Parasites are not necessarily harmful to their hosts and are often one of the main driving forces for the functioning of a particular habitat. Aliens can change the behavior of their hosts and force them to take actions that fundamentally affect the lives of other wild animals.
Adultery, or parasitism, is one of the greatest inventions of earthly life, although victims of adultery would probably disagree with this view. In any case, the indisputable fact is that in terms of the variety of life forms, there have not been many bigger breakthroughs than when one organism discovered that the interior or surface of another organism can serve as its environment for life.
Alien as a weapon
The word parasite is often understood as a synonym for small, rapidly multiplying and, moreover, bloodthirsty creatures. However, many aliens are not small and grow to respectable sizes. For example, the alien worm Placentonema gigantissimum lives in the placenta of whales, which measures only a third of a millimeter on average, but grows to a length of almost nine meters. Even the fertility of illegitimate people does not have to be dizzying. For example, ladybugs (Cyclopoida) parasitizing on the surface of the bodies of some aquatic invertebrates lay only a few dozen eggs in their lifetime.
Even with the harmfulness of the parasites, it is not so clear-cut, because infection with a parasite sometimes carries a clear advantage. For example, North American white-tailed deer (Odocoileus virginianus) are often infected with the parasitic worm of the species Parelaphostrongylus tenuis and tolerate the parasite attack without serious consequences. The fact that the alien belt is present in their bodies becomes evident only when American elk (Alces alces) try to penetrate the territory densely populated by deer. They are, on the other hand, highly sensitive to the worm, and when they come into contact with the eggs of an alien worm, which deer excrete with their droppings, it has fatal consequences for them. This is how deer use their own girdle to keep an unpleasant elk competitor away from their body.
Thus, an alien belt can completely reverse the outcome of a competition between two different species. Professionally, such help is called parasitic arbitrage, and it can be seen, for example, in cases where populations of the brown woodworm (Tribolium castaneum) and the warehouse woodworm (Tribolium confusum) collide. If it is a community of healthy rivals, the winner is usually the warehouse clerk, who completely displaces the competitor. If both beetles are infected with the single-celled parasite Adelina tribolii, their clash will be the opposite, and the brown beetle will be on top.
Just no nonsense
How parasites can completely affect the lives of their hosts is well documented by the invasive arthropod crab roothead (Sacculina carcini), which attacks sea crabs. The very invasion of the parasite is interesting. It cannot penetrate the hard outer shell, so it crawls along the surface of the crab’s body in search of a joint. However, even the joint gap is too narrow for it, so the roothead drops its own solid shell and pushes its way to safety under the carapace. Under the influence of the growing parasite, it gradually swells and forms a sac-like formation called externa.
Infected crabs do not grow or shed their shells. This is because the parasite prevents the host from spending energy and nutrients on activities that the rhizome itself does not need. Thanks to this, many sedentary animals remain on the surface of the crab’s body for a long time. Infected crabs offer these creatures a permanent home, and the presence of the parasite significantly changes the habitat in which the crab lives and contributes to the increase in its species richness.
The crab root head can secure an abundant supply of nutrients with yet another insidious trick. If the parasite infects a male crab, it quickly disrupts the hormone balance in his body and damages the gonads to the extent that it is practically equivalent to castration. The crab then does not waste energy and nutrients on the production of sex cells, and these reserves are again available to the roothead. A neutered male crab will even begin to change into female form, and if he meets a real male, he will perform hints of female courtship rituals. After a certain period of time, the ectoderm fills practically the entire crab’s bottom and also the external pouch. This is located on the bottom of the body in the place where the female crab normally lays a clutch of eggs. The male transformed into a female then takes care of this attachment in the same way as a real female takes care of her eggs.
Suicidal fish urge
Some outlaws have a pretty simple life. For example, female mosquitoes suck the blood of their victims and don’t have to worry too much about who they let in the vein. Other parasites, on the other hand, go through a complex metamorphosis, during which they switch between different types of hosts. Their main problem is the correct „transition“ from one victim to another. More than once they achieve this by influencing the behavior of their current host.
