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In
biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, Development ...

biology
and
medicine Medicine is the science Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity, awareness, or understanding of someone or something, such as facts ( descriptive knowledge), skills (proced ...

medicine
, a host is a larger
organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological ...

organism
that harbours a smaller
organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological ...

organism
; whether a
parasitic Parasitism is a Symbiosis, symbiotic biological interactions, relationship between species, where one organism, the parasite, lives on or inside another organism, the Host (biology), host, causing it some harm, and is adaptation (biology), ad ...
, a mutualistic, or a
commensalist
commensalist
''guest'' (
symbiont Symbiosis (from Ancient Greek, Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different Organism, biological organisms, be it Mutualism (biology ...
). The guest is typically provided with nourishment and shelter. Examples include
animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells ...

animal
s playing host to parasitic
worm Worms are many different distantly related bilateral animals that typically have a long cylindrical tube-like body, no limb Limb can refer to: *Limb (anatomy), an appendage of a human or animal *Limb Music, a record label *Limb (album), an ...

worm
s (e.g.
nematodes The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the valu ...
), cells harbouring
pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a Germ theory ...
ic (disease-causing)
virus A virus is a submicroscopic infectious agent In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecu ...

virus
es, a
bean A bean is the seed of one of several of the , which are used as vegetables for human or animal food. They can be cooked in many different ways, including boiling, frying, and baking, and are used in many traditional dishes throughout th ...

bean
plant hosting mutualistic (helpful)
nitrogen-fixing bacteria
nitrogen-fixing bacteria
. More specifically in
botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the Ancient Greek wo ...

botany
, a host plant supplies food resources to micropredators, which have an evolutionarily stable relationship with their hosts similar to
ectoparasitism Parasitism is a Symbiosis, symbiotic biological interactions, relationship between species, where one organism, the parasite, lives on or inside another organism, the Host (biology), host, causing it some harm, and is adaptation (biology), ad ...
. The host range is the collection of hosts that an organism can use as a partner.


Symbiosis

Symbiosis Symbiosis (from Ancient Greek, Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different Organism, biological organisms, be it Mutualism (biolog ...

Symbiosis
spans a wide variety of possible relationships between organisms, differing in their permanence and their effects on the two parties. If one of the partners in an association is much larger than the other, it is generally known as the host. In
parasitism Parasitism is a close relationship between species In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, ...

parasitism
, the parasite benefits at the host's expense. In
commensalism Commensalism is a long-term biological interaction In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their phy ...

commensalism
, the two live together without harming each other, while in mutualism, both parties benefit. Most parasites are only parasitic for part of their life cycle. By comparing parasites with their closest free-living relatives, parasitism has been shown to have evolved on at least 233 separate occasions. Some organisms live in close association with a host and only become parasitic when environmental conditions deteriorate. A parasite may have a long-term relationship with its host, as is the case with all endoparasites. The guest seeks out the host and obtains food or another service from it, but does not usually kill it. In contrast, a
parasitoid In evolutionary ecology, a parasitoid is an organism that lives in close association with its host (biology), host at the host's expense, eventually resulting in the death of the host. Parasitoidism is one of six major evolutionarily stable str ...
spends a large part of its life within or on a single host, ultimately causing the host's death, with some of the strategies involved verging on
predation Predation is a biological interaction In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physical en ...

predation
. Generally, the host is kept alive until the parasitoid is fully grown and ready to pass on to its next life stage. A guest's relationship with its host may be intermittent or temporary, perhaps associated with multiple hosts, making the relationship equivalent to the
herbivory File:Land_Snail_radula_tracks.jpg#, 250px, Tracks made by terrestrial gastropods with their radulas, scraping green algae from a surface inside a greenhouse A herbivore is an animal anatomically and physiologically adapted to eating plant mater ...
of a wild-living animal. Another possibility is that the host–guest relationship may have no permanent physical contact, as in the
brood parasitism Brood parasites are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. It is a synonym for "Outline ...
of the
cuckoo Cuckoos are s in the Cuculidae family, the sole in the order Cuculiformes . The cuckoo family includes the , , s, s, s, s and . The coucals and anis are sometimes separated as distinct families, the and respectively. The cuckoo order Cuculi ...

cuckoo
.


Hosts to parasites

Parasites follow a wide variety of evolutionary strategies, placing their hosts in an equally wide range of relationships. Parasitism implies
host–parasite coevolution Host–parasite coevolution is a special case of coevolution In biology, coevolution occurs when two or more species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, a ...
, including the maintenance of
gene polymorphism Genes which control hair colour are polymorphic. A gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen coined the word gene to describe the Mendelian_inheritance#History, Mendelian units of heredity..." (Greek language, Greek) mean ...
s in the host, where there is a trade-off between the advantage of resistance to a parasite and a cost such as disease caused by the gene.


Types of hosts

*Definitive or primary host - an organism in which the
parasite Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives on or inside another organism, the Host (biology), host, causing it some harm, and is adaptation (biology), adapted structurally to this w ...
reaches the adult stage and reproduces sexually, if possible. This is the final host. *Secondary or intermediate host - an organism that harbors the sexually immature parasite and is required by the parasite to undergo development and complete its life cycle. It often acts as a vector of the parasite to reach its definitive host. For example, ''
Dirofilaria immitis ''Dirofilaria immitis'', also known as heartworm or dog heartworm, is a parasitic Parasitism is a Symbiosis, symbiotic biological interactions, relationship between species, where one organism, the parasite, lives on or inside another or ...

Dirofilaria immitis
'', the heartworm of dogs, uses the mosquito as its intermediate host until it matures into the infective L3 larval stage. It is not always easy or even possible to identify which host is definitive and which secondary. As the life cycles of many parasites are not well understood, sometimes the subjectively more important organism is arbitrarily labelled as definitive, and this designation may continue even after it is found to be incorrect. For example, sludge worms are sometimes considered "intermediate hosts" for
salmonid Salmonidae is a family In human society A society is a Social group, group of individuals involved in persistent Social relation, social interaction, or a large social group sharing the same spatial or social territory, typically ...
whirling disease ''Myxobolus cerebralis'' is a myxosporean parasite Parasitism is a symbiotic relationship between species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as ...
, even though the myxosporean parasite reproduces sexually inside them. In
trichinosis Trichinosis, also known as trichinellosis, is a parasitic disease caused by roundworms of the '' Trichinella'' type. During the initial infection, invasion of the intestines The gastrointestinal tract, (GI tract, GIT, digestive tract, diges ...
, a disease caused by
roundworm The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the valu ...

roundworm
s, the host has reproductive adults in its digestive tract and immature juveniles in its
muscle Skeletal muscles (commonly referred to as muscles) are organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly cat ...

muscle
s, and is therefore both an intermediate and a definitive host. *Paratenic host - an organism that harbors the sexually immature parasite but is not necessary for the parasite's
development cycle In software engineering, a software development process is the process of dividing software development work into smaller, parallel or sequential steps or subprocesses to improve Software design, design, Software product management, product mana ...
to progress. Paratenic hosts serve as "dumps" for non-mature stages of a parasite in which they can accumulate in high numbers. The trematode ''
Alaria americana ''Alaria americana'' is a species of a trematode in a family Diplostomidae. All of these species infect carnivorous mammals by living in their small intestines as mature worms. ''A. americana'' are most frequently found in temperate regions, predo ...
'' may serve as an example: the so-called mesocercarial stages of this parasite reside in
tadpole A tadpole is the larval stage A larva (plural larvae ) is a distinct juvenile form many animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With ...
s, which are rarely eaten by the definitive canine host. The tadpoles are more frequently preyed on by
snakes Snakes are elongated, limbless, carnivorous A carnivore , meaning "meat Meat is animal flesh that is eaten as food. Humans have hunted and killed animals for meat since prehistoric times. The advent of civilization allowed the domes ...

snakes
, in which the mesocercariae may not undergo further development. However, the parasites may accumulate in the snake paratenic host and infect the definitive host once the snake is consumed by a canid. The nematode ''
Skrjabingylus nasicola ''Skrjabingylus nasicola'' is a species of parasitic nematode The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-parasitic nematodes being kn ...
'' is another example, with slugs as the intermediate hosts, shrews and rodents as the paratenic hosts, and mustelids as the definitive hosts. *Dead-end, incidental, or accidental host - an organism that generally does not allow transmission to the definitive host, thereby preventing the parasite from completing its development. For example, humans and horses are dead-end hosts for
West Nile virus West Nile virus (WNV) is a single-stranded RNA virus that causes West Nile fever. It is a member of the family ''Flaviviridae'', from the genus ''Flavivirus'', which also contains the Zika virus, dengue virus, and yellow fever virus. The virus i ...
, whose life cycle is normally between culicine
mosquito Mosquitoes are members of a group of almost 3,600 species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defin ...

mosquito
es and birds. People and horses can become infected, but the level of virus in their blood does not become high enough to pass on the infection to mosquitoes that bite them. * Reservoir host - an organism that harbors a
pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a Germ theory ...
but suffers no ill effects. However, it serves as a source of infection to other species that are susceptible, with important implications for
disease A disease is a particular abnormal condition that negatively affects the structure A structure is an arrangement and organization of interrelated elements in a material object or system A system is a group of Interaction, interactin ...
control. A single reservoir host may be reinfected several times.


Plant hosts of micropredators

Micropredation is an evolutionarily stable strategy within parasitism, in which a small predator lives parasitically on a much larger host plant, eating parts of it. The range of
plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel ...

plant
s on which a
herbivorous A herbivore is an animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All ...

herbivorous
insect feeds is known as its host range. This can be wide or narrow, but it never includes all plants. A small number of insects are
monophagous A generalist species is able to thrive in a wide variety of environmental conditions and can make use of a variety of different resources A resource is a source or supply from which a benefit is produced and that has some utility. Resources can b ...
, feeding on a single plant. The
silkworm ''Bombyx mori'', the domestic silk moth, is an insect from the moth Moths are a paraphyletic In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and all descendants of that ancestor excluding a ...

silkworm
larva is one of these, with
mulberry ''Morus'', a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In ...
leaves being the only food consumed. More often, an insect with a limited host range is oligophagous, being restricted to a few closely related species, usually in the same plant family. The
diamondback moth The diamondback moth (''Plutella xylostella''), sometimes called the cabbage moth, is a moth species of the family Plutellidae and genus ''Plutella''. The small, grayish-brown moth sometimes has a cream-colored band that forms a diamond along it ...
is an example of this, feeding exclusively on
brassica ''Brassica'' () is a genus of plants in the cabbage and mustard Mustard may refer to: Food and plants * Mustard (condiment) Mustard is a condiment made from the seeds A seed is an Plant embryogenesis, embryonic plant enclosed in a te ...
s, and the larva of the potato tuber moth feeds on potatoes, tomatoes and tobacco, all members of the same plant family,
Solanaceae The Solanaceae , or nightshades, are a family (biology), family of flowering plants that ranges from annual and perennial herbs to vines, lianas, epiphytes, shrubs, and trees, and includes a number of agricultural crops, medicinal plants, spice ...
. Herbivorous insects with a wide range of hosts in various different plant families are known as
polyphagous Feeding is the process by which organisms, typically animals, obtain food. Terminology often uses either the suffixes -vore, -vory, or -vorous from Latin ''vorare'', meaning "to devour", or -phage, -phagy, or -phagous from Greek φαγε ...
. One example is the
buff ermine The buff ermine (''Spilarctia luteum'') is a moth Moths are a paraphyletic group of insects that includes all members of the Order (biology), order Lepidoptera that are not Butterfly, butterflies, with moths making up the vast majority of the ...
moth whose larvae feed on
alder Alder is the common name of a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classific ...

alder
,
mint MiNT is Now TOS (MiNT) is a free software Free software (or libre software) is computer software distributed under terms that allow users to run the software for any purpose as well as to study, change, and distribute it and any adapted ver ...
,
plantain Plantain may refer to: * Cooking banana, banana cultivars in the genus ''Musa'' whose fruits are generally used in cooking * True plantains, a group of cultivars of the genus ''Musa'' * ''Plantago'', small inconspicuous plants commonly called plant ...
,
oak An oak is a tree In botany, a tree is a perennial plant with an elongated Plant stem, stem, or trunk (botany), trunk, supporting branches and leaves in most species. In some usages, the definition of a tree may be narrower, including on ...

oak
,
rhubarb Rhubarb is the fleshy, edible stalks ( petioles) of species and hybrids (culinary rhubarb) of '' Rheum'' in the family Polygonaceae The Polygonaceae are a family In human society A society is a Social group, group of individuals ...
, ,
blackberry The blackberry is an edible fruit produced by many species in the genus ''Rubus'' in the family (biology), family Rosaceae, hybrids among these species within the subgenus ''Rubus'', and hybrids between the subgenera ''Rubus'' and ''Idaeobatus'' ...

blackberry
, , ,
nettle{{redirect, Nettle Nettle is part of the English name of many plants with stinging hairs, particularly those of the genus '' Urtica''. It is also part of the name of plants which resemble ''Urtica'' species in appearance but do not have stinging hai ...

nettle
and
honeysuckle Honeysuckles (''Lonicera'', ; syn. ''Caprifolium'' Mill.) are arching shrub A shrub (or bush, but this is more of a gardening term) is a small- to medium-sized perennial woody plant. Unlike herbaceous plants, shrubs have persistent wood ...

honeysuckle
. Plants often produce toxic or unpalatable
secondary metabolite Secondary metabolites, also called specialised s, s, secondary products, or s, are s produced by , , or which are not directly involved in the normal , , or of the organism. Instead, they generally mediate ecological , which may produce a sel ...
s to deter herbivores from feeding on them. Monophagous insects have developed specific adaptations to overcome those in their specialist hosts, giving them an advantage over polyphagous species. However, this puts them at greater risk of extinction if their chosen hosts suffer setbacks. Monophagous species are able to feed on the tender young foliage with high concentrations of damaging chemicals on which polyphagous species cannot feed, having to make do with older leaves. There is a trade off between offspring quality and quantity; the specialist maximises the chances of its young thriving by paying great attention to the choice of host, while the generalist produces larger numbers of eggs in sub-optimal conditions. Some insect micropredators migrate regularly from one host to another. The hawthorn-carrot aphid overwinters on its primary host, a tree, and migrates during the summer to its secondary host, a plant in the .


Host range

The host range is the set of hosts that a parasite can use as a partner. In the case of human parasites, the host range influences the
epidemiology Epidemiology is the study and analysis of the distribution (who, when, and where), patterns and risk factor, determinants of health and disease conditions in defined populations. It is a cornerstone of public health, and shapes policy decisions ...
of the parasitism or disease.


Host range of viruses

For instance, the production of
antigenic shift 300px, NIAID illustration of potential influenza genetic reassortment Antigenic shift is the process by which two or more different strains of a virus A virus is a submicroscopic infectious agent that Viral replication, replicates only i ...
s in
Influenza A virus ''Influenza A virus'' causes influenza Influenza, commonly known as "the flu", is an infectious disease An infection is the invasion of an organism's body Tissue (biology), tissues by Pathogen, disease-causing agents, their multipli ...

Influenza A virus
can result from pigs being infected with the virus from several different hosts (such as human and bird). This co-infection provides an opportunity for mixing of the viral genes between existing strains, thereby producing a new viral strain. An
influenza vaccine Influenza vaccines, also known as flu shots or flu jabs, are vaccines that protect against infection by influenza viruses. New versions of the vaccines are developed twice a year, as the influenza virus rapidly changes. While their effectiveness ...

influenza vaccine
produced against an existing
viral strain Viral means "relating to virus A virus is a submicroscopic infectious agent that Viral replication, replicates only inside the living Cell (biology), cells of an organism. Viruses infect all types of life forms, from animals and plants to ...
might not be effective against this new strain, which then requires a new influenza vaccine to be prepared for the protection of the human population.


Non-parasitic associations


Mutualistic hosts

Some hosts participate in fully mutualistic interactions with both organisms being completely dependent on the other. For example,
termite Termites are Eusociality, eusocial insects that are classified at the Taxonomy (biology), taxonomic rank of infraorder Isoptera, or alternatively as Taxonomic rank#All ranks, epifamily Termitoidae, within the order Blattodea (along with cockroa ...

termite
s are hosts to the
protozoa Protozoa (singular protozoon or protozoan, plural protozoa or protozoans) is an informal term for a group of single-celled eukaryote Eukaryotes () are organism In biology, an organism () is any organic, life, living system that ...

protozoa
that live in their gut and which digest
cellulose Cellulose is an organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior ...

cellulose
, and the human
gut flora Gut or guts may refer to: Anatomy * Abdomen, the region of the body below the thorax but above the pelvic region * Beer gut, slang for an obese stomach * Gastrointestinal tract, the system of digestive organs in humans and other animals * Hu ...
is essential for efficient
digestion Digestion is the breakdown of large insoluble food Food is any substance consumed to provide Nutrient, nutritional support for an organism. Food is usually of plant, animal or Fungus, fungal origin, and contains essential nutrients, such a ...
. Many corals and other marine invertebrates house
zooxanthellae Zooxanthellae is a colloquial term for single-celled dinoflagellate The dinoflagellates (Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a count ...

zooxanthellae
, single-celled algae, in their tissues. The host provides a protected environment in a well-lit position for the algae, while benefiting itself from the nutrients produced by
photosynthesis Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Conversion (Doctor Who audio), "Conversion" (''Doctor Who'' audio), an episode of the audio drama ' ...

photosynthesis
which supplement its diet. '''', a deep sea giant tubeworm, has an obligate mutualistic association with internal, sulfide-oxidizing, bacterial symbionts. The tubeworm extracts the chemicals that the bacteria need from the sediment, and the bacteria supply the tubeworm, which has no mouth, with nutrients. Some
hermit crab Hermit crabs are anomuran decapod crustacean Crustaceans (Crustacea ) form a large, diverse arthropod An arthropod (, (gen. ποδός)) is an invertebrate Invertebrates are animals that neither possess nor develop a vertebral column (c ...

hermit crab
s place pieces of
sponge Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the Diploblasts. They are Multicellular organism, multicellular organisms that have bodies full of pores and channels allowing water ...

sponge
on the shell in which they are living. These grow over and eventually dissolve away the mollusc shell; the crab may not ever need to replace its abode again and is well-camouflaged by the overgrowth of sponge. An important hosting relationship is
mycorrhiza A mycorrhiza (from Ancient Greek, Greek μύκης ', "fungus", and ῥίζα ', "root"; pl. mycorrhizae, mycorrhiza or mycorrhizas) is a mutual symbiosis, symbiotic association between a fungus and a plant. The term mycorrhiza refers to the role ...

mycorrhiza
, a symbiotic association between a fungus and the roots of a vascular host plant. The fungus receives carbohydrates, the products of photosynthesis, while the plant receives phosphates and nitrogenous compounds acquired by the fungus from the soil. Over 95% of plant families have been shown to have mycorrhizal associations. Another such relationship is between and certain nitrogen-fixing bacteria called
rhizobia '' bacteria Rhizobia are diazotrophic bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a ...

rhizobia
that form nodules on the roots of the plant. The host supplies the bacteria with the energy needed for nitrogen fixation and the bacteria provide much of the nitrogen needed by the host. Such crops as
bean A bean is the seed of one of several of the , which are used as vegetables for human or animal food. They can be cooked in many different ways, including boiling, frying, and baking, and are used in many traditional dishes throughout th ...

bean
s,
pea The pea is most commonly the small spherical seed A seed is an embryonic plant enclosed in a protective outer covering. The formation of the seed is part of the process of reproduction Reproduction (or procreation or breeding) is ...

pea
s,
chickpea The chickpea or chick pea (''Cicer arietinum'') is an annual Annual may refer to: *Annual publication, periodical publications appearing regularly once per year **Yearbook **Literary annual *Annual plant *Annual report *Annual giving *Annual, Mo ...

chickpea
s and
alfalfa Alfalfa () (''Medicago sativa''), also called lucerne, is a perennial A perennial plant or simply perennial is a plant Plants are predominantly photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the ...

alfalfa
are able to fix nitrogen in this way, and mixing
clover Clover or trefoil are common names for plants of the genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining ( ...

clover
with
grass Poaceae () or Gramineae () is a large and nearly ubiquitous family In , family (from la, familia) is a of people related either by (by recognized birth) or (by marriage or other relationship). The purpose of families is to maintain ...

grass
es increases the yield of pastures. Neurotransmitter
tyramine Tyramine ( ) (also spelled tyramin), also known under several other names, is a naturally occurring trace amine Trace amines are an endogenous group of trace amine-associated receptor 1 (TAAR1) agonists – and hence, monoaminergic neuromodu ...

tyramine
produced by commensal '' Providencia'' bacteria, which colonize the gut of the nematode ''
Caenorhabditis elegans ''Caenorhabditis elegans'' () is a free-living transparent nematode The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-parasitic nematodes a ...

Caenorhabditis elegans
'', bypasses the requirement for its host to biosynthesise tyramine. This product is then probably converted to
octopamine Octopamine (molecular formula C8H11NO2; also known as norsynephrine, ''para''-octopamine and others) is an organic chemical closely related to norepinephrine, and synthesized biologically by a homologous pathway. Its name derives from the fact t ...

octopamine
by the host enzyme tyramine β-hydroxylase and manipulates a host sensory decision.


Hosts in cleaning symbiosis

Hosts of many species are involved in
cleaning symbiosis Cleaning symbiosis is a mutually beneficial association between individuals of two species, where one (the cleaner) removes and eats parasites and other materials from the surface of the other (the client). Cleaning symbiosis is well-known among ...
, both in the sea and on land, making use of smaller animals to clean them of parasites. Cleaners include fish, shrimps and birds; hosts or clients include a much wider range of fish, marine reptiles including turtles and iguanas, octopus, whales, and terrestrial mammals. The host appears to benefit from the interaction, but biologists have disputed whether this is a truly mutualistic relationship or something closer to parasitism by the cleaner.


Commensal hosts

Remora The remora , sometimes called suckerfish, is any of a family (Echeneidae) of Actinopterygii, ray-finned fish in the order Carangiformes. Depending on species, they grow to long. Their distinctive first dorsal fins take the form of a modified ov ...

Remora
s (also called suckerfish) can swim freely but have evolved suckers that enable them to adhere to smooth surfaces, gaining a free ride (
phoresis hitching a ride on a fly. File:Phoresy_edited.png, A pseudoscorpion on the leg of a crane fly Phoresis or phoresy is a non-permanent, commensalistic interaction in which one organism (a phoront or phoretic) attaches itself to another (the host) ...
), and they spend most of their lives clinging to a host animal such as a whale, turtle or shark. However, the relationship may be mutualistic, as remoras, though not generally considered to be
cleaner fish Cleaner fish are fish that show a specialist feeding strategy by providing a service to other species, referred to as clients, by removing dead skin, ectoparasites, and infected tissue from the surface or gill chambers. This example of cleaning s ...

cleaner fish
, often consume parasitic
copepod Copepods (; meaning "oar-feet") are a group of small crustaceans found in nearly every freshwater and saltwater habitat (ecology), habitat. Some species are planktonic (inhabiting sea waters), some are benthos, benthic (living on the ocean floor), ...

copepod
s: for example, these are found in the stomach contents of 70% of the
common remora The common remora (''Remora remora'') is a Pelagic zone, pelagic marine fish belonging to family Echeneidae. The dorsal fin, which has 22 to 26 soft rays, acts as a suction cup, creating a vacuum to allow it to attach to larger marine animals, su ...

common remora
. Many
mollusc Mollusca is the second-largest phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number ...
s,
barnacle A barnacle is a type of arthropod An arthropod (, (gen. ποδός)) is an invertebrate animal having an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Euarthropoda,Reference ...

barnacle
s and Polychaete, polychaete worms attach themselves to the carapace of the Atlantic horseshoe crab; for some this is a convenient arrangement, but for others it is an obligate form of commensalism and they live nowhere else.


History

The first host to be noticed in ancient times was human: human parasites such as hookworm are recorded from ancient Egypt from 3000 BC onwards, while in ancient Greece, the Hippocratic Corpus describes human bladder worm. The medieval Persian physician Avicenna recorded human and animal parasites including roundworms, threadworms, the Guinea worm and tapeworms. In Early Modern times, Francesco Redi recorded animal parasites, while the microscopist Antonie van Leeuwenhoek observed and illustrated the protozoan ''Giardia lamblia'' from "his own loose stools". Hosts to mutualistic symbionts were recognised more recently, when in 1877 Albert Bernhard Frank described the mutualistic relationship between a fungus and an alga in lichens.


See also

* PHI-base (Pathogen-Host Interaction database) * Generalist and specialist species * Host cell protein


References

{{DEFAULTSORT:Host (Biology) Biological interactions Parasitology Disease ecology