What are endocrine-disrupting chemicals (EDCs)?

Endocrine-disrupting chemicals are synthetic compounds that interfere with hormone signaling pathways in organisms. They can disrupt reproduction, development, and survival in both vertebrates and invertebrates. Testosterone and flutamide are both listed as environmental hormone substances by the U.S. Environmental Protection Agency.

Why are rotifers used to study endocrine disruptors?

Rotifers like Brachionus calyciflorus are widely used in ecotoxicology because they have a global distribution, a short life cycle with multiple generations observable in days, are easy to culture in the lab, and produce resting eggs that can be stored and hatched on demand for reproducible experiments.

What is mixis rate in rotifers?

Mixis rate is the proportion of female rotifers that switch from asexual to sexual reproduction. It is a key parameter in reproductive toxicity studies because it determines how many resting eggs a rotifer population can produce for long-term survival.

How does testosterone affect rotifer reproduction?

Low concentrations of testosterone (25 µg/L) increased population growth rate through a hormetic response, while higher concentrations suppressed it. Testosterone at all tested concentrations significantly reduced resting egg production per mictic female, which threatens long-term population survival.

How does flutamide affect rotifer reproduction?

Flutamide alone reduced the mixis rate (sexual reproduction switch) in rotifers. At higher concentrations (7.5 µg/L) combined with testosterone, it significantly decreased fertilization rates. Flutamide combined with testosterone also suppressed resting egg production, particularly at higher concentrations.

What happens when testosterone and flutamide are combined?

The combination produced antagonistic effects. The most striking finding was that 5 µg/L flutamide with 25 or 50 µg/L testosterone reduced the mixis rate to 2.2 times lower than flutamide alone. For resting eggs, low flutamide with testosterone increased production (antagonism), but higher flutamide reversed this effect. These interactions would be missed in single-chemical testing.

What is hormesis in ecotoxicology?

Hormesis is a phenomenon where a mild stressor triggers a compensatory overcorrection in an organism, resulting in stimulation at low doses and suppression at higher doses. In this study, low testosterone boosted rotifer population growth while higher concentrations suppressed it.

Why is resting egg production an important endpoint?

Resting eggs are dormant stages that allow rotifer populations to survive harsh environmental conditions. Reduced resting egg production compromises the population's long-term resilience. In this study, resting egg production was the most sensitive reproductive endpoint to both testosterone and flutamide exposure.

Do rotifers have androgen receptors?

No androgen receptor has been definitively identified in rotifers yet. However, transcriptome surveys show steroid receptors, biosynthetic enzymes, and signal transduction elements are present. Researchers have found a progesterone receptor in rotifers, and the evidence suggests androgen-like signaling pathways exist, though the exact molecular mechanism remains unknown.

What are the practical implications for environmental risk assessment?

The study highlights that single-chemical testing is insufficient for accurate risk assessment because chemical mixtures can produce antagonistic or additive effects. It also shows that population growth rate alone can be misleading, and endpoints like resting egg production should be included in chronic toxicity tests. The results support using rotifers as screening tools for EDC testing.

Testosterone and Flutamide Disrupt Rotifer Reproduction: What Ecotoxicologists Need to Know

Endocrine-disrupting chemicals are exactly what they sound like — synthetic compounds that mess with hormone signaling. And they’re everywhere in modern waterways. Runoff from agriculture, industrial discharge, pharmaceutical waste — all of it carries compounds that weren’t part of these ecosystems fifty years ago.

Two of the more interesting ones are testosterone and flutamide. Testosterone is an androgen. Flutamide is an anti-androgen, used clinically to block androgen receptors. Both are on the U.S. EPA’s list of environmental hormone substances, and both find their way into aquatic systems.

The question is: what happens when tiny freshwater animals get exposed to them, alone or together?

A recent study on the rotifer Brachionus calyciflorus took a close look at that question. The answer, as usual, is more complicated than you’d expect.

Why rotifers

Rotifers are microscopic, but they’re a big deal in ecotoxicology. B. calyciflorus specifically checks a lot of boxes for researchers:

Found in freshwater systems everywhere
Generations turn over in days, not months
Easy to keep alive in the lab
Their resting eggs can be stored and hatched on demand, which makes replication straightforward

But the real reason they’re useful has to do with how they reproduce. Monogonont rotifers switch between asexual and sexual reproduction. In the asexual phase, females just clone themselves through parthenogenesis. Then, under the right conditions, they flip to sexual mode — producing haploid eggs that become males if unfertilized, or resting eggs if fertilized.

Those resting eggs matter. They’re the species’ backup plan — dormant stages that sit through bad conditions and hatch when things improve. Mess with resting egg production, and you’re threatening the population’s ability to survive long-term.

What the experiment looked like

The researchers set up 16 treatment combinations, crossing four flutamide concentrations (0, 2.5, 5.0, and 7.5 µg/L) with four testosterone concentrations (0, 25, 50, and 75 µg/L). Then they tracked four things:

Population growth rate over 2 days
Mixis rate — how many females switched to sexual reproduction — at day 4
Fertilization rate at day 4
Resting egg production by day 7

That design let them tease apart the effects of each chemical and see whether they interacted.

Population growth: the hormesis curve

Low-dose testosterone (25 µg/L) actually made the rotifer population grow faster. That sounds backwards — a pollutant boosting reproduction? — but it’s a known phenomenon called hormesis. A mild stressor triggers a compensatory overreaction. Low doses stimulate; high doses suppress.

Add flutamide to the mix, and things got more complicated. Low flutamide with testosterone still increased population growth, but higher flutamide reversed that. Both chemicals had significant effects on their own. But combined, their interaction wasn’t statistically significant — meaning they weren’t really amplifying each other here.

Mixis rate: where things actually interacted

Mixis rate — the switch to sexual reproduction — was the endpoint where the interaction showed up most clearly.

Flutamide alone lowered the mixis rate. Testosterone alone raised it. But together, they did something neither did separately. The standout finding was 5 µg/L flutamide plus either 25 or 50 µg/L testosterone: the mixis rate dropped to less than half of what flutamide produced by itself.

That’s an antagonistic interaction with real consequences. The presence of testosterone made flutamide’s suppressive effect stronger, not weaker. If you were only testing one chemical at a time, you’d completely miss this.

Fertilization: a threshold effect

Fertilization rate was less sensitive to the combination. Lower flutamide (2.5 and 5.0 µg/L) with any testosterone level didn’t change much. But at 7.5 µg/L flutamide paired with 25, 50, or 75 µg/L testosterone, fertilization dropped noticeably.

One odd detail here: low flutamide alone (2.5 µg/L) tended to increase fertilization. Another hormetic blip. Both chemicals individually affected fertilization rates, but their combined effect wasn’t significant — they seem to work through separate pathways on this endpoint.

Resting eggs: the most vulnerable measure

This was the one that stood out. Testosterone at every concentration tested reduced the number of resting eggs produced per mictic female. Every single one.

That’s a big deal, because resting eggs are the rotifer’s long-term survival strategy. Cut those numbers, and the population loses its buffer against environmental stress.

The mixtures were more nuanced. Low flutamide (2.5 µg/L) with some testosterone levels actually increased resting egg production — an antagonistic effect, suggesting competition at the same target site. But at 7.5 µg/L flutamide with 25 µg/L testosterone, resting egg production fell significantly below control levels. The two-way ANOVA confirmed that both chemicals individually and their combined exposure all significantly affected resting egg numbers.

What this means for how we assess risk

A few things stand out.

First, single-chemical testing isn’t enough. Standard ecotoxicity tests look at one compound at a time. But in the real world, aquatic organisms are swimming in mixtures. This study shows that testing one chemical in isolation can either overestimate or underestimate risk depending on what endpoint you’re measuring.

Second, resting egg production should get more attention. Population growth rate might look normal, but if resting eggs are compromised, the population is in trouble over the long haul. Short-term tests can miss that.

Third, rotifers are more relevant to human endocrinology than most people realize. A 39 kDa protein induced during sexual reproduction in rotifers shares N-terminal sequences with steroid-induced proteins from human ovarian follicular fluid. The parallels between invertebrate and vertebrate hormone signaling are closer than they look.

Steroid signaling isn’t just a vertebrate thing

Vertebrate-type steroids have been found in arthropods for years, though nobody is entirely sure what they’re doing there. In rotifers, researchers have identified a progesterone receptor. Transcriptome scans show steroid receptors, biosynthetic enzymes, and signal transduction elements.

What’s interesting is that estrogens seem to play a smaller role in rotifer endocrine signaling than androgens and progesterone-like compounds. That flips the vertebrate-centric expectation on its head.

Similar patterns show up in other invertebrates. In Daphnia magna, long-term testosterone exposure reduced fecundity and increased aborted eggs. In the eastern mud snail, androgen and estrogen receptor-like proteins influence reproductive tract development. Across different branches of the animal tree, steroid signaling looks like an ancient, conserved system.

For researchers and regulators

A few practical points:

Use multiple endpoints. Population growth alone can mislead you. Include sexual reproduction parameters like mixis rate and resting egg production in chronic tests.
Test mixtures. Antagonistic interactions can hide real-world risks. Single-chemical assessments may underestimate harm from combined exposures.
Watch for hormesis. Low-dose stimulation doesn’t mean a chemical is safe. It can signal compensatory responses that break down at higher concentrations or longer exposure times.
Rotifers make good screening tools. Short generation time, easy culture, sensitive reproductive endpoints — they’re practical for both basic research and regulatory screening.

What’s still unknown

The molecular mechanism behind these effects isn’t settled yet. Whether it goes through a classic androgen receptor or some unknown steroid-binding protein, nobody knows. The rotifer transcriptome data is still incomplete, and no androgen receptor has been definitively identified in rotifers.

Future work worth doing:

Finding and characterizing the androgen receptor in rotifers at the molecular level
Looking at transgenerational effects through resting egg viability
Testing realistic environmental mixtures with multiple EDCs at once
Building standardized rotifer-based screening protocols for regulatory use

The rotifer is a tiny animal, but it’s telling us something important about how endocrine disruptors work across species. Worth paying attention to.