Cienciaes.com: The incredible submersible louse.

2020-08-04 09:50:35

When we talk about pets, we rarely think about lice. However, these faithful bloodsuckers have been with us since the very moment that human beings appeared on Earth.
About five thousand species of lice have been identified, all of them obligate parasites, that is, forced to live at all times of their miserable existence attached to the skin of some warm-blooded animal. More than 4,000 species of lice live by infesting birds. There is, apparently, no species of bird that is free of it. Yes, penguins have lice too. No less than fifteen different species are capable of infesting them.

About 800 species of lice infest mammals. In this case, there are some that lice leave alone, among which are monotremes (platypuses and echidnas, which live in Australia), pangolins and bats.

Human beings have three types of lice: head lice, body lice, and crab lice, or pubic lice. The head louse has been with us since before humans and chimpanzees began different evolutionary paths, from their common ancestor, at least seven million years ago. Interestingly, the body louse seems to be unique to humans. This louse is, in reality, a subspecies of the head louse and begins its evolution from the moment in which the human being begins to cover his body with fur or clothing, which allows him to colonize it.

Genetic studies carried out with body and head lice have thus made it possible to discover that human beings have been covering their bodies for some 80,000 years, at the very least, perhaps even tens of thousands of years more. In any case, these studies clearly indicate that Homo sapiens dressed thousands of years before they left Africa and colonized the entire planet.
The pubic lice story is different. This one does not seem to come from the other two, but humans acquired it about 3 or 4 million years ago from a gorilla louse. It is not known, nor do I dare to imagine, how this acquisition could have happened, but a song from the last century comes to mind, by the French singer-songwriter George Brassens, titled gare au gorille, of which several versions have been produced in Spanish. The song is about the adventures of a gorilla who escapes from the zoo and if you listen carefully I’m sure it will provide you with some interesting clues.

Condemned to evolve together.

Be that as it may, just like head and body lice, once acquired from the gorilla, pubic lice begin a co-evolution with us. This is one of the most important characteristics of these insect species. Throughout their evolution, lice have had to adapt to the particular living conditions of the species they parasitize and have had to evolve together with them when those conditions changed.

This joint evolution is very evident in the case of lice that parasitize marine mammals. Although penguins also have lice, these birds do not dive to the enormous depths, nor for the long times, that certain species of seals, lions and elephant seals do. These last animals can dive up to two thousand meters deep. How have marine mammal lice adapted to their way of life?

This issue was not fully known. Various possibilities have been considered. The first is that the lice of these animals die when they dive to high depths. In order to survive, the lice of these species may have been able to adapt to reproduce rapidly and move quickly from one animal to another before drowning. After all, only the strongest adult marine animals dive to great depths; young animals do not. Another possibility is that lice leave their hosts when they submerge and may survive long enough in the ocean to parasitize another nearby animal.

Yet another possibility, however, is that marine mammal lice have undergone adaptations that allow them to survive submerged for long periods and under extremely high pressure conditions. Remember that atmospheric pressure doubles for every 10 meters of depth in seawater, so a depth of 2,000 meters means a pressure of around 200 atmospheres.

A group of Argentine researchers is now carrying out a series of interesting experiments to try to find out what kind of adaptations the species of lice on marine mammals have acquired. In them, using a hydrostatic apparatus, they subject several specimens of lice, separated from the skin of sea elephant pups, to pressures of 80 to 200 atmospheres and lack of oxygen, submerged in seawater. The lice were also subjected to sudden pressure changes, such as those that occur when marine animals rapidly submerge or rapidly rise to the surface.
The results of these experiments are clear. Adult lice, and also the nymphs of these insects, that is, insects still immature in the process of becoming adults, are capable of withstanding pressures of up to 200 atmospheres without being crushed or drowned. One of these lice was mistakenly subjected to a pressure of 450 atmospheres for several minutes, and still survived. Likewise, lice tolerate sudden changes in pressure. These high pressures and changes are tolerated independently, that is, they are autonomous adaptations that do not depend on the protection that they could somehow receive from their host’s skin.

At the moment, the authors can only speculate about the reasons for such amazing properties of sea lice. They mention three: The presence of small scales on these lice, which could provide mechanical resistance to crushing, the ability to slow down their metabolism to reduce oxygen consumption, and the ability to absorb oxygen from seawater. New studies will be necessary to determine exactly the role of these potential adaptations, as well as the genes responsible for them.

Referencia: Maria Soledad Leonardi et al (2020). Under pressure: the extraordinary survival of seal lice in the depth of the sea. Journal of Experimental Biology.

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