The investigators sampled sediments on 9 June 2024 at two sites in each of the Malá Outrata, Velký Ústavní and Doktorovský ponds, which are hydrologically connected to the Blanice River and used for common carp production. They extracted 2,028 invertebrates and processed them as 579 samples: 214 individually processed large chironomid larvae longer than 7 mm, 145 pools of five small chironomid larvae shorter than 7 mm, and 220 pools of five oligochaetes. Large and small chironomids were operational morphogroups based on size, colouration and appearance rather than formal taxonomic groups.

Each sample was homogenised, and DNA was extracted with a modified protocol using bead beating and silica beads to disrupt resistant microsporidian spores. The researchers amplified approximately 400 base pairs of small-subunit ribosomal DNA with V1F and 530R primers, generated paired-end 250-base-pair Illumina MiSeq reads, removed chimeras and clustered sequences into OTUs at 97% similarity. To reduce possible false positives, they retained only OTUs represented by at least 50 reads, a threshold that may also exclude genuine low-abundance taxa. Their phylogenetic analysis combined 66 newly obtained sequences with 565 published sequences in a maximum-likelihood analysis of 2,648 nucleotide positions.

Among the 66 detected OTUs, 38 belonged to Enterocytozoonida, 12 to Nosematida, 11 to Amblyosporida and five to Neopereziida; Glugeida was not detected. Enterocytozoonida accounted for more than 62% of reads, including 57% from the Arlequin group. Nosematida represented more than 30%, driven by Encephalitozoonidae, while Amblyosporida and Neopereziida accounted for approximately 7% and 0.4%, respectively. The analysis found 51 OTUs without close matches in reference databases or previous studies. Three OTUs matched described species, four matched previously reported freshwater environmental sequences and five corresponded to undescribed lineages or undescribed species.

Sample-level microsporidian prevalence varied substantially among sites and host groups. It was highest overall in oligochaetes, at 69 positive samples out of 220, or 31%, compared with 37 of 145, or 26%, for small chironomids and 21 of 214, or 19%, for large chironomids. Site-level prevalence ranged from 0% to 75% among small chironomids and from 6% to 57% among large chironomids, while oligochaete values ranged from 17% to 69%. Velký Ústavní generally had the highest prevalence, including 75% for small chironomids and 69% for oligochaetes at site 2. The sampling design could not distinguish temporal variation from small-scale spatial effects.

Within-sample diversity also differed among operational groups. Small chironomids had the highest Shannon and Simpson values, 1.89 and 0.741, while oligochaetes had the lowest, 1.11 and 0.506. COI barcoding showed that the small-chironomid fraction was compositionally broader, including tube-building Tanytarsini, vegetation- and detritus-associated larvae and predatory Tanypodinae, whereas the large fraction consisted of Chironomus species. The groups overlapped because smaller samples included early instars of Chironomus. These ecological and developmental differences may help explain the observed diversity patterns, although metabarcoding cannot by itself establish infection or causality.

Read distributions were generally concentrated in one host group. For 49 of 66 OTUs, more than 90% of reads came from a single morphogroup, indicating a dominant read association rather than confirmed host specificity. Encephalitozoonidae OTUs were predominantly associated with oligochaetes, whereas Amblyosporida OTUs were mainly associated with chironomids. Only four OTUs were shared by all three ponds, while 14 were shared by all three host groups. Enterocytozoonida and Neopereziida included OTUs distributed across multiple groups, showing that apparent host association varied within the detected lineages.

Two OTUs previously reported from Daphnia were concentrated in chironomids. OTU2 was assigned to Conglomerata obtusa, with more than 99% of its reads associated with large and small chironomids, although it was detected rarely in the oligochaete Limnodrilus hoffmeisteri. COI barcoding identified Chironomus plumosus, C. annularius and C. curabilis in positive samples. OTU8 matched Microsporidium sp. MIC1, previously reported from Daphnia cucullata, and 99% of its reads were associated with chironomids, particularly C. plumosus and C. curabilis. These patterns are consistent with possible additional hosts in indirect life cycles, but they do not prove active infection; the authors note that low read counts could reflect ingested spores or exposure rather than infection.

The principal limitations are the short SSU rDNA target, which could not confidently resolve OTU39 to a described Nosema species, and the absence of direct parasite localisation or transmission experiments. Read abundance was used for relative comparisons, and pooled samples limited certainty about which individual host carried a detected lineage. The study therefore documents broad microsporidian diversity and candidate host associations in a geographically restricted set of ponds, while infection status, transmission routes and the ecological roles of the detected lineages remain unresolved.

Subject of Research: Microsporidian diversity and candidate host associations in benthic invertebrates from South Bohemian fish ponds

Article Title: Microsporidian diversity and host associations in aquatic invertebrate communities of South Bohemian fish ponds revealed by metabarcoding

Article References: Lisnerová, M., Nayak, S., van Beest, G. S., Bürgerová, M., Lövy, A., Pecková, H., Kyslík, J., Bartošová-Sojková, P., & Fiala, I. (2026). Microsporidian diversity and host associations in aquatic invertebrate communities of South Bohemian fish ponds revealed by metabarcoding. Metabarcoding and Metagenomics, 10, Article e201781. https://doi.org/10.3897/mbmg.10.201781

Image Credits: AI Generated

DOI: 10.3897/mbmg.10.201781

Keywords: microsporidia, metabarcoding, fish ponds, chironomids, oligochaetes, environmental DNA, SSU rDNA, host associations, South Bohemia, freshwater parasites, Microsporidian, diversity

The worms in question belong to a group of gutless marine oligochaetes, thin, short animals smaller than a pinhead that live in sediments in tropical and temperate seas around the world. Their name is not an exaggeration. Over the course of evolution, these worms have lost their mouth, their gut and their excretory organs entirely. They cannot eat in any conventional sense, and they have no way of disposing of metabolic waste on their own. Instead, they host dense communities of chemosynthetic bacteria directly beneath their skin. These bacterial tenants fix carbon from compounds such as sulfide, provide the worm with nutrition, and process the host’s waste products. The association is so tight that neither partner can persist without the other: the worm is a walking, burrowing habitat, and the bacteria are its kitchen and its waste treatment plant rolled into one.

Given this level of interdependence, one might expect the partnership to be locked in place, with each worm species bound to a fixed, unchanging set of bacterial partners. The new study shows that the reality is more nuanced and, in some ways, the opposite of what intuition would predict. Within a single worm species, the bacterial communities are indeed very consistent. Worms of the same species collected from ocean regions far apart from one another harbor the same, or very similar, suites of symbionts. At the level of the species, the partnership looks rock solid, as stable as any mutualism described in the literature.

But that stability dissolves the moment researchers compare different host species, even ones that are close evolutionary relatives. Closely related worm species often host very different bacterial communities, tailored to the specific conditions of their local habitat. The authors use the market metaphor to make sense of this pattern: a shopper visiting a market in a different city will still find staple foods, fruits and vegetables, but the specific basket they assemble will reflect local supply and local preferences. In the same way, each worm species solves its nutritional needs with a specific, locally adaptive community of symbionts. The functional requirements of the partnership remain constant, while the identities of the partners providing those functions can change dramatically from one host lineage and one location to another.

Even more surprising is what the study reveals about fidelity over time. The worms do not simply inherit a fixed bacterial community and pass it on unchanged. They regularly take up new symbionts from their environment, and occasionally they drop some of the ones they already carry. As Mankowski reports, the worms like to try out new market stalls every now and then, adjusting to the local supply. This ongoing turnover means that the symbiosis is not a closed, hermetically sealed system but an open, dynamic marketplace in which partners are continually recruited, tested and, in some cases, released. The community as a whole remains stable in its overall composition and function, while its individual members come and go.

Why would an animal that is utterly dependent on its bacteria take the apparent risk of not being completely loyal to them? Dubilier, who has studied these partnerships for decades, admits that the pattern was genuinely surprising, since dependence on a partner would seem to demand unwavering fidelity. The most plausible answer lies in the diversity of metabolic abilities among the bacterial partners. Each symbiont species brings a different set of biochemical skills to the table. By periodically acquiring new bacteria, a worm can tap into new sources of food and energy, expanding its metabolic repertoire. In habitats where nutrients are sometimes scarce or fluctuate over time, this ability to sample and incorporate new metabolic capabilities could make the difference between thriving and merely surviving. Flexibility, in other words, is not a weakness in the partnership but a form of insurance.

The evidence behind these conclusions is unprecedented in scale. The researchers analyzed samples collected during three decades of expedition work, spanning nearly 250 worms representing 63 species from 17 different locations across the world’s oceans. No comparable survey of these symbioses has ever been attempted before. A dataset of this breadth was essential for separating genuine patterns from local accidents: only by comparing worms across species, regions and habitats could the team demonstrate that community stability within species coexists with dramatic turnover between species and through time.

The study also owes its existence to a quiet revolution in laboratory technology. Gruber-Vodicka, who led the project at the Max Planck Institute in Bremen and now runs his own laboratory at Kiel University, recalls that the first and seminal metagenomic analyses of gutless oligochaetes required thousands of worms to yield enough genetic material for a single analysis, a constraint that made comparative work across species and locations practically impossible. Recent technical innovations have changed that equation completely. Today, researchers can work with single individuals of animals smaller than a pinhead, recovering the genomes of their symbionts and reconstructing the metabolic potential of entire bacterial communities from one worm at a time. That leap in sensitivity is what transformed a long-standing question about symbiotic stability into an answerable one.

The broader significance of the work extends well beyond these particular worms. Symbioses between animals and chemosynthetic or nutritional bacteria are widespread in nature, from deep-sea tube worms and hydrothermal vent mussels to insects fed by bacterial endosymbionts, and a central puzzle in evolutionary biology has been how such intimate dependencies persist over millions of years without becoming brittle. The new study suggests a general principle: stability in symbiosis need not come from rigidity. A partnership can remain functionally stable even as its membership changes, provided the exchange of benefits continues. The worms are loyal partners to their bacteria in the short term, but always open to new possibilities in the long run, and that combination of commitment and adaptability keeps the relationship resilient as environments shift.

The evolutionary success of this strategy is written in the worms’ distribution. Despite an apparently restrictive lifestyle, lacking a mouth, a gut and excretory organs, and depending entirely on microbial partners for nutrition and waste management, gutless oligochaete communities have colonized all the world’s oceans and an impressive variety of habitats, from carbonate sands in Belize to sediments far beyond the tropics. Their shifting alliances, maintained over roughly 150 million years of evolution, have proven to be a remarkably durable recipe. In the marketplace of symbiosis, it turns out, the most stable customers are not those who never change their stalls, but those who know exactly what they need while remaining willing to shop around for it.

Subject of Research: Stability and flexibility of bacterial symbioses in gutless marine worms

Article Title: A symbiotic marketplace: Stability through flexibility

Article References: A symbiotic marketplace: Stability through flexibility. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: symbiosis, marine worms, microbiome, metagenomics, chemosynthetic bacteria, Science Advances, Max Planck Institute for Marine Microbiology, host-microbe interactions, evolution, biodiversity, oligochaetes, nutrient exchange