Parasitic relationships are those in which one species of plant or animal benefits at the expense of the other, sometimes without killing the host organism. It is a widespread phenomenon in nature that affects almost every creature on the planet, either as hosts or as parasites. Parasites are expensive to their hosts since they divert resources for their development, reproduction, and survival with no compensation for the hosts.
Parasitism has evolved in viruses, bacteria, protozoa, invertebrates, and vertebrate metazoans. Hence it is not limited to a few taxonomic groupings. Of course, this taxonomic richness is accompanied by a wide range of life cycles, host exploitation techniques, transmission modalities, and virulence levels.
Given the high cost of parasitism, hosts are likely to develop defence mechanisms to reduce parasitism’s negative impact on their fitness. Hosts have evolved several morphological adaptations that support this viewpoint.
Parasitism is a connection between two species in which one organism (parasite) lives on or within the other organism (host) and causes damage to the host. A parasite diminishes the fitness of its host while increasing its own, generally by getting food and shelter.
“Parasite” is derived from the Greek word parasitos, which means “one who feeds at another’s cost.” Parasitology is the study of parasites and parasitism.
Parasites can be found in every biological kingdom (animals, plants, fungi, protozoa, bacteria, viruses). Every parasite in the animal kingdom has a free-living equivalent. Mosquitoes, mistletoe, roundworms, all viruses, ticks, and the protozoan that causes malaria are examples of parasites.
Types of Parasitism
Parasitism is divided into several categories depending on its size, features, interactions with its hosts, and life cycles.
On the basis of the life cycle
1. Obligate parasitism: This is parasitism in which the parasite’s life cycle is entirely dependent on the host. Without the host, obligatory parasites would perish. As a result, they do not cause significant harm to the host. Obligate parasitism is seen in fungi, bacteria, and viruses. For instance, head lice, when removed from the human scalp, die.
2. Facultative parasitism: In this parasitism, the parasite is not fully dependent on the host to complete its life cycle and may exist without the host. Strongyloides stercoralis is a free-living worm that infects people and produces the illness strongyloidiasis.
On the basis of habitat
1. Ectoparasitism: It refers to parasites that dwell outside of the host’s body. Lice and ticks are examples.
2. Endoparasitism: Endoparasitism describes parasites that dwell inside the host’s body. Hookworms and nematodes are examples. Endoparasites are either intercellular, which means inhabiting spaces in the host’s body or intracellular, which implies inhabiting cells in the host’s body.
3. Mesoparasitism: Mesoparasitism describes parasites that penetrate the host’s exterior apertures.
On the basis of their strategies
1. Directly Transmitted Parasites: Directly transmitted parasites are parasites that reach their hosts’ bodies independently. Fleas and mites are two examples.
2. Trophically Transmitted Parasites: Trophically transmitted parasites are parasites that enter hosts’ bodies when they devour them. Trematodes and roundworms are two examples.
3. Vector Transmitted Parasites: Vector transmitted parasites rely on an intermediate host to transport them to their final destination. One example is a protozoan that causes sleeping sickness and is spread by insect bites.
On the basis of damage caused
1. Necrotrophic parasites: These are parasitoids and devour a portion of the host organism’s tissue until it dies from tissue loss or food deficiency.
2. Biotrophic parasites: These do not inflict enough harm to their hosts to kill them; they need to keep the host alive since they cannot exist in a dead one.
On the basis of a number of hosts
1. Monogenic parasites live in only one host and finish their life cycle there.
2. Digenetic parasites: Digenetic parasites require many hosts to complete their life cycle. Plasmodium vivax, the malaria-causing protozoa, is digenetic. It must be a parasite of both people and mosquitos to complete its life cycle.
On the basis of size
1. Macroparasites: These are parasites that are large enough to be seen with the naked eye.
2. Microparasites: These are too small to be seen and must be viewed under a microscope. They are generally unicellular, such as protozoa.
1. Brood parasitism: It is a kind of parasitism in which young parasites are nurtured by their hosts. Cuckoo is a good example. Most cuckoo species and cowbirds engage in particular parasitism known as brood parasitism. Rather than building their nests, these birds lay their eggs in the nests of other species and abandon them, hoping that adult birds of other species will nurture the abandoned young as their own. The cowbird’s parasitism is not involved in harming the host’s brood or the host; however, the cuckoo may remove one or more host eggs to lessen suspicion of its existence, and the juvenile cuckoo may heave the host’s eggs and nestlings from the nest.
2. Kleptoparasitism: Kleptoparasitism is a parasitic infection where the parasite steals the host’s food. Example: Food theft by skuas
3. Sexual parasitism: It is a kind of parasitism in which males are completely reliant on females for existence. Anglerfish is an example.
Parasites in humans
1. Protozoa: These are single-celled creatures that can replicate in humans. Infected food and drink, person-to-person contact, and insect bites are all ways for these parasites to spread. Plasmodium malaria, which causes malaria, and Cryptosporidium, which is ingestible, are examples of protozoa.
2. Helminths: These are parasitic worms commonly live in the human digestive tract. They cannot proliferate or divide within the human body, and they finally pass through the faeces. Examples of Helminths are Ascaris lumbricoides and hookworm parasites.
3. Ectoparasites: These are microscopic creatures that dwell on the exterior of a person’s body. Ticks, fleas, and lice are examples of these pests. These live on the host’s body surface and do not commonly cause disease in the host.
Parasites in plants
Parasites thrive in the presence of plants. To begin with, they don’t move too much. Plants are nutrient-dense, and if the parasite is tiny enough, they can also provide refuge. As a result, it’s no surprise that various plant parasites exist.
1. Aphids: Aphids feed on plant sap, frequently in excess, for the health of the plant they infest. Aphids are small and soft-bodied, making them easy prey for predators such as ladybugs, but they also have defences. Aphids are occasionally raised for their sweet secretions by ants, who guard them while they eat.
2. Gall Wasp: Cynipidae is a family of non-stinging wasps that create galls on plants. Galls on plants are essentially tumors, large balls of worthless tissue. Even though galls are worthless to plants, wasps use them to deposit their eggs, providing nourishment and shelter for their developing young.
3. Mistletoe: “Santalales” is a Christmas plant with some unfavorable eating habits. It connects to a host plant by a haustorium, a unique root that drains the host of water and nutrients. Mistletoe is parasitic, but it’s also poisonous.
Parasites in insects
Insects that parasitize other insects are known as entomophagous parasites. These parasites usually affect larvae or young insects. Some insects lay their eggs within the larvae of other insects; when the eggs hatch, the parasitic young kill and consume the larvae, receiving nourishment. The parent parasite may paralyse a host, which is subsequently consumed by the young.
This is frequent in wasps like Ampulex compressa, whose larvae feed immobilized cockroaches after the parent has stung them. Other wasps, such as Ropalidia romandi, burrow into their hosts’ abdomens. They don’t kill their hosts, but they can alter their look and behavior and render them sterile. Insect parasitism is quite prevalent. Almost all insect species are affected by at least one form of the parasite.
Parasites in fish
Various creatures parasitise fish, and different populations of the same species of fish living in the same region may sometimes be distinguished by their parasites. Copepods (small crustaceans), nematodes, and leeches are examples of parasitic organisms. Attach themselves to the gills of the fish and dwell there. Cymothoa exigua is a parasitic isopod (a tiny crustacean that feeds on fish). It gets into a fish’s mouth and rips the fish’s tongue out.
The isopod then takes up residence where the tongue once resided and becomes the new tongue. Although the host fish can feed and survive with an isopod in its mouth, the isopod eats a little bit of the fish’s blood and mucus while it is there. Bluestreak cleaner wrasses are cleaner fish. Bluestreak cleaner wrasses and other cleaner fish remove dead skin and parasites from other fish, even huge predatory fish that would otherwise consume them.
When individuals eat dishes that include uncooked fish, such as sushi, the parasites in these fish might infect humans, posing a health risk. However, in the developed world, illness from raw fish is uncommon, and some raw seafood is frozen overnight to avoid infection.
Parasites are “omnipresent agents of natural selection” and major factors in evolution and ecology. It is one of the important chapters of the CBSE Class 12 Biology. Parasites are difficult to place in food webs because of their complicated relationships: a trematode with several hosts for its various life cycle phases would hold many positions in a food web simultaneously, creating energy flow loops that would confuse the analysis. Parasites would also inhabit the top levels of every food web because practically every animal has (many) parasites.
Parasites contribute to the structure of biological ecosystems, as evidenced by their major involvement in food webs, competitive interactions, biodiversity patterns, and the management of keystone species. Despite this, scientists have just scratched the surface of parasites’ complicated involvement in community ecology. Increased disease appearance in animals, which might have ramifications for human health and well-being, makes this a particularly important moment to better integrate parasitism into community ecology and comprehend parasite functions in nature.