The study, published in the journal Subterranean Biology, revisited an extraordinary dataset assembled largely by the late Slovenian zoologist Professor Janez Matjašič, who documented the temnocephalid flatworms of the Dinaric Karst from the 1960s through the 1990s. His monographic work recorded 15 subterranean flatworm species from five genera living on crustacean hosts along this vast karst region, noting not only which hosts each species occupied but also the preferred attachment sites on host bodies. What his taxonomy never provided, however, was an ecological framework: nobody had formally analysed how specific these flatworms are to their hosts, or whether multiple flatworm species coexist peacefully or fight for space.

The research team extracted every available record from the literature, noting the flatworm species, the cave locality, the host species and the body part to which each worm attached. They then built bipartite interaction networks linking epizoans to hosts at several hierarchical levels: flatworm genera against host clades, flatworm species against host species, and flatworm species against host body parts. Cave shrimps of the genera Troglocaris, Spelaeocaris and Troglocaridella, along with amphipods of the speciose genus Niphargus, served as the crustacean hosts. Network metrics such as connectance, nestedness, specialization (H2′), generality and vulnerability were evaluated against null models generated by 999 random permutations of the interaction matrices, allowing the researchers to test whether observed patterns deviated from what chance alone would produce.

The verdict was strikingly clear: temnocephalid flatworms of the Dinaric Karst are generalists. At the genus level, network metrics did not deviate from random expectation, and at the species level the networks were actually more generalized than chance would predict. Only one potentially specialist relationship emerged from the node-level analysis. The species Stygodyticola hadzii, which lives exclusively on Niphargus amphipods, showed maximal specialization to that host genus, with a node-level specialization value of one and a statistically significant deviation from random networks. Every other flatworm species attached promiscuously to different cave shrimp species and to different body regions, from gills to appendages.

One intriguing exception hints at finer-scale structure. Subtelsonia perianalis showed a marginally significant preference for occupying the anal region of its shrimp hosts, a pattern the authors suggest may reflect specialization on a particular food source — the intestinal contents of cave shrimps — rather than on the host species itself. Apart from this perianal specialist, however, the flatworms showed no tendency to partition their hosts by body part, and the analysis found no association between flatworm species and the recently revised cave shrimp lineages. Host speciation, in other words, was not accompanied by flatworm coevolution, at least not detectably in the available records.

Why would these worms be so generalized? The authors point to the oligotrophic nature of cave environments. Groundwater communities are dominated by trophic generalists because narrow specialization is unlikely to be sustainable where food is scarce and populations are small. Epizoic life is already a specialization in itself — temnocephalids lack locomotory cilia, carry anterior tentacles and possess a posterior sucker-like attachment organ, adaptations that commit them to life on a host. Pushing specialization further, the argument goes, is simply no longer viable in the resource-poor darkness of caves. The low host specificity detectable at the order level — shrimp versus amphipod — likely evolved long ago, at the onset of the formation of the subterranean Dinaric fauna, and has persisted ever since.

But generalism breeds competition. The team analysed co-occurrence patterns among flatworm species at three scales: within caves, within host species, and within host body regions, using a model that compares observed co-occurrence frequencies against random expectations while accounting for species commonness. The results implied a modest but meaningful level of competition. Twenty-one flatworm species pairs co-occurred on the same host species less frequently than expected by chance, and twenty-three pairs co-occurred within the same host body region less often than random. Because the flatworms exhibit low host specificity, competitive interactions cannot be resolved through niche partitioning; the worms must instead compete directly for hosts and attachment sites. The authors speculate that in such a competitive system, maintaining high mobility may be advantageous, since individuals displaced from a host must rapidly find and colonize an alternative one.

Not every negative co-occurrence signals combat. Stygodyticola hadzii co-occurred with four other species less frequently than expected at the locality level, but because this species lives on a different host genus entirely, the pattern likely reflects host preference rather than competition. Co-occurrences that matched random expectation could indicate neutral population dynamics or an outcompetition-recolonization model, in which winners displace losers locally but recolonization keeps resetting the board. The authors are careful to frame all conclusions as working hypotheses rather than settled facts.

Several caveats temper the findings. The number of records per species is generally low, and flatworm records are disproportionately concentrated in the northwestern Dinaric Karst, most likely reflecting sampling bias that the limited data could not correct through rarefaction. More intensive and spatially balanced sampling could alter the observed patterns and potentially reveal stronger host or site specificity. Furthermore, the hypothesis of coevolution should not be dismissed before a molecular revision of the flatworms themselves. Cryptic species are ubiquitous in nature, and it seems unlikely that such small flatworms, lacking many visible diagnostic traits, are genetically homogeneous across a karst region where their shrimp hosts are highly genetically structured. Two temnocephalid species described from cave shrimps in the Caucasus offer a particularly interesting venue for exploring coevolution, given the relatively young connection between the Dinaric Karst and the Caucasus revealed by cave shrimp phylogeny.

The study also opens a roadmap for future research. Molecular delimitation of temnocephalid species, reconstruction of their phylogenetic relationships and improved spatial resolution of their distributions are all needed to confirm whether the apparent generalism holds up. Experimental studies of host choice, attachment site preference and movement capacity would rectify the limitations of a purely comparative, literature-based approach — no small challenge given the difficulties of experimenting on subterranean organisms. If the working hypotheses are confirmed, the Dinaric Karst flatworm-crustacean system will stand as a compelling example of how competition, rather than niche partitioning, shapes interactions in the resource-starved depths of Earth’s groundwater realms — a subterranean drama playing out one sucker-hold at a time.

Subject of Research: Epizoic interactions and host specificity of subterranean temnocephalid flatworms in the Dinaric Karst

Article Title: Subterranean temnocephalid flatworms of the Dinaric Karst are not picky about the hosts but compete for them: an analysis of literature data

Article References: Fišer, C., Premate, E., & Zakšek, V. (2026). Subterranean temnocephalid flatworms of the Dinaric Karst are not picky about the hosts but compete for them: an analysis of literature data. Subterranean Biology, 55, 95-108. https://doi.org/10.3897/subtbiol.55.186321

Image Credits: AI Generated

DOI: 10.3897/subtbiol.55.186321

Keywords: temnocephalid flatworms, Dinaric Karst, cave shrimps, Niphargus, host specificity, epizoic interactions, species competition, network analysis, subterranean biology, co-occurrence, oligotrophic caves, Janez Matjašič

The study, published in Nature Plants, brings together results from 19 CO₂-enrichment experiments conducted in glasshouses, growth chambers and field environments. The experiments included plant communities ranging from grass-dominated ecosystems to multispecies assemblages. By comparing plants grown alone with plants grown in mixtures, the researchers examined a question that has often been overlooked in climate-change experiments: does elevated CO₂ help all species equally, or does competition determine which plants are able to use the additional carbon?

The contrast was striking. When plants grew without neighbours, most species responded positively to elevated CO₂, or eCO₂. This response is biologically plausible because CO₂ is the raw material plants use during photosynthesis. Inside leaves, carbon dioxide is fixed into sugars through the Calvin cycle, providing the carbon skeletons needed to build new tissues. Higher atmospheric CO₂ can also reduce the amount of water plants lose while taking up carbon, because leaf pores known as stomata may not need to remain as open. Under controlled conditions, these effects can translate into faster growth and greater biomass.

Yet those benefits often disappeared when plants had to share their environment. More than half of the species examined showed neutral or negative responses to eCO₂ when growing in mixtures. In other words, a plant that grew better under elevated CO₂ by itself was not necessarily able to capitalize on the same conditions when surrounded by other species. The finding challenges the tendency to use single-species experiments as a guide to how natural plant communities will respond to a carbon-rich atmosphere.

Competition can block the conversion of extra carbon into additional growth in several ways. Plants may compete for sunlight, water, nitrogen and phosphorus, all of which are required to turn photosynthetic products into leaves, stems, roots and seeds. A plant may absorb more CO₂ and produce more sugars, but if nutrients are scarce, it may be unable to construct the proteins and tissues needed to use that carbon. Similarly, a taller or faster-growing neighbour may capture the available light before a smaller species can benefit, creating a physical barrier between elevated CO₂ and actual plant growth.

The community-level result was more subtle than a simple collapse in productivity. Although many individual species experienced neutral or negative responses, dominant species often drove modest increases in total mixture biomass under eCO₂. This means a plant community can appear to benefit from elevated CO₂ even while many of its members do not. A rise in total biomass therefore does not necessarily indicate that growth has been shared evenly, or that the community has become more productive in a way that benefits all species.

That distinction could have major implications for biodiversity. If elevated CO₂ consistently favours species that are already dominant, they may capture an even larger share of light, nutrients and space. Subdominant species could then face stronger suppression, reducing their growth and potentially altering the composition of plant communities over time. The result may be a greener-looking ecosystem with fewer species, rather than a uniformly healthier or more productive one.

The researchers’ trait analysis offers a clue about why some plants succeed while others fall behind. Acquisitive strategies, associated broadly with rapid resource capture and fast growth, were better predictors of performance under competition than a species’ intrinsic sensitivity to CO₂ alone. This suggests that the key question is not simply whether a plant can respond physiologically to elevated carbon dioxide. It is whether the plant can acquire enough light, water and nutrients quickly enough to turn that response into a competitive advantage.

The findings also expose a limitation in how climate models and ecological forecasts often treat CO₂ fertilization. If models assume that species respond independently, they may overestimate the benefits that rising CO₂ will deliver to plant communities. Real ecosystems are networks of interactions, and the outcome for one species depends partly on the traits and abundance of its neighbours. The same atmospheric change can therefore stimulate growth in an isolated plant, suppress a subordinate competitor and produce only a small increase in total community biomass.

As atmospheric CO₂ continues to rise, understanding this uneven distribution of benefits will become increasingly important. The new synthesis does not suggest that elevated CO₂ has no effect on vegetation. Instead, it shows that its effects are filtered through competition. Dominant plants may gain enough to lift overall biomass, while many less competitive species receive little benefit or even lose ground. The future of plant communities may therefore be shaped not by a universal CO₂ boost, but by an intensified struggle over which species can claim it.

Subject of Research: The effects of elevated carbon dioxide on plant growth and competition in multispecies communities.

Article Title: Interspecific competition negates CO₂ benefits for most plant species.

Article References: Raubenheimer, S.L., Simpson, K.J., Ripley, B.S. et al. “Interspecific competition negates CO₂ benefits for most plant species.” Nature Plants (2026). https://doi.org/10.1038/s41477-026-02360-2

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s41477-026-02360-2

Keywords: Elevated CO₂, climate change, plant competition, biodiversity, plant communities, ecosystem productivity, dominant species, subdominant species, CO₂ fertilization, plant traits