The Mites That Live and Breed on Your Face Have Anuses, Genome Study Finds

The Mites That Live and Breed on Your Face Have Anuses, Genome Study Finds
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An illustration of Demodex folliculorum.

An illustration of Demodex folliculorum.
Photo: Shutterstock (Shutterstock)

Scientists have finally unraveled the genetic secrets of humanity’s coziest roommates: Demodex folliculorum, also known as the skin mite. Among other things, the findings confirm that these mites actually do have anuses, contrary to previous speculation. They also indicate that the microscopic animals may not be as potentially harmful as commonly thought and that they’re evolving into co-dependent, symbiotic creatures that might provide us some benefits to boot.

D. folliculorum is actually one of two mite species that call us home, along with Demodex brevis. Both species are arachnids—more closely related to ticks than spiders—but D. folliculorum mites are the ones that usually reside (and mate) on our faces. These stubby worm-shaped critters live for two to three weeks, all the while embedded in our pores, clinging to our hair follicles, and primarily feeding off our sebum, the oily substance provided by our body to protect and moisturize the skin.

Despite virtually every person in the world having their own mite collection, there’s still much we don’t understand about them. But in a new study published Tuesday in the journal Molecular Biology and Evolution, researchers in Europe say they’ve now fully sequenced the genome of D. folliculorum—an accomplishment that might answer some lingering questions about their inner workings.

Some researchers have argued, for instance, that these mites lack an anus. Without an anus, the theory goes, their fecal waste simply accumulates inside them over their brief lifespan and is only released all at once when they die. Some have also speculated that an overabundance of mites can cause a skin condition known as rosacea, perhaps due to bacteria that’s released from this explosion of poop upon a mite’s death. Other research has cast doubt on that claim, though, and the researchers behind the new study say they’ve confirmed that mites do indeed have an anus.

Study author Alejandra Perotti, a researcher at the University of Reading in the UK, notes that the larger presence of mites in people who develop rosacea and other skin conditions may very well be a consequence of the condition and not its actual cause. And if mites aren’t leaving behind huge amounts of poop behind when they die, then there’s a less clear rationale as to how they would make us sick in the first place. Other studies, for what it’s worth, have continued to find a link between the mites and rosacea, though they may only be one of many triggers involved.

“It is easier and faster to just blame the mites,” she said in an email to Gizmodo.

The team’s other findings show that these mites have evolved to become incredibly lazy, genetically speaking, as a result of hitching their wagon to humans. They have a very simple genome compared to other related species, and they seem to be surviving with the bare minimum of cells and proteins needed to function (Their leg pairs are even powered by a single muscle cell each). They’ve lost the ability to survive exposure to ultraviolet light, which explains why they hunker deep down into our pores and only move and mate at night, and they don’t appear to even produce their own melatonin anymore, like many animals do—instead, they seem to mooch it from us. They’re also passed down from mother to child, often through breastfeeding, meaning that populations have relatively low genetic diversity. And their lack of natural predators, host competition, and generally sheltered existence suggests that the mites are only likely to lose more genes over time.

The researchers theorize that these trends could one day lead to the end of D. folliculorum mites as a distinct entity—a process that’s been observed with bacteria but never an animal, they say. Eventually, the mites might no longer live externally on our skin as parasites but instead become wholly internal symbiotes. If so, then we might be seeing the very beginnings of that transition taking place now, though this transformation likely wouldn’t be finished for a long time.

Regardless of the future fate of these mites, the scientists say they’re perhaps doing some good for us now. They might help clear the skin of excess dead cells and other materials, for instance, at least when their populations are kept in check. Perotti also hopes that their research will provide people “proper knowledge of these permanent companions, which have been blamed too long for our skin problems.”

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