Differential phoretic vector use among sympatric Caenorhabditis nematodes and an association with invasive nitidulid beetles in southwestern Germany

This study reveals that three sympatric *Caenorhabditis* nematodes in southwestern Germany exhibit differential phoretic vector use, with the newly described *C. apta* showing a particularly strong and specific association with invasive nitidulid beetles, especially *Stelidota geminata*, thereby providing evidence for group dispersal and establishing a foundation for understanding nematode-vector interactions.

The Gist

Imagine a tiny, invisible world of microscopic worms called Caenorhabditis living on rotting fruit in a German orchard. These worms are famous in science labs (especially one called C. elegans), but in the wild, they face a big problem: they are too small to travel far on their own. If their food source (the rotting fruit) runs out or dries up, they are stuck.

To survive, they need a "taxi."

This paper is a detective story about how three different species of these tiny worms, living right next to each other in the same orchard, have found very different taxis to get them to new homes.

The Three Roommates and Their Different Commutes

The researchers found three "roommate" worm species living on the same rotting apples:

1. The Sluggish Commuters (C. briggsae and C. remanei): These worms are like people who prefer walking or taking a bus. The study found they hitchhike on snails and slugs (gastropods) and woodlice (pill bugs). They don't seem to care much about which specific snail they ride, as long as it's a slow-moving, moist creature.
2. The Jet-Setters (C. apta): This is the star of the show. Unlike its roommates, this worm never got on a snail or a woodlouse. Instead, it exclusively hitched a ride on beetles. Specifically, it loves two types of beetles that have recently invaded Europe from other parts of the world.

The "Invasive Taxi" Mystery

The beetles carrying C. apta are Nitidulid beetles (often called sap beetles). Two specific species were found:

• The Native-Range Taxi (Stelidota geminata): This beetle is originally from North America.
• The Foreign Taxi (Epuraea ocularis): This beetle is originally from Southeast Asia.

Here is the twist: The worm C. apta seems to have a special, almost exclusive friendship with the North American beetle (Stelidota geminata).

• The "VIP" Treatment: On the North American beetle, the worms were found in huge numbers, often clustering together in little groups on the beetle's wing covers (elytra). It was like the beetle was a crowded party bus for the worms.
• The "Economy" Treatment: On the Asian beetle, the worms were there too, but in much smaller numbers.

The researchers suspect that C. apta might be an invasive species itself, having arrived in Europe "stowaway" style on the North American beetle. It's possible that as the beetle spread across Europe, the worm spread with it, riding along like a passenger who never got off the bus.

How They Hitchhike

The worms don't just sit on the beetle's back like a backpack. They are smart travelers:

• The "Dauer" State: When the fruit runs out, the worms enter a super-tough, dormant state called a "dauer" larva. Think of this as a worm entering "suspended animation" or "cryosleep." They can survive starvation and harsh weather while waiting for a ride.
• The Clustering: The researchers saw these sleeping worms clumping together in the little folds under the beetle's wings. It's like a group of friends huddling together in a corner of a moving train to stay warm and safe.
• The Drop-Off: Once the beetle lands on a new, fresh rotting fruit, the worms wake up, crawl off, and start a new colony.

Why This Matters

This study is important because it shows that even tiny, simple creatures have complex social lives and travel strategies.

• Not All "Taxi Services" Are Equal: Just because two worms live in the same orchard doesn't mean they use the same transport. They have evolved to prefer specific vehicles.
• Invasive Species Travel Together: It highlights how invasive insects (the beetles) can accidentally carry other invasive species (the worms) to new continents, potentially changing local ecosystems.
• The "Group Hug" Effect: The fact that C. apta travels in groups suggests they might have evolved a "collective dispersal" strategy, where sticking together increases their chances of survival, much like a flock of birds flying together.

The Bottom Line

In a German orchard, nature is playing a game of "Who rides with whom?"

• Snail-lovers stick to snails.
• Beetle-lovers stick to beetles.
• And one specific worm (C. apta) has found a perfect match in an invasive beetle, using it as a high-speed transport system to conquer new territories in Europe.

It's a reminder that even the smallest creatures have big stories about where they come from, where they are going, and who they are willing to hitch a ride with.

Technical Summary

1. Problem Statement

While the model organism Caenorhabditis elegans and its relatives are widely studied in laboratories, their natural history and ecological interactions remain poorly understood. Specifically, the mechanisms of phoresy (dispersal via hitchhiking on invertebrate vectors) are critical for Caenorhabditis to colonize ephemeral, microbe-rich habitats (rotting fruit). However, vector specificity for the vast majority of the >80 known Caenorhabditis species is unknown.

• The Gap: It is unclear whether sympatric (co-occurring) Caenorhabditis species share the same vectors opportunistically or exhibit specific vector preferences that drive their ecological separation.
• The Context: Three species (C. briggsae, C. remanei, and a newly described C. apta) co-occur in orchard meadows in southwestern Germany. Previous studies suggested C. briggsae and C. remanei associate with gastropods and isopods, but the vector for C. apta (a species in the Angaria group) was unknown.

2. Methodology

The study employed a multi-phase approach combining broad field surveys, targeted quantitative dissection, and phylogeographic analysis.

• General Invertebrate Survey:

  • Location: Orchard meadows near the University of Konstanz, Germany.
  • Sampling: 478 individual invertebrates (gastropods, isopods, and various arthropods) were collected from rotting fruits between June and October 2025.
  • Extraction: Invertebrates were placed on Nematode Growth Media (NGM) seeded with E. coli OP50. Nematodes emerging from the dauer stage were identified morphologically and via reproductive behavior (hermaphroditic vs. gonochoristic) and mating tests.
  • Verification: To rule out "reluctant" dauers that do not exit the vector on agar, a subset of gastropods and isopods was physically dissected and washed to ensure no C. apta was missed.

• Targeted Nitidulid Beetle Survey:

  • Focus: Two invasive nitidulid beetle species, Stelidota geminata and Epuraea ocularis, were identified as primary vectors for C. apta.
  • Quantitative Dissection: 374 beetles (177 E. ocularis, 197 S. geminata) from six sites were dissected. Researchers examined the elytra (wing covers) and internal body cavities.
  • Metrics Recorded: Presence/absence, abundance (count), life stage (dauer vs. adult), sex ratio, and location of nematodes on the beetle.
  • Temporal Analysis: Repeated sampling at a single site (Egg Orchard) from September to November 2025 to assess seasonal dynamics.

• Statistical Analysis:

  • Generalized Linear Models (GLMs) and Generalized Linear Mixed Models (GLMMs) were used in R to analyze differences in nematode prevalence, abundance, and sex ratios across beetle species, body parts, and sites.

• Phylogeographic Context:

  • Distribution data for C. apta and its relatives (Angaria group) were compiled (1990–2024) and compared with the native and invasive ranges of the two beetle vectors to hypothesize the origin of the association.

3. Key Contributions

1. Discovery of a Novel Vector Association: Identified a specific, strong association between the newly described Caenorhabditis apta and nitidulid beetles, specifically the invasive Stelidota geminata.
2. Demonstration of Vector Specificity: Provided evidence that sympatric Caenorhabditis species exhibit differential vector use. While C. briggsae and C. remanei were found on gastropods/isopods, they were never found on the abundant nitidulid beetles, and C. apta was never found on gastropods/isopods.
3. Characterization of Dispersal Behavior: Documented that C. apta dauers disperse in mixed-sex groups (collective dispersal), often clustering in the cavity formed by the epipleura of the beetle's elytra.
4. Formal Description of C. apta: The paper includes the formal description of Caenorhabditis apta sp. n., distinguishing it from its closest relative C. agridulce based on reproductive isolation and morphological traits (e.g., spiral mating, stout amber spicules).

4. Key Results

• Differential Vector Use:
  • C. briggsae and C. remanei were isolated from gastropods (slugs) and isopods, consistent with previous literature.
  • C. apta was exclusively isolated from Nitidulidae beetles. No C. apta was found on gastropods or isopods despite high sampling effort.
• Beetle Specificity and Abundance:
  • S. geminata carried significantly more C. apta dauers per individual than E. ocularis.
  • Prevalence: 68.5% of S. geminata individuals carried C. apta, compared to only 35.6% of E. ocularis.
  • Location: Dauers were primarily found clustered on the underside of the elytra, specifically in the epipleural cavity.
• Sex Ratio:
  • The sex ratio of C. apta on S. geminata was close to 1:1 (male:female), whereas E. ocularis showed a significant female bias (likely due to low sample sizes per beetle).
• Temporal Dynamics:
  • Both the proportion of beetles carrying nematodes and the number of nematodes per beetle decreased significantly from September to November, correlating with the end of the active season before the first freeze.
• Co-occurrence:
  • Other nematodes (e.g., Sheraphelenchus, Pristionchus) were occasionally found on the same beetles, suggesting potential multi-species phoresy, though C. apta was the dominant species on S. geminata.
• Invasion Hypothesis:
  • C. apta was first detected in Europe in 2011, coinciding with the spread of the invasive beetle S. geminata (native to North America). All other Angaria group species are native to the Americas. This suggests C. apta may have been introduced to Europe via S. geminata (co-invasion).

5. Significance

• Ecological Insight: The study challenges the hypothesis that Caenorhabditis species in the Elegans group are purely opportunistic generalists. Instead, it demonstrates that even closely related, sympatric species can evolve distinct vector preferences, which may be a mechanism for coexistence and niche partitioning.
• Evolutionary Implications: The strong association with an invasive beetle suggests a potential case of co-invasion. If C. apta is indeed an invasive species in Europe, this system offers a unique model to study the ecological and evolutionary consequences of range expansion and the introduction of novel competitors to native Caenorhabditis populations.
• Behavioral Biology: The observation of collective dispersal (grouping on the beetle) supports previous findings of "dauer towers" in C. apta, providing a natural context to study the evolution of social behaviors in nematodes.
• Methodological Benchmark: The study establishes a rigorous framework for characterizing vector specificity in nematodes, combining broad community surveys with targeted quantitative dissection and phylogeographic reconstruction.