suggesting that male floral scent elicits pollen-collecting behaviour. Epicephala pollination evolved multiple times in Phyllantheae, at least thrice accompanied by transition from sexual monomorphism to dimorphism in floral scent. This is the first example in which sexually dimorphic floral scent has evolved to signal an alternative reward provided by each sex, provoking the pollinator's legitimate altruistic behaviour.

Molecular phylogeny of echiuran worms (Phylum: Annelida) reveals evolutionary pattern of feeding mode and sexual dimorphism.

The Echiura, or spoon worms, are a group of marine worms, most of which live in burrows in soft sediments. This annelid-like animal group was once considered as a separate phylum because of the absence of segmentation, although recent molecular analyses have placed it within the annelids. In this study, we elucidate the interfamily relationships of echiuran worms and their evolutionary pattern of feeding mode and sexual dimorphism, by performing molecular phylogenetic analyses using four genes (18S, 28S, H3, and COI) of representatives of all extant echiuran families. Our results suggest that Echiura is monophyletic and comprises two unexpected groups: [Echiuridae+Urechidae+Thalassematidae] and [Bonelliidae+Ikedidae]. This grouping agrees with the presence/absence of marked sexual dimorphism involving dwarf males and the paired/non-paired configuration of the gonoducts (genital sacs). Furthermore, the data supports the sister group relationship of Echiuridae and Urechidae. These two families share the character of having anal chaetae rings around the posterior trunk as a synapomorphy. The analyses also suggest that deposit feeding is a basal feeding mode in echiurans and that filter feeding originated once in the common ancestor of Urechidae. Overall, our results contradict the currently accepted order-level classification, especially in that Echiuroinea is polyphyletic, and provide novel insights into the evolution of echiuran worms.

Molecular phylogeny of the bivalve superfamily Galeommatoidea (Heterodonta, Veneroida) reveals dynamic evolution of symbiotic lifestyle and interphylum host switching.

BACKGROUND: Galeommatoidea is a superfamily of bivalves that exhibits remarkably diverse lifestyles. Many members of this group live attached to the body surface or inside the burrows of other marine invertebrates, including crustaceans, holothurians, echinoids, cnidarians, sipunculans and echiurans. These symbiotic species exhibit high host specificity, commensal interactions with hosts, and extreme morphological and behavioral adaptations to symbiotic life. Host specialization to various animal groups has likely played an important role in the evolution and diversification of this bivalve group. However, the evolutionary pathway that led to their ecological diversity is not well understood, in part because of their reduced and/or highly modified morphologies that have confounded traditional taxonomy. This study elucidates the taxonomy of the Galeommatoidea and their evolutionary history of symbiotic lifestyle based on a molecular phylogenic analysis of 33 galeommatoidean and five putative galeommatoidean species belonging to 27 genera and three families using two nuclear ribosomal genes (18S and 28S ribosomal DNA) and a nuclear (histone H3) and mitochondrial (cytochrome oxidase subunit I) protein-coding genes. RESULTS: Molecular phylogeny recovered six well-supported major clades within Galeommatoidea. Symbiotic species were found in all major clades, whereas free-living species were grouped into two major clades. Species symbiotic with crustaceans, holothurians, sipunculans, and echiurans were each found in multiple major clades, suggesting that host specialization to these animal groups occurred repeatedly in Galeommatoidea. CONCLUSIONS: Our results suggest that the evolutionary history of host association in Galeommatoidea has been remarkably dynamic, involving frequent host switches between different animal phyla. Such an unusual pattern of dynamic host switching is considered to have resulted from their commensalistic lifestyle, in which they maintain filter-feeding habits even in symbiotic habitats. The results of the molecular phylogenetic analysis did not correspond with the current taxonomic circumscription. Galeommatidae and Lasaeidae were polyphyletic, and Basterotia, which is traditionally assigned to Cyamioidea, formed a monophyletic clade within Galeommatoidea.

Morphological and ecological adaptation of Basterotia bivalves (Galeommatoidea: Sportellidae) to symbiotic association with burrowing echiuran worms.

The burrows created by benthos in tidal flats provide various habitats to other organisms. Echiuran burrows are unique among these in being persistently disturbed by the host's undulating activity, but little is known on how symbionts adapt to such a unique habitat. We report here the morphological and ecological adaptation by two bivalve species of Basterotia (Sportellidae), including one new species, which are commensals with burrowing echiuran worms. The burrows of Ikedosoma gogoshimense were inhabited by Basterotia gouldi at intertidal gravelly mud flats in the central Seto Inland Sea, whereas those of Ochetostoma erythrogrammon were inhabited by Basterotia carinata n. sp. at an intertidal gravelly coral-sand flat at Amami-Ohshima Island. Both bivalve species were found embedded in the burrow wall with their posterior inhalant and exhalant apertures gaping to the burrow lumen, suggesting that they utilize the water currents created by host echiurans. The posteriorly robust, laterally inflated shell with developed carina is considered an adaptation to symbiotic life, as it is exposed to pressure caused by the host's persistent undulating activity. Females of Basterotia bivalves were larger than males, suggesting size-dependent sex change, and possessed brooded veligers in the ctenidium. Our findings suggest that species-specific intimate association with echiurans may be widespread among the Sportellidae bivalves, whose biology remains poorly understood.