# Bdelloid Rotifers (DD-641)

> Bdelloid rotifers (class Bdelloidea) are microscopic animals, usually well under a millimeter long. They live in fresh water and also in damp moss, lichen and soil films that often dry out. In a 2000 paper in Science, they were called the largest animal group in which males, hermaphrodites and meiosis are unknown. Females produce daughters from unfertilized eggs (parthenogenesis). Genetic studies suggest the group may have done without conventional sex for many millions of years, though 'ancient asexuality' is still debated. Bdelloids carry an unusually high share of genes picked up from bacteria, fungi and plants by horizontal gene transfer. A 2015 study estimated about 13 new foreign genes per lineage per million years, with more in species from habitats that often dry out. A 2018 reanalysis, however, found that claims of DNA swaps between bdelloid individuals came from sample contamination. When their habitat dries, bdelloids contract into a 'tun' and enter anhydrobiosis, a dried, dormant state, and revive when water returns. The best-documented dry survival is about 9 years in stored moss. In 2021, researchers revived an Adineta rotifer from Siberian permafrost dated to about 24,000 years old, though it had been frozen rather than dried, and it went on to clone itself in the lab.

- Department: Nature (also Science)
- First published: 2025-08-22
- Last updated: 2025-08-22
- Permalink: https://detailenjoyer.com/case/bdelloid-rotifers/
- Author: Marcus Easterling (DetailEnjoyer), The Detail Department

## On the timeline

- **2021**: Researchers revive an Adineta rotifer from 24,000-year-old Siberian permafrost

Full timeline: https://detailenjoyer.com/timeline/?case=bdelloid-rotifers

## Quick facts

- **DD-641-F01**: No bdelloid males have ever been found. _(Write-up · Exhibit A (No males known))_
- **DD-641-F02**: A bdelloid revived from 24,000-year-old permafrost. _(Write-up · Exhibit E (From the permafrost))_

## Write-up

No males for millions of years. Bdelloid rotifers clone themselves and survive drying out.

1. **No males known**: Bdelloidea is the largest animal group in which males, hermaphrodites and meiosis are unknown.
2. **Borrowed genes**: They pick up foreign genes from other organisms, about 13 per lineage per million years in one estimate.
3. **Not swapping with each other**: Claims of DNA exchange between bdelloids were traced to sample contamination.
4. **Drying out**: In anhydrobiosis they dry into a dormant 'tun'. The best-documented revival came after about 9 years dry.
5. **From the permafrost**: In 2021, an Adineta rotifer about 24,000 years old was revived from frozen Siberian soil and cloned itself.

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## Reel vs. record

- **Note**: 'No males observed in over 80 million years': no one has observed rotifers for millions of years. No males have ever been seen, and genetic evidence suggests asexual descent over many millions of years, but the exact age is an estimate and still debated.
- **Note**: 'Enduring decades without water': the best-documented dry survival is about 9 years. Older reports of 27 and 59 years are considered less reliable, because contamination couldn't be ruled out.
- **Note**: 'Freshwater': many bdelloids live in moss, lichen and soil films that dry out often, which is why drying out is so important to them.
- **Missing**: bdelloids have an unusually high share of genes taken from bacteria, fungi and plants (horizontal gene transfer), which keeps being added to.
- **Missing record**: in 2021 an Adineta rotifer was revived from 24,000-year-old Siberian permafrost. It had been frozen, not dried, and then reproduced by cloning.

## Video: Bdelloid Rotifers: No Males for Millions of Years

- Duration: 18.4 s · Published: 2025-08-22
- File: https://detailenjoyer.com/assets/videos/DNrIY3H5LUV.mp4

### Transcript

DID YOU KNOW? Bdelloid rotifers are microscopic freshwater invertebrates remarkable for their ability to survive extreme conditions, and for reproducing exclusively through parthenogenesis, with no males observed in over 80 million years. They can enter a desiccated, dormant state called anhydrobiosis, enduring decades without water. NOW YOU KNOW!

## Questions & answers

**Q: How do bdelloid rotifers reproduce?**  
A: Parthenogenesis. _(Source: Write-up · Exhibit A (No males known) · Mark Welch & Meselson (2000): Evidence for the evolution of bdelloid rotifers without sexual reproduction or genetic exchange, Science)_

**Q: What is anhydrobiosis?**  
A: A dried, dormant state. _(Source: Write-up · Exhibit D (Drying out) · Guidetti & Jönsson (2002): Long-term anhydrobiotic survival in semi-terrestrial micrometazoans, Journal of Zoology)_

**Q: How old was the rotifer revived from permafrost in 2021?**  
A: Siberian permafrost dated to about 24,000 years. _(Source: Write-up · Exhibit E (From the permafrost) · Shmakova et al. (2021): A living bdelloid rotifer from 24,000-year-old Arctic permafrost, Current Biology)_

**Q: Where do bdelloids get their foreign genes?**  
A: Horizontal gene transfer. _(Source: Write-up · Exhibit B (Borrowed genes) · Eyres et al. (2015): Horizontal gene transfer in bdelloid rotifers is ancient, ongoing and more frequent in species from desiccating habitats, BMC Biology (PMC))_

**Q: What explained claims that bdelloids swap DNA with each other?**  
A: A 2018 reanalysis found contamination. _(Source: Write-up · Exhibit C (Not swapping with each other) · Wilson, Nowell & Barraclough (2018): Cross-contamination explains 'inter and intraspecific horizontal genetic transfers' between asexual bdelloid rotifers, Current Biology)_

## Sources

- [Mark Welch & Meselson (2000): Evidence for the evolution of bdelloid rotifers without sexual reproduction or genetic exchange, Science](https://www.science.org/doi/abs/10.1126/science.288.5469.1211)
- [Eyres et al. (2015): Horizontal gene transfer in bdelloid rotifers is ancient, ongoing and more frequent in species from desiccating habitats, BMC Biology (PMC)](https://ncbi.nlm.nih.gov/pmc/articles/PMC4632278/)
- [Wilson, Nowell & Barraclough (2018): Cross-contamination explains 'inter and intraspecific horizontal genetic transfers' between asexual bdelloid rotifers, Current Biology](https://www.cell.com/current-biology/fulltext/S0960-9822(18)30706-1)
- [Shmakova et al. (2021): A living bdelloid rotifer from 24,000-year-old Arctic permafrost, Current Biology](https://www.cell.com/current-biology/fulltext/S0960-9822%2821%2900624-2)
- [Guidetti & Jönsson (2002): Long-term anhydrobiotic survival in semi-terrestrial micrometazoans, Journal of Zoology](https://doi.org/10.1017/s095283690200078x)

## Cite this case file

Easterling, Marcus. "Bdelloid Rotifers." The Detail Department (DetailEnjoyer), case file DD-641, last updated Aug 22, 2025. https://detailenjoyer.com/case/bdelloid-rotifers/