Parasitism is a type of symbiosis where one partner benefits at the expense of another. It takes many forms and is common throughout the plant and animal kingdoms. A few examples are examined below.
Parasites live at the expense of their hosts, consuming resources, fluids and tissues to facilitate survival and reproduction. Some are obligate parasites, meaning that they die or fail to complete their life cycle when removed from their host. Others, known as facultative parasites may survive separately as free-living organisms.
Parasites come in a range of sizes but are generally classed either as macro or microparasites. The first may be seen by the unaided eye, and the second cannot. Many familiar parasites such as mites and bedbugs are macroparasites. Microparasites include tiny organisms such as bacteria and protozoa.
Some parasites are digenetic and require more than one host to complete their lifecycles. Malaria and tapeworms are two common examples. Parasites needing only once host are called monogenic. Parasites may also be classified by where and how they live.
Ectoparasites live on the outside of a host's body. Fleas, ticks and lice are common and familiar examples. Plants too suffer from ectoparasites. These include insects such as aphids, fungi and even other plants.
Endoparasites live inside of a host's body. Some are introduced via vectors such as fleas or mosquitos. Others may be accidentally consumed or picked up from the environment. There are two subtypes of endoparasites, intracellular and intercellular. Intracellular parasites live in the cells of the host. Bacteria of the genus Rickettsia are a potentially devastating example and cause diseases such as spotted fevers and typhus. Intercellular parasites live in the body of the host, but outside of the cells. Tapeworms, hookworms and roundworms are common and debilitating intercellular parasites.
Plants may also be infested by endoparasites such as nematodes. Many nematodes are beneficial, but those that infest plants may kill or stunt growth, with a potentially negative impact on crops.
Mesoparasites live in the body openings of the host such as the mouth, ear or cloaca. While these are often microparasites, such as yeasts and other fungi, some can be relatively large. A startling example isCymothoa exigua or tongue-eating louse. This crustacean enters a fish's mouth, severs its tongue and anchors itself where the tongue had been. It remains there for the life of the fish who continues to eat and live as before. The parasite consumes the blood and mucus of its host.
Brood parasites occur among egg-laying species such as birds, insects and fish. The parasite species doesn't feed directly on its host, but it uses the host's resources and energy to raise its young. The cuckoo is the best-known example. A female cuckoo lays an egg in the nest of another bird with similar appearing eggs. The egg hatches, and the young cuckoo is raised by unsuspecting foster parents. Often the cuckoo hatches first and dominates its "siblings" or even push them from the nest.
Social parasites live among social insects such as termites, ants and bees. They are similar to brood parasites, but they take advantage of an entire colony rather than just an individual. Many insects have evolved to mimic the appearance of another species. This allows them to enter the nest, lay eggs and leave them to be reared by the colony. Once again, the parasite passes on its genes with little expenditure of its own.
Parasitism is a successful and fascinating evolutionary strategy.