Skin mites that mate on our faces at night are slowly merging with humans

If you are reading this, you are probably not alone.

Most people on earth are habitats for mites, which spend most of their short lives burrowing upside down in our hair follicles, mostly on the face. In fact, man is the only habitat for Demodex Folliculorum. They are born of us, they feed on us, they mate with us and they die of us.

Your entire life cycle revolves around chewing on your dead skin cells before kicking the tiny bucket.

That’s how reliable it is D. folliculorum New research suggests the microscopic mites are in the process of evolving from an ectoparasite to an internal symbiont that shares a mutually beneficial relationship with its hosts (that’s us).

In other words, these mites are gradually merging with our bodies so that they now live inside us permanently.

Scientists have now sequenced the genomes of these ubiquitous little beasts, and the results show that their human-centric existence could induce changes not seen in other mite species.

“We found that these mites have a different arrangement of genes for body parts than other similar species as they adapt to sheltered pore life,” explained invertebrate biologist Alejandra Perotti of the University of Reading in the UK.

“These changes to their DNA have resulted in some unusual body traits and behaviors.”

Demodex subpageD. folliculorum seen in a potassium hydroxide preparation of human skin. (KV Santosh/Flickr, CC BY 2.0)

D. folliculorum is actually a fascinating little creature. Human skin debris is its only source of food, and it spends most of its two-week lifespan foraging for it.

The individuals only emerge at night, under cover of darkness, to meticulously slowly crawl across the skin to find a mate and hopefully copulate before returning to the safe darkness of a follicle.

Their tiny bodies are just a third of a millimeter long, with a collection of tiny legs and a mouth at one end of a long, sausage-shaped body – just right for scraping down human hair follicles to get at the tasty nouns inside.

Work on the mite’s genome, co-led by Marin and geneticist Gilbert Smith of Bangor University in the UK, revealed some of the intriguing genetic traits that drive this lifestyle.

Because their lives are so hectic – they have no natural enemies, no competition and no contact with other mites – their genome has been stripped down to the bare essentials.

Their legs are powered by three single-celled muscles, and their bodies have the absolute minimum amount of protein, just what is needed to survive. It is the smallest number ever seen in its larger group of related species.

This reduced genome is the reason for some of D. folliculorum‘s other strange trivial offenses too. For example the reason why it only comes out at night. Among the lost genes are those responsible for protecting against UV radiation and those that wake animals up in daylight.

They are also unable to produce the hormone melatonin, which is found in most living organisms with various functions; In humans, melatonin is important for regulating the sleep cycle, but in small invertebrates it induces mobility and reproduction.

This doesn’t seem to have hindered D. folliculorum, however; It can harvest melatonin, which is secreted by its host’s skin at dusk.

Demodex folliculorum dorsal penisThat’s not convenient. (Smith et al., Mol. biol. development., 2022)

Unlike other mites, their reproductive organs D. folliculorum have moved to the front of their bodies, with male mites’ penises pointing forward and up from their backs. This means he has to position himself under the female while they perch precariously on a hair to mate, which they do all night AC/DC style (presumably).

But while mating is fairly important, the potential gene pool is very small: there are very few opportunities to expand genetic diversity. This could mean that the mites are headed for an evolutionary dead end.

Interestingly, the team also found that the mites have the largest number of cells in their bodies during the nymphal developmental stage, between larva and adults. When they transition to adulthood, they lose cells — the first evolutionary step, the researchers say, in an arthropod species’ journey to a symbiotic lifestyle.

One might wonder what possible benefits humans can derive from these particular animals; something else the researchers found may partially point to the answer. Scientists have thought so for years D. folliculorum does not have an anus but instead collects waste matter in its body which explodes out when the mite dies, causing skin diseases.

Demodex folliculorum anusThe arrow points to the mite’s anus, and now you’re probably on some sort of watch list. (University Reading)

The team found that this is simply not the case. The mites actually have tiny little assholes; Your face is unlikely to be covered in mite feces that were expelled posthumously.

“Mites have been blamed for many things,” said zoologist Henk Braig of Bangor University and National University of San Juan in Argentina. “The long association with humans may suggest that they may also play a simple but important useful role, such as keeping the pores on our face unclogged.”

The research was published in molecular biology and evolution.