Lyrodus Pedicellatus Research Articles

Closing the genome of Teredinibacter turnerae T7902 by long-read nanopore sequencing.

We present the complete closed circular genome sequence derived from Oxford Nanopore sequencing of the shipworm endosymbiont Teredinibacter turnerae T7902 (DSM 15152, ATCC 39867), originally isolated from the shipworm Lyrodus pedicellatus (1). This sequence will aid in the comparative genomics of shipworm endosymbionts and the understanding of host-symbiont evolution.

Occurrence and Distribution of Shipworms (Mollusca: Bivalvia: Teredinidae) and Two New Records of <i>Bankia</i> species along the Coast of Hokkaido, Japan

Shipworms of the family Teredinidae were collected from driftwood along the coastal areas of Hokkaido between October 2020 and November 2021. Identification of species recorded a total of five species pervaded wooden debris: Teredo navalis Linnaeus, 1758, Lyrodus pedicellatus (Quatrefages, 1849), Bankia setacea (Tryon, 1863), Bankia bipennata (Turton, 1819), and Bankia carinata (Gray, 1827), the latter two were warm-water elements and considered a new record of species discovered from the temperate zone of Hokkaido waters. This suggests the possibility that ocean currents and suitable environmental conditions caused the settlement of these teredinids in driftwood along the vast coastal stretches of Hokkaido. Also, B. bipennata and B. carinata have been most likely distributed along the Sea of Japan and brought accidentally by typhoons from the Kuroshio region and Tsushima and Tsugaru currents to Hokkaido, respectively. Moreover, biogeographic patterns within Teredinidae from other coastal areas in the country and elsewhere were examined from the available literature.

Characterisation of the enzyme transport path between shipworms and their bacterial symbionts

BackgroundShipworms are marine xylophagus bivalve molluscs, which can live on a diet solely of wood due to their ability to produce plant cell wall-degrading enzymes. Bacterial carbohydrate-active enzymes (CAZymes), synthesised by endosymbionts living in specialised shipworm cells called bacteriocytes and located in the animal’s gills, play an important role in wood digestion in shipworms. However, the main site of lignocellulose digestion within these wood-boring molluscs, which contains both endogenous lignocellulolytic enzymes and prokaryotic enzymes, is the caecum, and the mechanism by which bacterial enzymes reach the distant caecum lumen has remained so far mysterious. Here, we provide a characterisation of the path through which bacterial CAZymes produced in the gills of the shipworm Lyrodus pedicellatus reach the distant caecum to contribute to the digestion of wood.ResultsThrough a combination of transcriptomics, proteomics, X-ray microtomography, electron microscopy studies and in vitro biochemical characterisation, we show that wood-digesting enzymes produced by symbiotic bacteria are localised not only in the gills, but also in the lumen of the food groove, a stream of mucus secreted by gill cells that carries food particles trapped by filter feeding to the mouth. Bacterial CAZymes are also present in the crystalline style and in the caecum of their shipworm host, suggesting a unique pathway by which enzymes involved in a symbiotic interaction are transported to their site of action. Finally, we characterise in vitro four new bacterial glycosyl hydrolases and a lytic polysaccharide monooxygenase identified in our transcriptomic and proteomic analyses as some of the major bacterial enzymes involved in this unusual biological system.ConclusionBased on our data, we propose that bacteria and their enzymes are transported from the gills along the food groove to the shipworm’s mouth and digestive tract, where they aid in wood digestion.

Out of taxonomic crypsis: A new trans-arctic cryptic species pair corroborated by phylogenetics and molecular evidence

Cryptic species are a common phenomenon in cosmopolitan marine species. The use of molecular tools has often uncovered cryptic species occupying a fraction of the geographic range of the original morphospecies. Shipworms (Teredinidae) are marine bivalves, living in drift and fixed wood, many of which have a conserved morphology across cosmopolitan distributions. Herein novel and GenBank mitochondrial (cytochrome c oxidase subunit I) and nuclear (18S rRNA) DNA sequences are employed to produce a phylogeny of the Teredinidae and delimit a cryptic species pair in the Psiloteredo megotara complex. The anatomy, biogeography, and ecology of P. megotara, Psiloteredo sp. and Nototeredo edax are compared based on private and historic museum collections and a thorough literature review. Morphological and anatomical characters of P. megotara from the North Atlantic and Psiloteredo sp. from Japan were morphologically indistinguishable, and differ in pallet architecture and soft tissue anatomy from N. edax. The two Psiloteredo species were then delimited as genetically distinct species using four molecular-based methods. Consequently, the Northwest Pacific species, Psiloteredo pentagonalis, first synonymized with N. edax and then with P. megotara, is resurrected. Nototeredo edax, P. megotara and P. pentagonalis are redescribed based upon morphological and molecular characters. Phylogenetic analysis further revealed cryptic species complexes within the cosmopolitan species Bankia carinata and possibly additional cryptic lineages within the cosmopolitan Lyrodus pedicellatus.

Discovery, activity and characterisation of an AA10 lytic polysaccharide oxygenase from the shipworm symbiont Teredinibacter turnerae

BackgroundThe quest for novel enzymes for cellulosic biomass-degradation has recently been focussed on lytic polysaccharide monooxygenases (LPMOs/PMOs), Cu-containing proteins that catalyse the oxidative degradation of otherwise recalcitrant polysaccharides using O2 or H2O2 as a co-substrate.ResultsAlthough classical saprotrophic fungi and bacteria have been a rich source of lytic polysaccharide monooxygenases (LPMOs), we were interested to see if LPMOs from less evident bio-environments could be discovered and assessed for their cellulolytic activity in a biofuel context. In this regard, the marine shipworm Lyrodus pedicellatus represents an interesting source of new enzymes, since it must digest wood particles ingested during its natural tunnel boring behaviour and plays host to a symbiotic bacterium, Teredinibacter turnerae, the genome of which has revealed a multitude of enzymes dedicated to biomass deconstruction. Here, we show that T. turnerae encodes a cellulose-active AA10 LPMO. The 3D structure, at 1.4 Å resolution, along with its EPR spectrum is distinct from other AA10 polysaccharide monooxygenases insofar as it displays a “histidine-brace” catalytic apparatus with changes to the surrounding coordination sphere of the copper. Furthermore, TtAA10A possesses a second, surface accessible, Cu site 14 Å from the classical catalytic centre. Activity measurements show that the LPMO oxidises cellulose and thereby significantly augments the rate of degradation of cellulosic biomass by classical glycoside hydrolases.ConclusionShipworms are wood-boring marine molluscs that can live on a diet of lignocellulose. Bacterial symbionts of shipworms provide many of the enzymes needed for wood digestion. The shipworm symbiont T. turnerae produces one of the few LPMOs yet described from the marine environment, notably adding to the capability of shipworms to digest recalcitrant polysaccharides.

Lyrodus mersinensis sp. nov. (Bivalvia: Teredinidae) another cryptic species in the Lyrodus pedicellatus (Quatrefages, 1849) complex.

New data from barcode index numbers (BINs) and 28S rRNA gene sequences confirm a cryptic species pair in Lyrodus pedicellatus from the eastern Mediterranean and European Atlantic coasts. Therefore, it is paramount to associate the new species to a scientific name for a reliable reference system of biological information. To this end, we describe Lyrodus mersinensis sp. nov., another cryptic species in the L. pedicellatus complex, and redescribe the `true´ L. pedicellatus. Both the description and redescription are based on molecular diagnostic characters obtained from sequences of the mitochondrial cytochrome c oxidase subunit I (COI) and nuclear 28S rRNA genes. The 18S rRNA gene sequences did not yield diagnostic characters to distinguish these species. A morphological diagnosis of pedicellatus-like Lyrodus species is also provided.

A preliminary study on the growth rate of Lyrodus pedicellatus (Bivalvia: Teredinidae) within the first 5 months after settlement

Very little is known about the growth rates of Lyrodus pedicellatus, or their behaviour within English waters, and due to difficulties with morphological identification it is highly likely that its presence has often been confused with Teredo navalis. Over 5 months the growth rate of L. pedicellatus was monitored within a laboratory environment, where tanks were kept consistent with the temperature along the south coast of England. Each month, test panels were X-rayed and the tunnel length and volume was recorded using CAD. During these months, L .pedicellatus achieved an average tunnel length growth rate of 7.14mm/month−1, furthermore the tunnel volume growth varied across all individuals but L. pedicellatus was able to increase volume growth rate on a month by month basis. Results produced low numbers of juveniles growths, and growth rates appeared slow in comparison to other Teredinids, additionally, it appears that for a large portion of the year, the temperatures along the south coast of England are very close to L. pedicellatus’ minimum reproductive parameters, thus inhibiting their full reproductive potential. Nonetheless, their life history strategy and presence along the English coastline is worrying for many maritime structures and consequently, L. pedicellatus should be regarded as an invasive species whose abundance and behaviour needs to be closely monitored.

Uncovering the molecular mechanisms of lignocellulose digestion in shipworms

Lignocellulose forms the structural framework of woody plant biomass and represents the most abundant carbon source in the biosphere. Turnover of woody biomass is a critical component of the global carbon cycle, and the enzymes involved are of increasing industrial importance as industry moves away from fossil fuels to renewable carbon resources. Shipworms are marine bivalve molluscs that digest wood and play a key role in global carbon cycling by processing plant biomass in the oceans. Previous studies suggest that wood digestion in shipworms is dominated by enzymes produced by endosymbiotic bacteria found in the animal’s gills, while little is known about the identity and function of endogenous enzymes produced by shipworms. Using a combination of meta-transcriptomic, proteomic, imaging and biochemical analyses, we reveal a complex digestive system dominated by uncharacterized enzymes that are secreted by a specialized digestive gland and that accumulate in the cecum, where wood digestion occurs. Using a combination of transcriptomics, proteomics, and microscopy, we show that the digestive proteome of the shipworm Lyrodus pedicellatus is mostly composed of enzymes produced by the animal itself, with a small but significant contribution from symbiotic bacteria. The digestive proteome is dominated by a novel 300 kDa multi-domain glycoside hydrolase that functions in the hydrolysis of β-1,4-glucans, the most abundant polymers in wood. These studies allow an unprecedented level of insight into an unusual and ecologically important process for wood recycling in the marine environment, and open up new biotechnological opportunities in the mobilization of sugars from lignocellulosic biomass.

Developmental dynamics of myogenesis in the shipworm Lyrodus pedicellatus (Mollusca: Bivalvia).

BackgroundThe shipworm Lyrodus pedicellatus is a wood-boring bivalve with an unusual vermiform body. Although its larvae are brooded, they retain the general appearance of a typical bivalve veliger-type larva. Here, we describe myogenesis of L. pedicellatus revealed by filamentous actin labelling and discuss the data in a comparative framework in order to test for homologous structures that might be part of the bivalve (larval) muscular ground pattern.ResultsFive major muscle systems were identified: a velum retractor, foot retractor, larval retractor, a distinct mantle musculature and an adductor system. For a short period of larval life, an additional ventral larval retractor is present. Early in development, a velum muscle ring and an oral velum musculature emerge. In late stages the lateral and dorsal mantle musculature, paired finger-shaped muscles, an accessory adductor and a pedal plexus are formed. Similar to other bivalve larvae, L. pedicellatus exhibits three velum retractor muscles, but in contrast to other species, one of them disappears in early stages of L. pedicellatus. The remaining two velum retractors are considerably remodelled during late larval development and are most likely incorporated into the elaborate mantle musculature of the adult.ConclusionsTo our knowledge, this is the first account of any larval retractor system that might contribute to the adult bodyplan of a (conchiferan) mollusk. A comparative analysis shows that a pedal plexus, adductors, a larval velum ring, velum retractors and a ventral larval retractor are commonly found among bivalve larvae, and thus most likely belong to the ground pattern of the bivalve larval musculature.Electronic supplementary materialThe online version of this article (doi:10.1186/s12983-014-0090-9) contains supplementary material, which is available to authorized users.

Biodegradation of wood exposed in the marine environment: Evaluation of the hazard posed by marine wood-borers in fifteen European sites

The activity of marine wood-borers causes great destruction in maritime wooden structures. Therefore, the aim of this study was to evaluate the hazard posed by marine wood-borers in fifteen European sites, to assist authorities and researchers concerned with the protection of wood in the sea. In northern Europe, Teredo navalis is the species that poses the highest borer hazard while in the Atlantic coast of southern Europe Lyrodus pedicellatus is the most destructive species, with the exception of two sites in Portugal. In these sites, Limnoria tripunctata was more destructive than L. pedicellatus. In the Mediterranean both T. navalis and L. pedicellatus pose a very high borer hazard to wooden structures.Salinity and temperature emerged as the environmental conditions that best explain the occurrence and abundance of wood boring species in the sites surveyed. Three of the species highlighted in this study are warm water species. Therefore their activity might increase in the future, due to global warming. Considering that wood is still a very valuable material for construction, its use for maritime construction should be favoured. Thus research to improve the durability of wooden materials in the marine environment is of paramount importance.

Developmental dynamics of myogenesis in the shipworm Lyrodus pedicellatus (Mollusca: Bivalvia)

BackgroundThe shipworm Lyrodus pedicellatus is a wood-boring bivalve with an unusual vermiform body. Although its larvae are brooded, they retain the general appearance of a typical bivalve veliger-type larva. Here, we describe myogenesis of L. pedicellatus revealed by filamentous actin labelling and discuss the data in a comparative framework in order to test for homologous structures that might be part of the bivalve (larval) muscular ground pattern.ResultsFive major muscle systems were identified: a velum retractor, foot retractor, larval retractor, a distinct mantle musculature and an adductor system. For a short period of larval life, an additional ventral larval retractor is present. Early in development, a velum muscle ring and an oral velum musculature emerge. In late stages the lateral and dorsal mantle musculature, paired finger-shaped muscles, an accessory adductor and a pedal plexus are formed. Similar to other bivalve larvae, L. pedicellatus exhibits three velum retractor muscles, but in contrast to other species, one of them disappears in early stages of L. pedicellatus. The remaining two velum retractors are considerably remodelled during late larval development and are most likely incorporated into the elaborate mantle musculature of the adult.ConclusionsTo our knowledge, this is the first account of any larval retractor system that might contribute to the adult bodyplan of a (conchiferan) mollusk. A comparative analysis shows that a pedal plexus, adductors, a larval velum ring, velum retractors and a ventral larval retractor are commonly found among bivalve larvae, and thus most likely belong to the ground pattern of the bivalve larval musculature.

Diversity, environmental requirements, and biogeography of bivalve wood-borers (Teredinidae) in European coastal waters.

BackgroundBivalve teredinids inflict great destruction to wooden maritime structures. Yet no comprehensive study was ever carried out on these organisms in European coastal waters. Thus, the aims of this study were to: investigate the diversity of teredinids in European coastal waters; map their past and recent distributions to detect range expansion or contraction; determine salinity-temperature (S-T) requirements of species; flag, for future monitoring, the species that pose the greatest hazard for wooden structures.ResultsA total of nine teredinid species were found established in European coastal waters. Seven were considered cryptogenic, of unknown origin, and two were considered alien species. Teredo navalis and Nototeredo norvagica were the species with the widest distribution in European waters. Recently, T. navalis has been reported occurring further east in the Baltic Sea but it was not found at a number of sites on the Atlantic coast of southern Europe. The Atlantic lineage of Lyrodus pedicellatus was the dominant teredinid in the southern Atlantic coast of Europe. In the Mediterranean six teredinid species occurred in sympatry, whereas only three of these occurred in the Black Sea. The species that pose the greatest hazard to wooden maritime structures in European coastal areas are T. navalis and the two lineages of L. pedicellatus.ConclusionsCombined data from field surveys and from the literature made it possible to determine the diversity of established teredinid species and their past and recent distribution in Europe. The environmental requirements of species, determined using climatic envelopes, produced valuable information that assisted on the explanation of species distribution. In addition, the observed trends of species range extension or contraction in Teredo navalis and in the two lineages of Lyrodus pedicellatus seem to emphasise the importance of temperature and salinity as determinants of the distribution of teredinids, whereas their life history strategy seems to play an important role on competition.Teredo navalis and pedicellatus-like Lyrodus species should be monitored due to their destructive capability. The two alien species may expand further their distribution range in Europe, becoming invasive, and should also be monitored.

Microbial Distribution and Abundance in the Digestive System of Five Shipworm Species (Bivalvia: Teredinidae)

Marine bivalves of the family Teredinidae (shipworms) are voracious consumers of wood in marine environments. In several shipworm species, dense communities of intracellular bacterial endosymbionts have been observed within specialized cells (bacteriocytes) of the gills (ctenidia). These bacteria are proposed to contribute to digestion of wood by the host. While the microbes of shipworm gills have been studied extensively in several species, the abundance and distribution of microbes in the digestive system have not been adequately addressed. Here we use Fluorescence In-Situ Hybridization (FISH) and laser scanning confocal microscopy with 16S rRNA directed oligonucleotide probes targeting all domains, domains Bacteria and Archaea, and other taxonomic groups to examine the digestive microbiota of 17 specimens from 5 shipworm species (Bankia setacea, Lyrodus pedicellatus, Lyrodus massa, Lyrodus sp. and Teredo aff. triangularis). These data reveal that the caecum, a large sac-like appendage of the stomach that typically contains large quantities of wood particles and is considered the primary site of wood digestion, harbors only very sparse microbial populations. However, a significant number of bacterial cells were observed in fecal pellets within the intestines. These results suggest that due to low abundance, bacteria in the caecum may contribute little to lignocellulose degradation. In contrast, the comparatively high population density of bacteria in the intestine suggests a possible role for intestinal bacteria in the degradation of lignocellulose.

Investigating the taxonomy and systematics of marine wood borers (Bivalvia : Teredinidae) combining evidence from morphology, DNA barcodes and nuclear locus sequences

Marine wood-boring teredinids, some of the most destructive wood borers in the sea, are a particularly difficult group to identify from morphological features. While in most bivalve species shell features are used as diagnostic characters, in the teredinids shell morphology shows high intraspecific variation and thus identification is based almost entirely on the morphology of the pallets. In the present study we aimed at improving ‘taxonomic resolution’ in teredinids by combining morphological evidence with mitochondrial and nuclear DNA sequences, respectively Cytochromec oxidase subunitI and small subunit rRNA 18S gene, to generate more rigorous and accessible identifications. DNA barcodes of Atlantic and Mediterranean populations of Lyrodus pedicellatus diverged by ~20%, suggesting cryptic species in the morphospecies L. pedicellatus. The low intraspecific divergence found in barcodes of specimens of Nototeredo norvagica (0.78%) confirms that Atlantic and Mediterranean forms of N. norvagica, the latter sometimes reported as Teredo utriculus, are the same species. Teredothyra dominicensis was found for the first time in the Mediterranean. A match was obtained between our 18S sequences and sequences of T. dominicensis from Netherlands Antilles, confirming that T. dominicensis in the Mediterranean is the same species that occurs in the Caribbean. There were differences in 18S sequences between Bankia carinata from the Mediterranean and Caribbean, which may indicate cryptic species.

Changes in the wood boring community in the Tagus Estuary: a case study

In recent years an increased shift in the geographical range of species has been documented, which coincides with global warming. On the Portuguese coast the raising of sea-surface temperatures and salinity has also been affecting the intertidal species range. The aim of the present study was to assess changes in the wood boring community present in the Tagus Estuary by comparing data from present surveys with historical records from the 1960s and 1970s. A shift in the activity of wood boring species was observed in the Tagus Estuary. In recent surveys, the main agents of wood destruction in the Tagus were Limnoria tripunctata and Limnoria quadripunctata. In the 1960s and 1970s only L. tripunctata was reported as occurring in the Tagus estuary but it was not considered at threat to wooden structures exposed in the sea. The present predominance of limnoriids in the Tagus might be related to several factors but possibly the most important was the increase in salinity in the area, as limnoriids appear to be restricted to waters with salinities closed to that of seawater. The teredinid species also changed in the area. In the 1960s and 1970s Teredo navalis was reported as the only teredinid species occurring in the Tagus. In recent surveys however, this species was not found, instead two other teredinid species were present Lyrodus pedicellatus and Nototeredo norvagica . The absence of T. navalis in recent surveys might be related to competition with L. pedicellatus.

Fouling and boring organisms deteriorating various European and tropical woods at Turkish coastline

This study aims to investigate the diversity of fouling and boring organisms damaging wood material at Turkish coasts. Trials were carried out at six harbour sites throughout the seas surrounding Turkey. Various Euopean and tropical wood samples were hanged down at a depth of six meters in the sea for a period of one year. Identification of the organisms obtained from wood panels revealed the presence of five wood borer and 26 fouling species. A°skenderun harbour had the highest boring organism diversity (five species) and it was followed by Trabzon and Finike harbours (three species) and Bandirma, Eregli and Alacati harbours (two species). The two molluscan boring species, Teredo navalis and Lyrodus pedicellatus were observed at all harbour sites, but Nototeredo norvegica was at Trabzon and A°skenderun harbours only. Bankia carinata was obtained only at A°skenderun harbour and the crustacean wood borer Limnoria tripunctata was found at Finike and A°skenderun harbours. All native tree species, except for the olive, were significantly damaged by fouling and boring organisms. Key words: Wood destroyer organism, mollusca, crustacea, Turkey coasts.

Incidence and severity of marine borer attack at different depths at Mtongwe Jetty Pontoon Mombasa, Kenya

AbstractThe activity of marine borers was investigated at Mtongwe Jetty Pontoon, Mombasa, Kilindini harbour using untreated panels of Pinus patula (Schlecht) submerged to different depths. Twenty test panels replicated over 7 months were each strung in three ladder‐like frames using nylon twine and suspended vertically, in such a way that the top most panels were 25 cm below the water surface at low and high tide. Monthly observations were made between January and July for the vertical incidence and extent of marine borer attack following the technique of Bobat (1995). Marine borers were collected from test panels, identified and counted. In addition, the percentage weight loss for panels at different depths was determined at the end of 7 months. Teredo fulleri (Clapp) was dominant on panels near water surface while Martesia striata (Linne) and Lyrodus pedicellatus (Quatrefages) were predominant at the bottom. The results showed that attack began within the first month of exposure and was severest by the seventh month. The vertical incidence and extent of attack decreased with increasing depth of panel exposure and was negatively correlated with depth. This is attributed to changes in hydrographic conditions.

Performance of Different Wood Protection Methods in Tropical Marine Waters, Krishnapatnam Harbour, India

The performance of different wood protection methods viz., diffusion, double diffusion, sap displacement, boucherie and pressure process was assessed in tropical marine waters at Krishnapatnam harbour on the east coast of India. Among non-pressure processes, the diffusion process was the most effective against wood borers and results were similar to the pressure process, while other treatment methods were not effective in marine waters. Control logs were severely damaged within one year, while treated ones showed enhancement in the durability compared to controls by a factor of two to three times. Wood borer attack and fouling settlement on logs was recorded at regular intervals. In all, 9 species of borer's viz. Martesia striata, M. nairi, Teredo furcifera, T. fulleri, T. parksi, Lyrodus pedicellatus, Bankia campanellata, B. carinata and B. rochi were identified. Among these, M. striata, T. furcifera, L. pedicellatus and B. campanellata played an important role in damage of wooden logs. In the case of fouling organisms, barnacles, oysters and bryozoans were dominant groups, while other groups settled sporadically.

Seasonal Settlement of Wood Borers in Krishnapatnam Harbour, Andhra Pradesh, East Coast of India

Observations on seasonal settlement patterns and the abundance of wood borers on test panels were conducted at Krishnapatnam harbour on the East Coast of India. Eighteen species of borers were identified on test panels which include Martesia striata, M. nairi among pholadids; Bactranophorus thoracites, Dicyathifer manni, Nausitora hedleyi, Teredothyra smithi, Teredo furcifera, T. parksi, T. clappi, T. bartschi, T. fulleri, Lyrodus pedicellatus, L. affinis, L. massa, L. bipartitus, Bankia campanellata, B. rochi and B. carinata among the teredinids. Among these, M. striata, T. furcifera, T. fulleri, L. pedicellatus and B. carinata settlement was recorded almost throughout the year. Teredo parksi, T. bartschi, T. smithi, T. clappi, L. bipartitus, L. affinis, N. hedleyi, B. thoracites, M. nairi, B. rochi and B. campanellata settlement was restricted to certain months of the year. Dicyathifer manni and L. massa settled throughout the year, but discontinuity in recruitment was noticed on test panels. Among these, M. striata, M. nairi, T. furcifera, T. fulleri, L. pedicellatus and B. campanellata were predominant species which played an important role in the damage of test panels, while the intensity of other species was sporadic. Borer abundance was slightly higher during high saline months and shipworms causes more damage to the test panels.

Extensive Variation in Intracellular Symbiont Community Composition among Members of a Single Population of the Wood-Boring Bivalve Lyrodus pedicellatus (Bivalvia: Teredinidae)

Shipworms (wood-boring bivalves of the family Teredinidae) harbor in their gills intracellular bacterial symbionts thought to produce enzymes that enable the host to consume cellulose as its primary carbon source. Recently, it was demonstrated that multiple genetically distinct symbiont populations coexist within one shipworm species, Lyrodus pedicellatus. Here we explore the extent to which symbiont communities vary among individuals of this species by quantitatively examining the diversity, abundance, and pattern of occurrence of symbiont ribotypes (unique 16S rRNA sequence types) among specimens drawn from a single laboratory-reared population. A total of 18 ribotypes were identified in two clone libraries generated from gill tissue of (i) a single specimen and (ii) four pooled specimens. Phylogenetic analysis assigned all of the ribotypes to a unique clade within the gamma subgroup of proteobacteria which contained at least five well-supported internal clades (phylotypes). By competitive quantitative PCR and constant denaturant capillary electrophoresis, we estimated the number and abundance of symbiont phylotypes in gill samples of 13 individual shipworm specimens. Phylotype composition varied greatly; however, in all specimens the numerically dominant symbiont belonged to one of two nearly mutually exclusive phylotypes, each of which was detected with similar frequencies among specimens. A third phylotype, containing the culturable symbiont Teredinibacter turnerae, was identified in nearly all specimens, and two additional phylotypes were observed more sporadically. Such extensive variation in ribotype and phylotype composition among host specimens adds to a growing body of evidence that microbial endosymbiont populations may be both complex and dynamic and suggests that such genetic variation should be evaluated with regard to physiological and ecological differentiation.