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Thyasira flexuosa. Light micrographs and TEM of gill filaments at different times during the sulphide experiment. (A) TEM of a bacteriocyte in a freshly collected specimen. Abundant bacteria (b) are visible, and are held in extracellular pockets delimited by microvilli (mv); c: cytoplasm of bacteriocyte; h: hemocoel. (B) TEM of a bacteriocyte in a specimen maintained for 3 wk in low sulphide conditions. A few symbionts (b) are still present extracellularly, but more are being degraded (db) within the bacteriocyte cytoplasm (c); m: bacteriocyte cell membrane; mv: microvilli. (C) TEM of a bacteriocyte in a specimen maintained for 3 wk in high sulphide conditions. A few symbionts (b) are located extracellularly; c: bacteriocyte cytoplasm; h: hemocoel; mv: microvilli. (D) Light micrograph of a gill filament in a specimen maintained for 2 wk in low sulphide. Very few symbionts are visible in the bacteriocytes (bc). (E) Gill filaments in a specimen maintained for 2 wk in high sulphide; b: bacteria; bc: bacteriocyte; g: granules. (F) Gill filaments in a specimen maintained for 3 wk in low sulphide; bc: bacteriocyte; g: granule; n: nucleus of bacteriocyte. (G) Gill filaments in a specimen maintained for 3 wk in high sulphide; bc: bacteriocyte; n: nucleus
Source publication
Many bivalve species with chemoautotrophic symbionts have mixotrophic diets and some of their nutritional requirements are met by particulate feeding. The symbionts require reduced compounds (such as sulphide) for their autotrophic production. As the concentration of both particulate food and sulphide can vary in their habitat, it has been suggeste...
Contexts in source publication
Context 1
... flexuosa. TEM observation showed bacte- ria in freshly collected specimens (Fig. 3A), as well as in samples fixed during the experiment (Fig. 3B,C). After 3 wk, very few bacteria were visible among the microvilli; several bacteria in various stages of degra- dation were seen within the bacteriocytes (Fig. ...
Context 2
... flexuosa. TEM observation showed bacte- ria in freshly collected specimens (Fig. 3A), as well as in samples fixed during the experiment (Fig. 3B,C). After 3 wk, very few bacteria were visible among the microvilli; several bacteria in various stages of degra- dation were seen within the bacteriocytes (Fig. ...
Context 3
... flexuosa. TEM observation showed bacte- ria in freshly collected specimens (Fig. 3A), as well as in samples fixed during the experiment (Fig. 3B,C). After 3 wk, very few bacteria were visible among the microvilli; several bacteria in various stages of degra- dation were seen within the bacteriocytes (Fig. ...
Context 4
... semi-thin sections showed symbiont density decreasing during the experiment, with very few remaining after 2 and 3 wk (Fig. 3D-G). The percent symbiont coverage of bacteriocytes decreased at a faster rate in the low sulphide treatment (Fig. 4). The difference in percent symbiont coverage between indi- viduals maintained in low and high sulphide treat- ments (with sampling time not considered) was nearly significant (t-test, p = ...
Citations
... First, they harbor bacteria either among the microvilli outside the epithelial cells of gills or, more integrally, in the apical vesicles delimited by the cell membrane and microvilli (hereafter for extracellular symbiosis to distinguish from the previous type of epi-symbiosis) [20]. In addition, thyasirids show a fluctuating dependence upon symbiontderived nutrients [28,29]. Thus, extracellular symbiotic thyasirids might be regarded as intermediate between endosymbiotic and asymbiotic bivalves and may exhibit distinct evolutionary adaptations. ...
... Therefore, the highly expressed gene family of these transporters in C. bisecta could emphasize the nutritional function of its gill tissues. Moreover, gene families coding for enzymes related to the filter-feeding lifestyle in mollusks, such as the glycosyl hydrolase and chymotrypsins, are found either contracted or missing in C. bisecta, suggesting that the clam had relied on symbiosis for nutrition for a long time (Additional file 1: Supplementary note 5, Additional file 2: Table S12) [22,28,[63][64][65]. ...
... In conclusion, the massive expansion in phagocytosis is in line with the theory that thyasirids periodically engulf and digest symbionts [28,76]. In addition, the gene families related to phagocytosis divergently evolved among different bivalves. ...
Background
Bivalves have independently evolved a variety of symbiotic relationships with chemosynthetic bacteria. These relationships range from endo- to extracellular interactions, making them ideal for studies on symbiosis-related evolution. It is still unclear whether there are universal patterns to symbiosis across bivalves. Here, we investigate the hologenome of an extracellular symbiotic thyasirid clam that represents the early stages of symbiosis evolution.
Results
We present a hologenome of Conchocele bisecta (Bivalvia: Thyasiridae) collected from deep-sea hydrothermal vents with extracellular symbionts, along with related ultrastructural evidence and expression data. Based on ultrastructural and sequencing evidence, only one dominant Thioglobaceae bacteria was densely aggregated in the large bacterial chambers of C. bisecta , and the bacterial genome shows nutritional complementarity and immune interactions with the host. Overall, gene family expansions may contribute to the symbiosis-related phenotypic variations in different bivalves. For instance, convergent expansions of gaseous substrate transport families in the endosymbiotic bivalves are absent in C. bisecta . Compared to endosymbiotic relatives, the thyasirid genome exhibits large-scale expansion in phagocytosis, which may facilitate symbiont digestion and account for extracellular symbiotic phenotypes. We also reveal that distinct immune system evolution, including expansion in lipopolysaccharide scavenging and contraction of IAP (inhibitor of apoptosis protein), may contribute to the different manners of bacterial virulence resistance in C. bisecta .
Conclusions
Thus, bivalves employ different pathways to adapt to the long-term co-existence with their bacterial symbionts, further highlighting the contribution of stochastic evolution to the independent gain of a symbiotic lifestyle in the lineage.
... Apart from O. webbi, frenulates overall are quite common in fjords; we recovered S. fiordicum and S. ekmani from Kvalsund and Nordfjord, and these species have been recorded even in very dense aggregations across different fjords [21,23,39,61]. Frenulates are not the only group of chemosymbiotic animals either; thyasirid bivalves are common Arctic benthic macrofauna that can also form symbiotic associations with chemosynthetic bacteria [22,78,79]. ...
... Thyasirids are not obligately chemosymbiotic, and different individuals within single populations can have different degrees of reliance on chemosynthetic partners versus filter feeding [78][79][80][81][82]. In other words, thyasirids can rely both on the direct consumption of organic matter and chemosynthesis for their nutrition. ...
We used ancient DNA (aDNA) extraction methods to sequence museum voucher samples of Oligobrachia webbi, a frenulate siboglinid polychaete described from a northern Norwegian fjord over fifty years ago. Our sequencing results indicate a genetic match with the cryptic seep species, Oligobrachia haakonmosbiensis (99% pairwise identity for 574 bp mtCOI fragments). Due to its similarity with O. webbi, the identity of O. haakonmosbiensis has been a matter of debate since its description, which we have now resolved. Furthermore, our results demonstrate that chemosynthesis-based siboglinids, that constitute the bulk of the biomass at Arctic seeps are not seep specialists. Our data on sediment geochemistry and carbon and nitrogen content reveal reduced conditions in fjords/sounds, similar to those at seep systems. Accumulation and decomposition of both terrestrial and marine organic matter results in the buildup of methane and sulfide that apparently can sustain chemosymbiotic fauna. The occurrence of fjords and by extension, highly reducing habitats, could have led to Arctic chemosymbiotic species being relatively generalist with their habitat, as opposed to being seep or vent specialists. Our stable isotope analyses indicate the incorporation of photosynthetically derived carbon in some individuals, which aligns with experiments conducted on frenulates before the discovery of chemosynthesis that demonstrated their ability to take up organic molecules from the surrounding sediment. Since reduced gases in non-seep environments are ultimately sourced from photosynthetic processes, we suggest that the extreme seasonality of the Arctic has resulted in Arctic chemosymbiotic animals seasonally changing their degree of reliance on chemosynthetic partners. Overall, the role of chemosynthesis in Arctic benthos and marine ecosystems and links to photosynthesis may be complex, and more extensive than currently known.
... Though the frenulates (and moniliferans in the case of HMMV) represent the entirety of the macrofaunal chemosynthesis-based community presently known at Arctic seeps, small thyasirids (maximum 5 mm length) are also highly abundant at Arctic seeps (Åström et al., 2016, 2019). The family of Thyasiridae bivalves includes chemosymbiotrophic species associated with sulfur-oxidizing symbionts (Dufour, 2005;Dufour and Felbeck, 2006;Duperron et al., 2013), and a high abundance of thyasirids at seep sites is suggestive of these bivalves harboring chemosynthetic symbionts. Furthermore, thyasirids are extremely flexible regarding the nature of their symbiotic associations and exhibit a wide range of different dietary adaptations, from microbial syntrophy and chemosymbiosis to mixotrophy and heterotrophy (Dando and Spiro, 1993;Dufour, 2005;Taylor and Glover, 2010;Duperron et al., 2013). ...
Cold-seep benthic communities in the Arctic exist at the nexus of two extreme environments; one reflecting the harsh physical extremes of the Arctic environment and another reflecting the chemical extremes and strong environmental gradients associated with seafloor seepage of methane and toxic sulfide-enriched sediments. Recent ecological investigations of cold seeps at numerous locations on the margins of the Arctic Ocean basin reveal that seabed seepage of reduced gas and fluids strongly influence benthic communities and associated marine ecosystems. These Arctic seep communities are mostly different from both conventional Arctic benthic communities as well as cold-seep systems elsewhere in the world. They are characterized by a lack of large specialized chemo-obligate polychetes and mollusks often seen at non-Arctic seeps, but, nonetheless, have substantially higher benthic abundance and biomass compared to adjacent Arctic areas lacking seeps. Arctic seep communities are dominated by expansive tufts or meadows of siboglinid polychetes, which can reach densities up to >3 × 10⁵ ind.m–2. The enhanced autochthonous chemosynthetic production, combined with reef-like structures from methane-derived authigenic carbonates, provides a rich and complex local habitat that results in aggregations of non-seep specialized fauna from multiple trophic levels, including several commercial species. Cold seeps are far more widespread in the Arctic than thought even a few years ago. They exhibit in situ benthic chemosynthetic production cycles that operate on different spatial and temporal cycles than the sunlight-driven counterpart of photosynthetic production in the ocean’s surface. These systems can act as a spatio-temporal bridge for benthic communities and associated ecosystems that may otherwise suffer from a lack of consistency in food quality from the surface ocean during seasons of low production. As climate change impacts accelerate in Arctic marginal seas, photosynthetic primary production cycles are being modified, including in terms of changes in the timing, magnitude, and quality of photosynthetic carbon, whose delivery to the seabed fuels benthic communities. Furthermore, an increased northward expansion of species is expected as a consequence of warming seas. This may have implications for dispersal and evolution of both chemosymbiotic species as well as for background taxa in the entire realm of the Arctic Ocean basin and fringing seas.
... In contrast to other clams, thyasirids maintain their symbionts among the microvilli of gill epithelial cells, as described in some mussels; such extracellular symbioses have been considered more primitive than intracellular symbioses [6,[21][22][23]. Chemosymbiotic thyasirids are mixotrophs that appear to rely on particulate food to a greater extent when symbiont abundance is low [24], or at times when environmental sulfide concentrations are low [25]. All thyasirid symbionts identified to date are gammaproteobacteria [23,25,26]. ...
... This relationship has been observed over multiple years in three sampling sites, and all specimens of T. cf. gouldi OTUs 1 and 2 whose gills have been examined using thin sectioning and transmission electron microscopy (i.e., over 200 specimens) harboured large populations of visibly homogenous bacterial symbionts [20,24,27,28]. Phylogenetic analysis using 16S rRNA gene sequences have identified three distinct symbiont phylotypes (A -C) hosted by the two symbiotic T. cf. ...
Background
Next-generation sequencing has opened new avenues for studying metabolic capabilities of bacteria that cannot be cultured. Here, we provide a metagenomic description of chemoautotrophic gammaproteobacterial symbionts associated with Thyasira cf. gouldi, a sediment-dwelling bivalve from the family Thyasiridae. Thyasirid symbionts differ from those of other bivalves by being extracellular, and recent work suggests that they are capable of living freely in the environment.
Results
Thyasira cf. gouldi symbionts appear to form mixed, non-clonal populations in the host, show no signs of genomic reduction and contain many genes that would only be useful outside the host, including flagellar and chemotaxis genes. The thyasirid symbionts may be capable of sulfur oxidation via both the sulfur oxidation and reverse dissimilatory sulfate reduction pathways, as observed in other bivalve symbionts. In addition, genes for hydrogen oxidation and dissimilatory nitrate reduction were found, suggesting varied metabolic capabilities under a range of redox conditions. The genes of the tricarboxylic acid cycle are also present, along with membrane bound sugar importer channels, suggesting that the bacteria may be mixotrophic.
Conclusions
In this study, we have generated the first thyasirid symbiont genomic resources. In Thyasira cf. gouldi, symbiont populations appear non-clonal and encode genes for a plethora of metabolic capabilities; future work should examine whether symbiont heterogeneity and metabolic breadth, which have been shown in some intracellular chemosymbionts, are signatures of extracellular chemosymbionts in bivalves.
... Thyasirids have a much wider distribution than other chemosymbiotic bivalve families; they are found from coastal to hadal depths, in different types of sediments, and from both the poles to the equator. They may show a wide variation in their anatomical characters and in the extent of their nutritional reliance upon symbionts (Dufour and Felbeck, 2006). The generic definitions within the Thyasiroidea have been recognized to be problematic (Payne and Allen, 1991;Oliver and Sellanes, 2005). ...
Chemosymbiotic micro- and macro-fauna related to cold-seep sites were recovered in the Palmahim Disturbance (PD), offshore Israel, during EU EUROFLEETS2 SEMSEEP Cruise, by box-coring and Remotely Operated Vehicle (ROV) dives. No live macrofauna was identified in the collected sediments, with the exception of the seep-related crustacean Calliax lobata (de Gaillande and Lagardère, 1966). Numerous Calliax claws testify the past colonization of these soft bottoms by several generations of this ghost shrimp. After sediment sieving on 1 mm, we identified gastropods belonging to the families Trochidae, Eucyclidae, unassigned Seguenzioidea (genus Anekes), Rissoidae, Elachisinidae, Raphitomidae, Mangeliidae, Architectonicidae, Orbitestellidae, and Acteonidae. The identified bivalves belong to the families Nuculidae, Yoldiidae, Mytilidae, Lucinidae, Thyasiridae, Semelidae, Kelliellidae, Vesicomyidae, Xylophagidae, and Cuspidariidae. A seep-related group of chemosymbiotic molluscs was detected, including: Taranis moerchii (Malm, 1861), Lurifax vitreus Warén and Bouchet, 2001, Idas ghisottii Warén and Carrozza, 1990, Lucinoma kazani Salas and Woodside, 2002, Thyasira biplicata (Philippi, 1836), Isorropodon perplexum Sturany, 1896, and the newly described Vesicomyid species Waisiuconcha corsellii n. sp., that represents also the first record of the genus Waisiuconcha in recent Mediterranean sediments.
The ROV dives recorded local patches of several m² of seafloor covered by dead shells of L. kazani, with a density of up to about 200 loose shells per square meter. The potential occurrence of seep-related foraminifera, among low-oxygen tolerant species, was explored by comparison with previously sampled adjacent localities, and lead to the identification of Chilostomella oolina, Globobulimina affinis and G. pseudospinescens as potential foraminiferal seep indicators in the southeastern Mediterranean Sea. The absence of live, seep-related fauna in surface sediments in the PD, where seepage has been confirmed, suggests intermittent activity and a pause or decline of the investigated seeps.
... gouldi gills, as observed for Solemya velum (Scott et al. 2004). Like other symbiotic thyasirids (Dufour and Felbeck 2006), symbiotic T. cf. gouldi are likely mixotrophs, assimilating organic carbon from other sources besides their symbionts, which influences the δ 13 C of gills and other tissues. ...
... gouldi, the exploratory mixing model and fatty acid signature suggested a mixotrophic diet, with bacteria making a more important contribution than SPOM (including phytoplankton). A mixotrophic diet has been suggested for thyasirids, on the basis of δ 13 C values that differed widely between sampling sites and dates (Spiro et al. 1986;Dando and Spiro 1993), and because the abundance of symbionts varies according to SPM availability (Dufour and Felbeck 2006) and temporally (Laurich et al. 2015). We found no evidence of temporal trends in isotope ratios in the symbiotic T. cf. ...
Many bivalves living in symbiosis with chemoautotrophic bacteria are mixotrophs, deriving nutrients both from their symbionts and from external sources. The bivalve family Thyasiridae contains symbiotic and asymbiotic species that live infaunally and presumably suspension-feed. Symbiotic thyasirids use their foot to construct numerous elongate and ramified burrows, or pedal tracts, in surrounding sediments; pedal tracts were interpreted as conduits for accessing reduced sulfur required by gill-associated symbionts. Recently, some asymbiotic thyasirids were found to form similar burrows, and this behaviour, accompanied by the irrigation of pedal tracts, was thought to be associated with deposit feeding. We used stable isotope analyses to examine the food sources of three taxa of co-occurring symbiotic and asymbiotic thyasirids sampled from a fjord in Newfoundland, Canada at different dates and sampling sites. We considered suspended particulate organic matter, sedimentary organic matter, and sulfur-oxidizing bacteria as potential food sources, and found that sediment organic matter characteristics varied significantly according to site but not sampling date, and that suspended organic matter near the seafloor was of relatively low quality and abundance at all dates and sites. The non-gill tissues of symbiotic and asymbiotic thyasirids had similar carbon isotopic compositions, but nitrogen isotope signatures were heavier and less variable in asymbiotic taxa than in the symbiotic Thyasira cf. gouldi. The two asymbiotic thyasirid species studied herein appeared to vary in their feeding strategies, with Parathyasira appearing to mostly farm sulfur-oxidizing bacteria along burrow linings and then collect them using their foot (a mode of bivalve deposit-feeding known as pedal feeding). This work expands our understanding of the breadth of feeding strategies in the widespread and often abundant thyasirid bivalves, and provides support for microbial farming along burrow linings in some thyasirids.
... Previous paleoecological reconstructions suggest that P. platinus lived in oxygen-depleted environments and possibly harbored thioautotrophic symbionts, because specimens of this species are often found in organic and pyrite-rich beds (Sageman and Bina, 1997;Kauffman et al., 2007). With a certain degree of variability, most chemosymbiotic bivalves are mixotrophic organisms and obtain nutrients and carbon from feeding POM and thioautotrophy (Page et al., 1991;Le Pennec et al., 1995;Stewart et al., 2005;Dufour and Felbeck, 2006). The metabolism of thioautotrophic symbionts is based on the energy derived from the oxidation of H 2 S, which is used to sequester ambient CO 2 into organic compounds (Stewart et al., 2005). ...
Platyceramus platinus was a giant inoceramid bivalve that inhabited the outer shelf environments of the Western Interior Seaway (WIS) in North America. With axial heights typically exceeding 1 m, the shells of this species potentially serve as a unique high-resolution geochemical proxy archive for Late Cretaceous paleoclimate. Here we present the first sclerochronological investigation of P. platinus shells to evaluate the usefulness of this species as an archive of short-term (e.g., seasonal to inter-annual) paleoenvironmental variability. We analyzed the growth patterns, the stable oxygen (δ¹⁸O) and carbon (δ¹³C) isotope values of well-preserved P. platinus shell fragments from the Santonian Niobrara Formation at Monument Rocks (Kansas, USA), a National Natural Landmark. A series of diagenetic tests, including cathodoluminescence (CL), scanning electron microscopy (SEM), and geochemical (LA-ICP-MS) analysis, confirmed the good state of preservation of the material. Shell microgrowth patterns suggested lunar daily (circalunidian) growth and that P. platinus grew nearly uninterruptedly throughout the year. Assuming a δ¹⁸Ow value of −3.45 ± 0.26‰, reconstructions based on shell δ¹⁸O data suggest average seasonal temperature variations between 12.5 ± 3.0 and 25.5 ± 1.1 °C and a mean annual temperature of 17.0 ± 4.1 °C for the outer shelf environment of the WIS. Repeated sudden negative δ¹³C shifts of up to 2.00‰ and Mn-rich shell growth bands (Mn/Ca ratios up to 90.21 μmol/mol) suggest that P. platinus filter-fed on suspended organic detritus which sank from the upper water column during episodic events. The availability of large amounts of suspended food, however, slowed shell accretion rates of P. platinus. This shell growth behavior combined with the positive δ¹³C values (0.03–3.96‰) possibly indicate a chemosymbiotic lifestyle that allowed P. platinus to survive under oxygen-depleted conditions at the seafloor.
... Furthermore, chemosymbiosis can be both present and absent within multiple thyasirid lineages, with bacterial symbiosis showing a lack of conservation at both the genus (Southward 1986) and species levels . Symbiotic thyasirids retain the ability to feed heterotrophically, and the relevance of symbiotically derived organic carbon to the host can fluctuate as a function of sulphide availability (Dufour and Felbeck 2006), and time of year (Donval et al. 1989;Dando and Spiro 1993;Laurich et al. 2015). It is not known whether thyasirids may use other strategies besides symbiont endocytosis and lysis to control the size of their symbiont populations. ...
... The cell membrane of the host appears partially ruptured (rm). Cyt: cytoplasm of bacteriocyte specimens (data not shown), supporting previous observations of declining symbiont populations in thyasirids maintained under low sulphide and particulate food conditions (Dufour and Felbeck 2006). Temporal trends in symbiont division rate were inconsistent: although we found a significant interaction between thiosulphate treatment and time, no consistent patterns emerged from our data, possibly due to the lack of thiosulphate replenishment in the T. cf. ...
Stable associations between marine invertebrates and their chemosynthetic bacterial symbionts are predicated on both the adequate transfer of resources and the restriction of bacterial cells to a finite population within host tissues. In symbioses between thyasirid bivalves and thiotrophic bacteria, symbionts are extracellular, acquired from a free-living pool, and periodically endocytosed and digested by host bacteriocytes. Thyasirid symbionts require reduced sulphur to fuel their autotrophic metabolism; the host makes this energy source accessible to symbionts through its burrowing and irrigation behaviours. Here, we demonstrate that unlike the bacterial symbionts of many chemosymbiotic bivalves, those of Thyasira flexuosa and T. cf. gouldi divide while associated with host gill epithelial cells, possibly constituting a second method of symbiont replacement in thyasirid bivalves alongside environmental acquisition. Furthermore, exposure of T. flexuosa and T. cf. gouldi to elevated concentrations of thiosulphate, a reduced sulphur species used by many sulphur-oxidizing bacterial symbionts, results in the rapid onset of bacterial division and expansion of symbiont populations within host gills and can result in host mortality. These results highlight the flexible nature of symbioses in thyasirid bivalves and the possibility for symbiosis breakdown in some thyasirid lineages. Furthermore, results suggest a role for behaviours such as sulphide mining and irrigation in maintaining and controlling a stable population of bacterial symbionts in this family.
... In contrast to other clams, thyasirids maintain their symbionts among the microvilli of gill epithelial cells, as described in some mussels; such extracellular symbioses have been considered more primitive than intracellular symbioses (6,(21)(22)(23). Chemosymbiotic thyasirids are mixotrophs that appear to rely on particulate food to a greater extent when symbiont abundance is low (24), or at times when environmental sulfide concentrations are low (25). All thyasirid symbionts identified to date are gammaproteobacteria (23,25,26). ...
Next-generation sequencing has opened new avenues for studying metabolic capabilities of bacteria that cannot be cultured. Here, we provide a metagenomic description of a chemoautotrophic gammaproteobacterial symbiont associated with Thyasira cf. gouldi, a sediment-dwelling bivalve from the family Thyasiridae. Symbionts of thyasirids differ from those of other bivalves by being located outside rather than inside gill epithelial cells, and recent work suggests that they are capable of living freely in the environment. The T. cf. gouldi symbiont genome shows no signs of genomic reduction and contains many genes that would only be useful outside the host, including flagellar and chemotaxis genes. The thyasirid symbiont may be capable of sulfur oxidation via both the sulfur oxidation and dissimilatory sulfate reduction pathways, as observed in other bivalve symbionts. In addition, genes for hydrogen oxidation and dissimilatory nitrate reduction were found, suggesting varied metabolic capabilities under a range of redox conditions. The genes of the tricarboxylic acid cycle are also present, along with membrane bound sugar importer channels, suggesting that the bacteria may be mixotrophic. In this study, we have generated the first thyasirid symbiont genomic resources and lay the groundwork for further research in tracking the changes required for life as a bivalve symbiont.
... Although accessing sulphides may be the primary function of pedal tracts in symbiotic thyasirids , these bivalves may also use pedal feeding to gain nutrients. Chemosymbiotic thyasirids are mixotrophs, nutritionally relying on more than just their symbionts, as evidenced by differences in stable isotope composition among conspecifics (Spiro et al., 1986; Dando & Spiro, 1993) and variability in symbiont abundance (Dufour & Felbeck, 2006). In populations of the symbiotic Thyasira cf. ...
The bivalve family Thyasiridae includes species living in symbiosis with chemoautotrophic, sulphur-oxidizing bacteria and others that are asymbiotic. Chemosymbiotic thyasirids create extensive ramifying burrows (pedal tracts), presumably to acquire reduced sulphur for their symbionts. Here, we investigate whether asymbiotic thyasirids may also form pedal tracts. We compared the behaviour of asymbiotic (Parathyasira sp. and Thyasira cf. gouldi operational taxonomic unit 3) and symbiotic (Thyasira cf. gouldi operational taxonomic units 1 and 2) thyasirids from Bonne Bay, Newfoundland Canada, maintained in thin tanks in flow-through aquaria. Photographs and X-radiographs of thin tanks showed that all thyasirids established pedal tracts, with no discernable difference in the depth, total length or number of pedal tracts among taxa. We interpret thyasirid pedal tract formation as an early adaptation for pedal feeding, likely combined with the farming of chemosynthetic bacteria along burrow walls. Pedal tract formation could also be a precursor to chemosymbiosis establishment in the Thyasiridae.
