Parasitism

Parasitism is a close biological relationship between two different species. In this interaction, one organism acts as the parasite, while the other is called the host. The parasite lives on or inside the host for a period of time to gain resources. This relationship is harmful to the host, which may suffer from disease or reduced fitness.

Unlike predators that kill their prey immediately, most parasites do not kill their hosts right away. The entomologist E. O. Wilson described them as "predators that eat prey in units of less than one." This means they take only small portions of the host to survive.

Parasites use various biological strategies to exploit their hosts. One major way to classify them is by where they live. Endoparasites live inside the host's body, such as intestinal tapeworms.

Ectoparasites live on the external surface of the host, like lice or ticks. Some organisms, called mesoparasites, enter an opening in the host and remain partly embedded.

Parasites can also be classified by their life cycles. An obligate parasite must use a host to complete its life cycle. In contrast, a facultative parasite can live without a host if it chooses.

There are six major parasitic strategies used to exploit animal hosts. Parasitic castration is one extreme method where the parasite destroys the host's ability to reproduce.

For example, the barnacle genus Sacculina attacks the gonads of crabs. This diverts the crab's energy away from making offspring and toward growing the parasite. Another strategy is trophically-transmitted parasitism. In this case, the parasite is passed to a new host when the current host is eaten.

This often involves complex life cycles with both intermediate and definitive hosts.

Other strategies include vector-transmitted parasitism and direct transmission. Vector-transmitted parasites rely on a third party, like a mosquito, to carry them between hosts.

For instance, the Plasmodium parasite causes malaria and is moved by biting insects.

Direct transmission occurs when parasites move through simple contact without a third party. This includes many lice and mites that live on animals. Finally, parasitoidism involves insects that eventually kill their hosts, and micropredation involves small, brief attacks.

Parasites are often highly specialized to live on specific hosts. Many species are host-specific, meaning they can only survive on one particular type of animal. To find these hosts, parasites look for specific "host cues." These cues can be chemical or physical signals like heat, moisture, or exhaled carbon dioxide.

Some parasites are also divided by size into microparasites and macroparasites. Microparasites are tiny organisms like viruses or protozoans that reproduce inside the host. Macroparasites are larger, multicellular organisms that often reproduce outside the host body.

Human history shows a long awareness of these organisms. Ancient civilizations in Egypt, Greece, and Rome recognized parasites like roundworms. In 1681, Antonie van Leeuwenhoek used his microscope to observe the protozoan Giardia lamblia. Later, Francesco Redi provided detailed descriptions of parasites like ticks and the sheep liver fluke.

The formal study of parasitology grew significantly during the 19th century as scientific tools improved.

Parasitism also appears frequently in human culture and literature. In 1733, Jonathan Swift used the concept in a poem to satirize poets. In the realm of fiction, the idea is often used to create tension or horror. Bram Stoker's 1897 novel Dracula features a parasite that drinks blood.

Science fiction films, such as the 1979 movie Alien, also explore parasitic life forms. These cultural examples show how the concept of one being living off another is deeply embedded in our imagination.