In biology, parasitism is a non-mutual relationship between species, where one species, the parasite, benefits at the expense of the other, the host. Traditionally parasite primarily meant an organism visible to the naked eye, or a macroparasite (such as a helminth). Microparasites are typically far smaller, such as protozoa, viruses, and bacteria. Examples of parasites include the plants mistletoe and cuscuta, and animals such as hookworms.
Unlike predators, parasites typically do not kill their host, are generally much smaller than their host, and often live in or on their host for an extended period. Both are special cases of consumer-resource interactions. Parasites show a high degree of specialization, and reproduce at a faster rate than their hosts. Classic examples include interactions between vertebrate hosts and tapeworms, flukes, the Plasmodium species, and fleas. Parasitoidy is an evolutionary strategy within parasitism in which the parasite eventually kills its host.
Parasites reduce host biological fitness by general or specialized pathology, from parasitic castration and impairment of secondary sex characteristics, to the modification of host behavior. Parasites increase their own fitness by exploiting hosts for resources necessary for their survival, in particular transmission. Although parasitism often applies unambiguously, it is part of a continuum of types of interactions between species, grading via parasitoidy into predation, through evolution into mutualism, and in some fungi, shading into being saprophytic.
People have known about parasites such as roundworms and tapeworms since ancient Egypt, Greece, and Rome. In Early Modern times, Antonie van Leeuwenhoek observed Giardia lamblia in his microscope in 1681, while Francesco Redi described endo- and ectoparasites including sheep liver fluke and ticks. Modern parasitology developed in the 19th century. In human culture, parasitism has negative connotations. These were exploited to satirical effect in Jonathan Swift‘s 1733 poem “On Poetry: A Rhapsody”, comparing poets to hyperparasitical “vermin”. In fiction, Bram Stoker‘s 1897 Gothic horror novel Dracula and its many later adaptations featured a blood-drinking parasite. Ridley Scott‘s 1979 film Alien was one of many works of science fiction to feature a terrifying parasitic alien species.
There are six major evolutionary strategies within parasitism. These apply to parasites whose hosts are plants as well as animals:
- Parasitic castrators feed on their host’s reproductive tissues, leaving other bodily processes largely intact, and therefore ensuring the host’s survival and the freedom of the parasite to remain in the host body for as long as the host continues to live.
- Directly transmitted parasites rely on happenstance encounters with members of their host species to feed and reproduce. They may spread from one host to another through skin-to-skin contact, or lie dormant until a host steps on or brushes against them.
- Trophically transmitted parasites have a life cycle involving two or more hosts. In their juvenile stage, they infect and often encyst in an animal; this is the intermediate host. When this animal is eaten by a predator, the parasite survives the digestion process and matures into an adult. This predator thus become the definitive host for the parasite. Some parasites are capable of modifying the behavior of their intermediate hosts in order to increase the chances of being eaten by a predator.
- Vector-transmitted parasites rely on a third party to carry them from one host to another. These are often microscopic non-animal parasites, namely protozoa, bacteria, or viruses, and their vectors are often parasitic arthropods such as fleas, lice, ticks and mosquitoes.
- Parasitoids kill their hosts and thus migrate between hosts frequently.
- Micropredators actively hunt for hosts in the manner of traditional predators. Micropredators choose hosts that are large but helpless or ineffective in resisting the attack. For example, a mosquito attacks animals too slow to protect themselves from the bite. Similarly, phytophagous scale insects, aphids, and caterpillars attack much larger plants, and serve as vectors of bacteria, fungi and viruses causing plant diseases, and plants defoliated by caterpillars may die, as in parasitoidy. Female scale insects are unable to move, so they are obligate parasites, permanently attached to their hosts.
These strategies for successful parasitism are adaptive peaks; many intermediate strategies are possible, but organisms in many different groups have consistently converged on these six, which are evolutionarily stable.
Parasites that live on the outside of the host, either on the skin or the outgrowths of the skin, are called ectoparasites. They are directly transmitted between hosts. Examples include lice, fleas, and some mites.
Those that live inside the host, including all parasitic worms (helminths), are called endoparasites. Endoparasites can exist in one of two forms: intercellular parasites (inhabiting spaces in the host’s body) or intracellular parasites (inhabiting cells in the host’s body). Coinfection by multiple parasites is common.
Autoinfection is the infection of a primary host with a parasite, particularly a helminth, in such a way that the complete life cycle of the parasite happens in a single organism, without the involvement of another host. This can occur with the intestinal parasite Strongyloides stercoralis. Strongyloidiasis involves premature transformation of noninfective larvae into infective larvae, which can penetrate the intestinal mucosa (internal autoinfection) or the skin of the perineal area (external autoinfection).
Those parasites living in an intermediate position, being half-ectoparasites and half-endoparasites, are called mesoparasites. For example, the cod worm Lernaeocera branchialis invades the gill tissue of a host fish, but quickly grows a tubelike structure that cuts through the body tissues until it reaches the fish’s heart, through which it robs the host of its blood. The rear end of the parasite remains outside, so that it can scatter eggs into the water.
A parasitoid sooner or later kills its prey, so this form of parasitism is close to predation. Idiobiont parasitoid wasps sting their prey on capture, either killing them outright or paralyzing them immediately. The prey is then carried to a nest, an egg is laid on or in it, and the parasitoid develops rapidly. Koinobiont parasitoid wasps lay their eggs in young hosts, usually larvae, which are allowed to go on growing, so the host and parasitoid develop together for an extended period. Some koinobionts regulate their host’s development hormonally, for example preventing it from pupating or making it moult whenever the parasitoid is ready to moult.
A hyperparasite or epiparasite feeds on another parasite, as exemplified by a protozoan living in a helminth parasite. The term is used slightly more loosely to refer also to parasitoids whose hosts are either parasites or parasitoids. Hyperparasitoids may be facultative or obligate, and the young may develop inside or outside the host’s body, usually a larva.
Social parasites take advantage of interactions between members of social organisms such as ants, termites, and bumblebees. Examples include the large blue butterfly, Phengaris arion. Its larvae employ mimicry to parasitize certain species of ants, Bombus bohemicus, a bumblebee which invades the hives of other species of bee and takes over reproduction, their young raised by host workers, and Melipona scutellaris, a eusocial bee whose virgin queens escape killer workers and invade another colony without a queen. An extreme example of social parasitism is the ant species Tetramorium inquilinum of the Alps, which lives exclusively on the backs of other species of Tetramorium host ants. With tiny and weakened bodies, they have evolved for a single task: holding on to their host, since if they fall off, they will die.
In kleptoparasitism (from Greek κλέπτης (kleptes), thief), parasites appropriate food gathered by the host. An example is the brood parasitism practiced by cowbirds, whydahs, cuckoos, and black-headed ducks which do not build nests of their own and leave their eggs in nests of other species. The host behaves as a “babysitter” as they raise the young as their own. If the host removes the cuckoo’s eggs, some cuckoos return and attack the nest to compel host birds to remain subject to this parasitism.
Intraspecific social parasitism may also occur, as in parasitic nursing, where some individual young take milk from unrelated females. In wedge-capped capuchins, higher ranking females sometimes take milk from low ranking females without any reciprocation. The high ranking females benefit at the expense of the low ranking females.
Parasitism can take the form of isolated cheating or exploitation among more generalized mutualistic interactions. For example, broad classes of plants and fungi exchange carbon and nutrients in common mutualistic mycorrhizal relationships; however, some plant species known as myco-heterotrophs cheat by taking carbon from a fungus rather than donating it.
An adelpho-parasite (from Greek αδελφός (adelphos), brother) is a parasite in which the host species is closely related to the parasite, often being a member of the same family or genus. An example of this is the citrus blackfly parasitoid, Encarsia perplexa, unmated females of which may lay haploid eggs in the fully developed larvae of their own species. These result in the production of male offspring. The marine worm Bonellia viridis has a similar reproductive strategy, although the larvae are planktonic.
In many animals, males are much smaller than females. In some species of anglerfish, such as Ceratias holboelli, the males are so small they have become sexual parasites, wholly dependent on females of their own species for survival, and unable to fend for themselves. The female nourishes the male and protects him from predators, while the male gives nothing back except the sperm that the female needs to produce the next generation.
A parasitic plant derives some or all of its nutritional requirements from another living plant. They make up about 1% of angiosperms and are in almost every biome in the world. All parasitic plants have modified roots, named haustoria (singular: haustorium), which penetrate the host plants, connecting them to the conductive system – either the xylem, the phloem, or both. This provides them with the ability to extract water and nutrients from the host. Parasitic plants are classified depending on where the parasitic plant latches onto the host and the amount of nutrients it requires. Some parasitic plants are able to locate their host plants by detecting chemicals in the air or soil given off by host shoots or roots, respectively. About 4,500 species of parasitic plant in approximately 20 families of flowering plants are known.
Species within Orobanchaceae (broomrapes) are some of the most economically destructive species on Earth. Species of Striga (witchweeds) are estimated to cost billions of dollars a year in crop yield loss annually, infesting over 50 million hectares of cultivated land within Sub-Saharan Africa alone. Striga infects both grasses and grains, including corn, rice and Sorghum, undoubtedly some of the most important food crops. Orobanche also threatens a wide range of important crops, including peas, chickpeas, tomatoes, carrots, and varieties of the genus Brassica (cabbages). Yield loss from Orobanche can reach 100%; despite extensive research, no method of control has been entirely successful.
Parasitic fungi derive some or all of their nutritional requirements from plants, other fungi, or animals, and unlike mycorrhizal fungi which have a mutualistic relationship with their host plants, they are pathogenic. For example, the honey fungi in the genus Armillaria grow in the roots of a wide variety of trees, and eventually kill them. They then continue to live in the dead wood, feeding saprophytically.
Many bacteria are parasitic, though since the result is infection and disease, sometimes leading to death, they are generally thought of as pathogens instead. Parasitic bacteria are extremely diverse, and infect their hosts by a variety of routes. To give a few examples, Bacillus anthracis, the cause of anthrax, is spread by contact with infected domestic animals; the bacillus’s spores, which can survive for years outside the body, can enter a host through an abrasion or may be inhaled. Borrelia, the cause of Lyme disease and relapsing fever, is transmitted by a vector, ticks of the genus Ixodes, from the diseases’ reservoirs in animals such as deer. Campylobacter jejuni, a cause of severe enteritis (gut inflammation), is spread by the fecal-oral route from animals, or by eating insufficiently cooked poultry, or by contaminated water. Haemophilus influenzae, an agent of bacterial meningitis and respiratory tract infections such as influenza and bronchitis, is transmitted by droplet contact. Treponema pallidum, the cause of syphilis, is spread by sexual intercourse.
Viruses are obligate intracellular parasites, characterized by extremely limited biological function, to the point where, while they are evidently able to infect all other organisms from bacteria and archaea to animals, plants and fungi, it is unclear whether they can themselves be described as living. Viruses consist of a strip of genetic material (DNA or RNA), covered in a protein coat and sometimes a lipid envelope. They thus lack all the usual machinery of the cell such as enzymes, relying entirely on the host cell’s ability to replicate DNA and synthesise proteins. Most viruses are bacteriophages, infecting bacteria, and it is possible that viruses are both extremely ancient, being at least as old as the first cells, and polyphyletic, having evolved from several entirely unrelated ancestors.
Competition favoring virulence
Competition between parasites can be expected to favor faster reproducing and therefore more virulent parasites, by natural selection. Parasites whose life cycle involves the death of the host, to exit the present host and sometimes to enter the next, evolve to be more virulent, and may alter the behavior or other properties of the host to make it more vulnerable to predators.
However, among competing parasitic insect-killing bacteria of the genera Photorhabdus and Xenorhabdus, virulence depended on the relative potency of the antimicrobial toxins (bacteriocins) produced by the two strains involved. When only one bacterium could kill the other, the other strain was excluded by the competition. But when caterpillars were infected with bacteria both of which had toxins able to kill the other strain, neither strain was excluded, and their virulence was less than when the insect was infected by a single strain.
Conversely, parasites whose reproduction is largely tied to their host’s reproductive success tend to become less virulent or mutualist, so that their hosts reproduce more effectively.
Parasite ecology is complex, usually involving hosts that have multiple parasites (multi-parasite hosts), parasites that have multiple hosts (multi-host parasites), and competition within a host. These interactions affect parasite and host reproduction and therefore evolution, including of the virulence of parasites and of methods of transmission. Reviewing the field, T. Rigaud and colleagues noted in 2010 that among the outcomes demonstrated empirically are that multiple infection can affect virulence, and can trigger evolutionary change; that the parasites involved in the same host may have contrasting transmission modes; and that assemblages of multiple parasites can be more virulent than would be expected from each individual parasite. Rigaud and colleagues also consider trade-offs of virulence among hosts (that adaptation for higher virulence in one host means lower virulence in others), predicting that when there are many hosts, the outcome will be non-specialist parasites with relatively low virulence. Where hosts differ in quality, Rigaud and colleagues predict that parasites should become optimally virulent in their primary host. On the other hand, they also predict that the more diverse the host community, the lower the incidence of parasites should be, because infecting hosts that are unsuitable or resistant means the loss of those parasites (wasted transmission).
Symbiosis (from Greek συμβίωσις “living together”, from σύν “together” and βίωσις “living“) is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasitic. The organisms may be of the same or of different species. In 1879, Heinrich Anton de Bary defined it as “the living together of unlike organisms”.
Symbiosis can be obligatory, which means that one or both of the symbionts entirely depend on each other for survival, or facultative (optional) when they can generally live independently.
Symbiosis is also classified by physical attachment; symbiosis in which the organisms have bodily union is called conjunctive symbiosis, and symbiosis in which they are not in union is called disjunctive symbiosis. When one organism lives on another such as mistletoe, it is called ectosymbiosis, or endosymbiosis when one partner lives inside the tissues of another, as in Symbiodinium in corals.
In 1877, Albert Bernhard Frank used the term symbiosis which previously had been used to depict people living together in community to describe the mutualistic relationship in lichens. In 1879, the German mycologist Heinrich Anton de Bary defined it as “the living together of unlike organisms.” The definition has varied among scientists with some advocating that it should only refer to persistent mutualisms, while others thought it should apply to any type of persistent biological interaction in other words mutualistic, commensalistic, or parasitic.
After 130 years of debate, current biology and ecology textbooks use the latter “de Bary” definition or an even broader definition where symbiosis means all species interactions, and the restrictive definition where symbiosis means only mutualism is no longer used.
Obligate versus facultative
Symbiosis relationships can be obligate, meaning that one or both of the symbionts entirely depend on each other for survival. For example, in lichens, which consist of fungal and photosynthetic symbionts, the fungal partners cannot live on their own.
The algal or cyanobacterial symbionts in lichens, such as Trentepohlia, can generally live independently, and their symbiosis is, therefore, facultative (optional).
Endosymbiosis is any symbiotic relationship in which one symbiont lives within the tissues of the other, either within the cells or extracellularly. Examples include diverse microbiomes, rhizobia, nitrogen-fixing bacteria that live in root nodules on legume roots; actinomycete nitrogen-fixing bacteria called Frankia, which live in alder root nodules; single-celled algae inside reef-building corals; and bacterial endosymbionts that provide essential nutrients to about 10%–15% of insects.
Ectosymbiosis, also referred to as exosymbiosis, is any symbiotic relationship in which the symbiont lives on the body surface of the host, including the inner surface of the digestive tract or the ducts of exocrine glands. Examples of this include ectoparasites such as lice, commensal ectosymbionts such as the barnacles which attach themselves to the jaw of baleen whales, and mutualist ectosymbionts such as cleaner fish.
Mutualism or interspecies reciprocal altruism is a relationship between individuals of different species where both individuals benefit. In general, only lifelong interactions involving close physical and biochemical contact can properly be considered symbiotic. Mutualistic relationships may be either obligate for both species, obligate for one but facultative for the other, or facultative for both.
A large percentage of herbivores have mutualistic gut flora to help them digest plant matter, which is more difficult to digest than animal prey. This gut flora is made up of cellulose-digesting protozoans or bacteria living in the herbivores’ intestines. Coral reefs are the result of mutualisms between coral organisms and various types of algae which live inside them. Most land plants and land ecosystems rely on mutualisms between the plants, which fix carbon from the air, and mycorrhyzal fungi, which help in extracting water and minerals from the ground.
An example of mutual symbiosis is the relationship between the ocellaris clownfish that dwell among the tentacles of Ritteri sea anemones. The territorial fish protects the anemone from anemone-eating fish, and in turn the stinging tentacles of the anemone protect the clownfish from its predators. A special mucus on the clownfish protects it from the stinging tentacles.
A further example is the goby fish, which sometimes lives together with a shrimp. The shrimp digs and cleans up a burrow in the sand in which both the shrimp and the goby fish live. The shrimp is almost blind, leaving it vulnerable to predators when outside its burrow. In case of danger the goby fish touches the shrimp with its tail to warn it. When that happens both the shrimp and goby fish quickly retreat into the burrow. Different species of gobies (Elacatinus spp.) also exhibit mutualistic behavior through cleaning up ectoparasites in other fish.
Another non-obligate symbiosis is known from encrusting bryozoans and hermit crabs. The bryozoan colony (Acanthodesia commensale) develops a cirumrotatory growth and offers the crab (Pseudopagurus granulimanus) a helicospiral-tubular extension of its living chamber that initially was situated within a gastropod shell.
A spectacular examples of obligate mutualism is between the siboglinid tube worms and symbiotic bacteria that live at hydrothermal vents and cold seeps. The worm has no digestive tract and is wholly reliant on its internal symbionts for nutrition. The bacteria oxidize either hydrogen sulfide or methane, which the host supplies to them. These worms were discovered in the late 1980s at the hydrothermal vents near the Galapagos Islands and have since been found at deep-sea hydrothermal vents and cold seeps in all of the world’s oceans.
There are many types of tropical and sub-tropical ants that have evolved very complex relationships with certain tree species.
Mutualism and endosymbiosis
During mutualistic symbioses, the host cell lacks some of the nutrients which the endosymbiont provides. As a result, the host favors endosymbiont’s growth processes within itself by producing some specialized cells. These cells affect the genetic composition of the host in order to regulate the increasing population of the endosymbionts and ensure that these genetic changes are passed onto the offspring via vertical transmission (heredity).
As the endosymbiont adapts to the host’s lifestyle the endosymbiont changes dramatically. There is a drastic reduction in its genome size, as many genes are lost during the process of metabolism, and DNA repair and recombination, while important genes participating in the DNA to RNA transcription, protein translation and DNA/RNA replication are retained. The decrease in genome size is due to loss of protein coding genes and not due to lessening of inter-genic regions or open reading frame (ORF) size. Species that are naturally evolving and contain reduced sizes of genes can be accounted for an increased number of noticeable differences between them, thereby leading to changes in their evolutionary rates. When endosymbiotic bacteria related with insects are passed on to the offspring strictly via vertical genetic transmission, intracellular bacteria go across many hurdles during the process, resulting in the decrease in effective population sizes, as compared to the free living bacteria. The incapability of the endosymbiotic bacteria to reinstate their wild type phenotype via a recombination process is called Muller’s ratchet phenomenon. Muller’s ratchet phenomenon together with less effective population sizes leads to an accretion of deleterious mutations in the non-essential genes of the intracellular bacteria. This can be due to lack of selection mechanisms prevailing in the relatively “rich” host environment.
Commensalism describes a relationship between two living organisms where one benefits and the other is not significantly harmed or helped. It is derived from the English word commensal, which is used of human social interaction. The word derives from the medieval Latin word, formed from com- and mensa, meaning “sharing a table.”
Commensal relationships may involve one organism using another for transportation (phoresy) or for housing (inquilinism), or it may also involve one organism using something another created, after its death (metabiosis). Examples of metabiosis are hermit crabs using gastropod shells to protect their bodies and spiders building their webs on plants.
A parasitic relationship is one in which one member of the association benefits while the other is harmed. This is also known as antagonistic or antipathetic symbiosis.
Parasitic symbioses take many forms, from endoparasites that live within the host’s body to ectoparasites that live on its surface. In addition, parasites may be necrotrophic, which is to say they kill their host, or biotrophic, meaning they rely on their host’s surviving. Biotrophic parasitism is an extremely successful mode of life. Depending on the definition used, as many as half of all animals have at least one parasitic phase in their life cycles, and it is also frequent in plants and fungi. Moreover, almost all free-living animals are host to one or more parasite taxa. An example of a biotrophic relationship would be a tick feeding on the blood of its host.
Amensalism is the type of relationship that exists where one species is inhibited or completely obliterated and one is unaffected by the other. There are two types of amensalism, competition and antibiosis. Competition is where a larger or stronger organism deprives a smaller or weaker one from a resource. Antibiosis occurs when one organism is damaged or killed by another through a chemical secretion. An example of competition is a sapling growing under the shadow of a mature tree. The mature tree can rob the sapling of necessary sunlight and, if the mature tree is very large, it can take up rainwater and deplete soil nutrients. Throughout the process, the mature tree is unaffected by the sapling. Indeed, if the sapling dies, the mature tree gains nutrients from the decaying sapling. Note that these nutrients become available because of the sapling’s decomposition, rather than from the living sapling, which would be a case of parasitism. An example of antibiosis is Juglans nigra (black walnut), secreting juglone, a substance which destroys many herbaceous plants within its root zone.
Amensalism is an interaction where an organism inflicts harm to another organism without any costs or benefits to the perpetrator. A clear case of amensalism is where sheep or cattle trample grass. Whilst the presence of the grass causes negligible detrimental effects to the animal’s hoof, the grass suffers from being crushed. Amensalism is often used to describe strongly asymmetrical competitive interactions, such as has been observed between the Spanish ibex and weevils of the genus Timarcha which feed upon the same type of shrub. Whilst the presence of the weevil has almost no influence on food availability, the presence of ibex has an enormous detrimental effect on weevil numbers, as they consume significant quantities of plant matter and incidentally ingest the weevils upon it.
Synnecrosis is a rare type of symbiosis in which the interaction between species is detrimental to both organisms involved. It is a short-lived condition, as the interaction eventually causes death. Because of this, evolution selects against synnecrosis and it is uncommon in nature. An example of this is the relationship between some species of bees and victims of the bee sting. Species of bees who die after stinging their prey inflict pain on themselves (albeit to protect the hive) as well as on the victim. This term is rarely used.
Symbiosis is increasingly recognized as an important selective force behind evolution, with many species having a long history of interdependent co-evolution.
In fact, the evolution of all eukaryotes (plants, animals, fungi, and protists) is believed under the endosymbiotic theory to have resulted from a symbiosis between various sorts of bacteria. This theory is supported by certain organelles dividing independently of the cell, and the observation that some organelles seem to have their own genome.
The biologist Lynn Margulis, famous for her work on endosymbiosis, contended that symbiosis is a major driving force behind evolution. She considered Darwin’s notion of evolution, driven by competition, to be incomplete and claimed that evolution is strongly based on co-operation, interaction, and mutual dependence among organisms. According to Margulis and Dorion Sagan, “Life did not take over the globe by combat, but by networking.”
Symbiosis played a major role in the co-evolution of flowering plants and the animals that pollinate them. Many plants that are pollinated by insects, bats, or birds have highly specialized flowers modified to promote pollination by a specific pollinator that is also correspondingly adapted. The first flowering plants in the fossil record had relatively simple flowers. Adaptive speciation quickly gave rise to many diverse groups of plants, and, at the same time, corresponding speciation occurred in certain insect groups. Some groups of plants developed nectar and large sticky pollen, while insects evolved more specialized morphologies to access and collect these rich food sources. In some taxa of plants and insects the relationship has become dependent, where the plant species can only be pollinated by one species of insect.