Symbiosis – Definition, Types, Examples

What is Symbiosis?

Symbiosis, derived from the Greek words “σύν” (sýn) meaning “together” and “βίωσις” (bíōsis) signifying “living,” refers to a prolonged biological interaction between two distinct species, known as symbionts. This interaction can manifest in various forms, ranging from mutualistic, where both entities benefit, to commensalistic, where one benefits without harming the other, and even parasitic, where one organism benefits at the expense of the other.

Historically, the term “symbiosis” was introduced in the context of lichens by Albert Bernhard Frank in 1877. A year later, the renowned mycologist Heinrich Anton de Bary provided a more comprehensive definition, describing it as the coexistence of dissimilar organisms. Over the decades, the interpretation of symbiosis has been a subject of scientific deliberation. While some scholars believed it should exclusively denote mutualistic relationships, others contended it should encompass all enduring biological interactions. Presently, the broader definition, which includes mutualism, commensalism, and parasitism, is widely accepted in the biological community.


Symbiotic relationships can be categorized based on their degree of dependency and physical association. In terms of dependency:

  1. Obligate Symbiosis: Here, the relationship is so intertwined that the survival of one or both organisms is contingent upon the presence of the other.
  2. Facultative Symbiosis: In this type, while the organisms may choose to engage in a symbiotic relationship, they are also capable of independent existence.

From a physical perspective, symbiosis can be:

  1. Endosymbiosis: This occurs when one symbiont resides within the other. Depending on the location, it can be further classified into intercellular (within cells) or extracellular (outside cells) symbiosis.
  2. Ectosymbiosis: In this relationship, one organism dwells on the external surface of the other, which could include areas like the digestive tract lining or exocrine glands.
  3. Conjunctive Symbiosis: This is characterized by the formation of a unified body by the symbionts.
  4. Disjunctive Symbiosis: All other physical arrangements that don’t result in a single body fall under this category.

In conclusion, symbiosis is a multifaceted biological phenomenon that has evolved over millions of years, enabling diverse species to coexist and adapt in ever-changing environments. The intricate nature of these relationships underscores the complexity and interdependence inherent in the natural world.

Definition of Symbiosis

Symbiosis refers to a long-term biological interaction between two distinct species, where at least one benefits and the relationship can be mutualistic, commensalistic, or parasitic.


Types of Symbiosis

There are present different types of symbiosis relationship such as;

1. Mutualism

  • Mutualism is a type of symbiosis in which both symbiotic partners benefit from the relationship, which often makes one or both of them much healthier.
  • Mutualism can take the form of a relationship between a resource and a service, a service and a resource, or a service and a service.
  • Resource-resource mutualisms, which are also called “trophic mutualisms,” happen when two organisms exchange one resource for another.
  • Most of the time, resource-resource mutualisms happen between an autotroph (an organism that makes food from sunlight) and a heterotroph (an organism which must absorb or ingest food to gain energy).
  • A mycorrhizal association is a symbiotic relationship between the roots of a plant and a fungus. This is a type of trophic mutualism.
  • The fungus takes over the roots of the plant and gets sugar and glucose from them. In exchange, the plant can take in more water and minerals because the fungi are better at it.
  • When a symbiotic partner gives a service in exchange for a resource, this is called a service-resource mutualism. The exchange between plants and the insects that pollinate them is one of the best-known examples of this.
  • Pollinators (insects, birds, moths, bats, etc.) visit plants to get energy-rich nectar. When they do this, they pollinate the plant and spread their own pollen to other plants of the same species.
  • Service-service interactions are a rare example of mutualistic symbiosis. As the name suggests, both members of a symbiotic relationship get something out of it, like a place to live or safety from predators.
  • For example, anemone fish (family: Pomacentridae) and sea anemones have a close relationship that protects both of them from predators.
  • The anemone fish have developed a thick layer of mucus on their skin to protect them from being stung by the anemone’s nematocysts. The anemone gives the fish a place to hide from predators and a place to breed, while aggressively chasing away other fish that might try to eat the ends of the tentacles, which are full of nutrients.
  • But some people say that there aren’t many true service-service mutualisms because the symbiosis usually involves sharing resources.
  • In the anemone-anemone fish mutualism, the nutrients in the anemone fish waste feed the symbiotic algae that live in the tentacles of the anemone and use photosynthesis to give the anemone energy. In this way, symbioses show how complicated life is and how fragile the balance is in ecosystems.

Example of Mutualism

a. Pistol shrimps and gobies

  • There are about 2,000 species of fish in the family Gobiidae, which includes the true gobies. Most of them are small and live on the bottom of the ocean. In some cases, gobies and pistol shrimps of the family Alpheidae get along well and help each other.
  • Pistol shrimp dig holes in the sand on the bottom of the ocean, which they sometimes share with a goby. Outside of the burrow, the pair stays close to each other. Often, the shrimp touches the fish with its sensitive antennae to stay in touch.
  • When the goby sees a possible predator, it sends out chemical signals and runs into the shared burrow to hide. The shrimp also uses these physical and chemical signals to figure out when it needs to hide. When the goby is moving around, it tells the shrimp that it’s probably safe to come out of its burrow.

b. Aphids and ants

  • Aphids are tiny insects that feed on plant sap and make honeydew, a sugary liquid, as a waste product. Many kinds of aphids work together with ants to get honeydew. The ants “milk” the aphids with their antennae to get the honeydew.
  • In exchange, some kinds of ants will protect the aphids from predators and disease-causing organisms. Some ants will move aphid eggs and young aphids underground to their nest. This makes it easier to get their honeydew, which is like a dairy farm for ants.
  • But some aphids have learned to take advantage of ants that look for honeydew. There are two types of Paracletus cimiciformis aphids: the round one that is milked and the flat one that looks like an ant. When the ants bring the flat people to their nest, the aphids will drink the ants’ larvae’s body fluid.
  • Honeydew is made by many different kinds of insects, like scale insects and some caterpillars, and is liked by many different kinds of animals, not just ants. Some geckos in Madagascar have been seen eating the honeydew that plant hoppers leave behind. Scientists aren’t sure if this is a case of mutualism or not because the gecko may keep planthopper-eating animals away.

c. Woolly bats and pitcher plants

  • Pitcher plants are carnivores that use nectar at the top of their tube-like bodies to attract insects and small vertebrates that they can eat. These animals fall into the plant’s stomach, which is filled with digestive juices, because there is something slippery on the edge.
  • You might think that animals would try to stay away from these plants if they could, but some bats climb inside them on their own.
  • Nepenthes hemsleyana is a tropical pitcher plant that grows in Borneo. It is known to be a home for woolly bats.
  • The bat gets a place to hide, and the plant gets the guano (faeces) that the little mammal leaves behind. This gives the plant the food it needs to keep living.
  • Tree shrews and Nepenthes lowii, a different kind of Bornean pitcher plant, have a similar relationship. Shrews climb up on the edge of the pitcher to get to the nectar. In exchange, the shrews drop their nutritious poop into the plant’s stomach. The hollow body of the plant acts a bit like a toilet bowl.

d. Coral and algae

  • Corals might look like rocks or plants, but they are actually animals that live in the ocean. Corals that build reefs get their bright colours from the zooxanthellae algae they share a relationship with.
  • Coral starts out as a tiny, free-swimming larva that grows into a polyp by attaching itself to a hard surface. The polyp reproduces and grows to make a colony by making many copies of itself that grow on top of each other and secrete a hard skeleton.
  • As corals get bigger, they get zooxanthellae from the environment around them. The coral gives the zooxanthellae a place to live and important nutrients for photosynthesis. In return, the zooxanthellae make sugars that the coral feeds on and oxygen as a byproduct.
  • Pollution and heat stress can make corals get rid of their algae, which turns the coral white. This is called coral bleaching. If the coral goes too long without algae, it can die because it usually can’t get enough food from its surroundings to meet its energy needs.

2. Commensalism

Commensalism is a form of symbiosis where one organism derives benefit, often in terms of food, shelter, or locomotion, without causing harm or providing any significant advantage to the other organism involved. This interaction is characterized by its neutrality for one party, while the other reaps the benefits.


A classic example of commensalism is the relationship between barnacles and whales. Barnacles affix themselves to the skin of whales, gaining the advantage of movement and access to nutrient-rich water currents. This allows them to feed efficiently. The whale, on the other hand, experiences no discernible benefit or harm from the barnacles’ presence.

The etymology of “commensalism” is rooted in the medieval Latin word meaning “to share food.” It is a combination of “com-” (with) and “mensa” (table). This term has been adapted into the English language to describe harmonious human interactions, emphasizing the idea of sharing or coexisting without detriment.


In the realm of biology, commensalism can manifest in various ways:

  1. Phoresy: One organism utilizes another for transportation.
  2. Inquilinism: An organism takes up residence within another, using it as a habitat.
  3. Metabiosis: An organism capitalizes on the remnants or structures left behind by another organism after its demise. For instance, hermit crabs utilize the vacant shells of gastropods for protection, and spiders often construct their webs on plants.

In essence, commensalism underscores the myriad ways organisms can interact in nature, where one party benefits without impacting the other significantly. It serves as a testament to the intricate balance and diversity of relationships within the biological world.


Examples of Commensalism

  • Remora fish have a disc on top of their heads that lets them attach to sharks, manta rays, and whales. When the bigger animal eats, the remora breaks off and eats the food that the bigger animal leaves behind.
  • Nurse plants are bigger plants that protect seedlings from the weather and herbivores so they can grow.
  • Tree frogs hide in plants to stay safe.
  • Once they are kicked out of a pack, golden jackals will follow a tiger to eat what it left behind.
  • Goby fish live on other sea animals. To hide from predators, they change colour to match the host animal.
  • Cattle egrets eat the bugs that cattle stir up when they graze. The animals are fine, but the birds get food.
  • The burdock plant makes spiny seeds that stick to animals’ fur or to people’s clothes. The animals are not affected by this way of spreading seeds, but it is important for the plants.

3. Amensalism

  • Amensalism is a non-symbiotic, asymmetric relationship between two species in which one species hurts or kills the other, while the other species has no effect on the first.
  • There are two different kinds of amensalism: competition and hostility (or antibiosis). In competition, a bigger or stronger organism takes something from a smaller or weaker one.
  • Antagonism is when one organism is hurt or killed by a chemical secretion from another organism. A seedling that grows in the shade of a big tree is an example of competition.
  • The older tree can block the younger tree’s access to sunlight, and if the older tree is very big, it can soak up rainwater and take away nutrients from the soil.
  • The sapling has no effect on the mature tree at any point in the process. In fact, if the sapling dies, the nutrients in the dead sapling are taken up by the full-grown tree.
  • An example of antagonism is the black walnut tree, Juglans nigra, which makes juglone, a chemical that kills many herbaceous plants in its root zone.
  • Amensalism is often used to describe interactions between competitors that are very different from each other. For example, the Spanish ibex and weevils of the genus Timarcha that eat the same type of shrub are two examples of such interactions.
  • Weevils don’t have much of an effect on how much food is available, but ibex do, because they eat a lot of plants and sometimes eat the weevils that are on them.

Example of Amensalism

  • When cows walk on grass, the grass gets trampled down. But this action does not help the cattle and does not hurt them either.
  • Amensalism also happens when an animal like a goat eats the same type of shrub as an insect (such as a beetle). When the goat eats the shrub, it doesn’t hurt itself. However, the beetle loses a lot of food and may be eaten by accident by the goat. Competition is the name for this kind of amensalism.
  • Antibiosis can be seen when Penicillium and bacteria work together. The mould Penicillium makes a substance called penicillin, which kills bacteria very quickly. This discovery led to the creation of penicillin, the first real antibiotic.
  • The way that the mould Penicillium kills some bacteria is a classic example of antibiosis. The mold’s secretion, called penicillin, is now a powerful medicine used to treat bacterial infections. Some higher plants make substances that stop nearby plants from growing or kill them outright. The black walnut (Juglans nigra) is a good example. It makes juglone, which kills many herbaceous plants in its root zone.

4. Parasitism

Parasitism represents a form of symbiosis where one organism, the parasite, benefits at the detriment of another, the host. This relationship is characterized by an imbalance, where the parasite derives sustenance or other advantages, often compromising the health or well-being of the host.

Unlike predation, where the prey is typically killed, parasitism does not always result in the immediate death of the host. In fact, it can be advantageous for the parasite to maintain the host’s viability for prolonged periods to ensure its own survival and reproduction. However, there are instances, as seen with parasitoids, where the host’s demise is an eventual consequence of the parasitic invasion.

Parasitism manifests in various forms:

  1. Ectoparasitism: External parasites, such as fleas, attach to the host’s surface, feeding on blood or other external resources. While they might cause discomfort, the more significant threat arises when these ectoparasites act as vectors, transmitting harmful pathogens like bacteria or viruses to the host.
  2. Endoparasitism: These parasites infiltrate the host’s body, deriving nutrients directly from the host’s tissues or influencing host behaviors to their advantage.
  3. Kleptoparasitism: Here, the parasite pilfers resources, such as food, that the host has gathered or produced.
  4. Brood Parasitism: A unique form of kleptoparasitism observed in certain birds, insects, and fish. The parasite deposits its eggs in the nest of the host, deceitfully passing on the responsibility of rearing its offspring to the host. This tactic can lead to reduced survival rates for the host’s offspring due to competition, rejection, or even direct harm by the parasitic young.

The evolutionary trajectory of parasitism is intriguing. It’s postulated that some parasitic relationships might have evolved from more mutualistic interactions. Over time, one partner might have started extracting more benefits without reciprocating, gradually transitioning into a parasitic mode of existence.

In summary, parasitism underscores the complexity of biological interactions, where one organism’s survival strategy hinges on the exploitation of another, often leading to intricate evolutionary arms races between host and parasite.

Examples of Parasitism

1. Parasitism in Humans

  • People can be parasitized by fungi, leeches, lice, viruses, protozoa, tapeworms, and other things.
  • Helminthes is one of the few organisms that live in the intestines of their hosts and can cause diseases like jaundice, malnutrition, diarrhoea, etc. Viruses and bacteria are the cause of all infections.

2. Parasitism in Plants

  • Aphids are little green insects that parasitize plants by feeding on their sap. Various species of fungus parasitize plants and rot fruits, vegetables, and grains.
  • The parasitic plants’ modified roots, known as haustoria, link to the xylem or phloem of the host plant and drain it of nutrients and water.

3. Parasitism in Insects

  • Parasitism is widespread among insects. Entomophagous parasites prey on insect larvae and young.
  • Several bug species lay their eggs within the larvae of other insect species. When the egg hatches, the hatchling consumes the larva for sustenance.

Examples of Symbiosis

1. Corals and Zooxanthellae

  • Corals are composed of organisms known as coral polyps. Inside the coral tissue, coral polyps have a highly sophisticated obligatory mutualistic symbiosis with photosynthesizing algae called zooxanthellae (pronounced “zoo-zan-THELL”).
  • The zooxanthellae absorb sunlight and convert it into oxygen and energy in the form of carbohydrates and lipids, which are then transmitted to the tissues of the coral and serve as nutrients for its survival and growth.
  • As a result of the metabolic activity of the coral, the zooxanthellae receive carbon dioxide, phosphorus, and nitrogen.
  • Although corals cannot exist without zooxanthellae, they may adjust the amount within their tissues by modifying the nutrition supply to the algae.
  • However, if the water temperature remains excessively high for an extended period of time, corals experience stress, expel all of their zooxanthellae, and are deprived of sufficient nutrients to survive. This causes coral to bleach.

2. Cordiceps

  • The Cordyceps fungus (family: Cordycipitaceae) is a very ubiquitous and lethal endoparasite of insects and other arthropods that infects its host and replaces all of its tissue with mycelium.
  • The ascocarp, the fruiting body, eventually erupts from the host and releases the reproductive spores. The majority of Cordyceps are adapted to a single host species. The “zombie fungus,” Ophiocordyceps unilateralis, is uniquely specialised to parasitize Camponotus leonardi carpenter ants.
  • The spores of the zombie fungus connect to the ant and use enzymes to digest its tissues; the fungus then manipulates the ant’s behaviour. The ant leaves its colony on the forest floor, climbs a plant’s stem, and embeds its mandibles abnormally deeply into a leaf.
  • Once hooked to the leaf, the ant is rendered immobile and the fungus begins to take over the tissues, preparing to reproduce via its fruiting body. The fungus is extremely intelligent, as it has elevated its victim in order to disseminate its spores further than if the host were on the ground.

3. Cleaner Fish

  • Numerous fish are infested with ectoparasites, which are produced in the water and attach to the skin and glands to feed on the blood of their hosts.
  • Some highly specialised species of fish have evolved a facultative mutualistic symbiosis with numerous species of larger fish in which they remove ectoparasites from the larger fish, so offering a “cleaning” function.
  • The Bluestreak Cleaner Wrasse (Labroides dimidiatus) is an example of a tropical fish that waits at “cleaning stations” to remove parasites off larger fish.
  • The cleaner fish execute a unique “dance” to attract the host fish and promote their cleaning service. Although the cleaner fish put themselves in apparent tremendous peril by swimming within the mouth cavities of even the most ferocious predators, their service is so effective that they are rarely damaged by the host fish and make repeated ‘customer’ visits.


Which term describes a symbiotic relationship where both organisms benefit?
a) Commensalism
b) Parasitism
c) Mutualism
d) Predation

In which type of symbiosis does one organism benefit while the other is neither harmed nor benefited?
a) Mutualism
b) Parasitism
c) Commensalism
d) Competition

Which organism is known to lay its eggs in the nests of other birds, an example of brood parasitism?
a) Sparrow
b) Cuckoo
c) Robin
d) Eagle

Lichens are a classic example of which type of symbiosis?
a) Mutualism
b) Commensalism
c) Parasitism
d) Competition

Which term describes a symbiotic relationship where one organism lives inside another?
a) Ectosymbiosis
b) Endosymbiosis
c) Kleptoparasitism
d) Phoresy

In which type of symbiosis does one organism harm another for its benefit?
a) Mutualism
b) Parasitism
c) Commensalism
d) Cooperation

Which of the following is NOT a type of symbiotic relationship?
a) Mutualism
b) Commensalism
c) Parasitism
d) Predation

Which term describes a symbiotic relationship where one organism steals food or resources from another?
a) Phoresy
b) Endosymbiosis
c) Kleptoparasitism
d) Ectosymbiosis

Which of the following is an example of ectosymbiosis?
a) Bacteria living in human intestines
b) Fungi living inside plant roots
c) Fleas on a dog
d) Mitochondria in animal cells

Coral reefs and the algae that live within them represent which type of symbiotic relationship?
a) Mutualism
b) Commensalism
c) Parasitism
d) Competition


What is symbiosis?

Symbiosis refers to any type of close and prolonged biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasitic.

How many types of symbiotic relationships are there?

There are three primary types of symbiotic relationships: mutualism (both organisms benefit), commensalism (one organism benefits, the other is unaffected), and parasitism (one organism benefits at the expense of the other).

Can symbiotic relationships evolve over time?

Yes, symbiotic relationships can evolve. For instance, a relationship that starts as commensalism can evolve into mutualism over time as both organisms begin to benefit.

What is an example of mutualism in nature?

An example of mutualism is the relationship between bees and flowering plants. Bees collect nectar from flowers as a food source, while the flowers get pollinated by the bees.

How does parasitism differ from predation?

In parasitism, the parasite benefits by exploiting the host, but the host is not necessarily killed. In predation, the predator kills and consumes the prey.

What is endosymbiosis?

Endosymbiosis is a type of symbiosis where one organism lives inside the other. A classic example is the mitochondria in eukaryotic cells, which are believed to have originated from ancient bacteria.

Can symbiotic relationships be harmful?

Yes, in parasitic relationships, one organism (the parasite) benefits at the expense of the other (the host), which can be harmed.

What is brood parasitism?

Brood parasitism is a type of parasitism where one organism (like the cuckoo bird) lays its eggs in the nests of other species. The host species then raises the intruding offspring, often at the expense of its own.

Are all symbiotic relationships permanent?

No, while many symbiotic relationships are long-term, some can be temporary. For instance, a cleaner fish might remove parasites from a larger fish, benefiting both, but they part ways after.

Why are symbiotic relationships important in ecosystems?

Symbiotic relationships play a crucial role in maintaining ecosystem balance. They can aid in nutrient cycling, protection against predators, and promoting biodiversity.


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