Skin mites that mate in our faces at night slowly blend in with humans

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

Most people on earth are habitats for mites that spend most of their short lives buried, with their heads first, in our hair follicles, primarily in their faces. In fact, humans are the only habitat for Demodex folliculorum. They are born on us, they live on us, they mate on us, and they die on us.

Their entire life cycle is about gnawing on your dead skin cells before kicking the tiny little bucket.

So dependent is D. folliculorum on humans for their survival, new research suggests that the microscopic mites are evolving from an ectoparasite to an internal symbiont – and one that shares a mutually beneficial relationship with their hosts (it’s us).

In other words, these mites gradually merge with our bodies so that they now live permanently in us.

Scientists have now sequenced the genomes of these ubiquitous little beasts, and the results show that their human-centered existence can create changes not seen in other species.

“We found that these mites have a different arrangement of body delgenerates than other similar species because they adapt to a sheltered life inside the pores,” explained invertebrate biologist Alejandra Perotti at the University of Reading in the UK.

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

demodex undersideD. 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. Detritus from human skin is its only food source, and it spends most of its two weeks of life hunting for it.

Individuals appear only at night, in the cover of darkness, to crawl painfully slowly across the skin to find a mate, and hopefully copulate before returning to the safe darkness of a follicle.

Their tiny bodies are only a third of a millimeter long, with a cluster of tiny bones and a mouth at one end of a long, sausage-shaped body – just right to scrub down human hair follicles to get the tasty names there.

The work on the mite’s genome, led by Marine and geneticist Gilbert Smith from Bangor University in the UK, revealed some of the fascinating genetic traits that drive this lifestyle.

Because their lives are so tough – they have no natural predators, no competition and no exposure to other mites – their genome has been reduced to just the essentials.

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

This reduced genome is the cause of some of D. folliculorumtheir other strange peccadilloes too. For example, the reason it only comes out at night. Among the genes that are lost are those that are responsible for protection against UV radiation, and those that wake animals in daylight.

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

This did not seem to have hindered D. folliculorum, men; it can harvest melatonin which is excreted from the skin of the host at dusk.

demodex folliculorum dorsal penisThis is not practical. (Smith et al., Mol. Biol. Evol., 2022)

Unlike other mites, their reproductive organs of D. folliculorum has moved towards the front of the body, with the male mite’s penis pointing forward and upward from the back. This means that he has to arrange under the female while they sit insecurely on a hair for mating, something they do all night, AC / DC style (presumably).

However, although mating is quite important, the potential gene pool is very small: there is very little chance of expanding genetic diversity. This may mean that the mites are heading towards an evolutionary dead end.

Interestingly, the team also found that at the nymph stage of development, between larva and adult, the mites are the largest number of cells in the body. As they progress to adulthood, they lose cells – the first evolutionary step, the researchers said, in the march for an arthropod species to a symbiotic lifestyle.

One may wonder what possible benefits humans can get from these peculiar animals; anything else the researchers found may partly suggest the answer. For years, scientists have believed it D. folliculorum does not have an anus, but instead collects waste in the body to explode when the mite dies, thus causing skin diseases.

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

The team found that this is simply not the case. The mites actually have tiny little assholes; Your face is probably not full of mite poop expelled posthumously.

“Mid has been blamed for many things,” said zoologist Henk Braig of the University of Bangor and the National University of San Juan in Argentina. “The long association with people may indicate that they may also have simple but important beneficial roles, such as keeping the pores of the face disconnected.”

The research is published in Molecular biology and evolution.