What Is Symbiosis in Biology?

We can define symbiosis as the close relationship and interactions between different biological organisms. Both parties use direct contact with the other, with some obtaining certain benefits to allow their survival. This is in contrast to animals of the same species which help each other due to their social order. For example, a mother dog raising her pups is not an example of symbiosis in nature.

The two organisms involved in a symbiotic relationship are known as ‘symbionts’. If the two organisms are of different sizes, the larger may be known as the ‘host’. Not all symbiotic relationships are necessary for survival. For this reason, we can categorize symbionts into two general groups:

• Facultative symbiosis: in these cases, the two species can survive without each other. It is sometimes known as option symbiosis. Facultative relationships demonstrate the adaptability of organisms to a variety of ecological niches.
  
  
• Obligatory symbiosis: one or more of the symbionts need the other to survive. Without them, their species will no longer be able to continue. This type of symbiosis highlights the interdependence of certain species.
  
  

Symbiotic relationships in biology were first named by the German botanist Heinrich Anton de Bary in 1879. His original definition of symbiosis was the ‘living together of dissimilar organisms’. This is a helpful, but very general sense of the term. For this reason, we need to look at different types of symbiosis in biology to better understand the concept. De Bary's work laid the foundation for our modern understanding of ecological interactions.

Symbiotic relationships can be classified based on several factors. These include how the symbiotic relationship occurs between the two individual organisms. As we have explained above, not all symbiotic relationships are essential for life. Sometimes the biological interactions occur to benefit both parties, sometimes only one individual. In fact, some symbiotic relationships can cause harm to one party while the other benefits.

Depending on the costs and benefits obtained by the species involved, we can distinguish between the following types of symbiosis in biology:

• Mutualism: it is often used as a synonym for symbiosis, although it is not exactly the same. Mutualistic relationships are those in which the two organisms involved obtain benefit. An example is the relationship between bees and flowers, where bees get nectar and flowers get pollinated.
  
  
• Commensalism: one of the species benefits from the other, although they do not cause harm in doing so. It means the host species can provide something to the other species without losing anything. It is relatively rare in nature. For instance, barnacles attaching to a whale benefit by gaining mobility, while the whale is unaffected.
  
  
• Parasitism: when one species (parasite) benefits at the expense of the other (host). In this case, it causes harm, but this can be to varying degrees. In some cases, the parasite may cause such minor harm that it does not affect the host's quality of life. In others, it can result in the death of the host. The malaria parasite is a classic example of parasitism, exploiting its host for survival.
  
  

The spatial relationship between the two symbiont organisms is also very important. This is in reference to the part of each species which interacts with the other. We can distinguish between the following:

• Endosymbiosis: if the organism lives inside the cells of the other symbiont or in the gaps between them. This often involves significant evolutionary adaptations.
  
  
• Ectosymbiosis: if the symbiont can survive outside the other, i.e. outside its cells. In this case, the symbiont can be found on the surface of the digestive tract, exocrine glands or externally on their host's body. This type of symbiosis illustrates the diversity of survival strategies in nature.
  
  

As we mentioned, some of these relationships are essential for life. For this reason, they can again be classified according to whether they are temporary (optional) or permanent (mandatory) relationships.

Depending on the way in which this relationship has been established, one can distinguish between symbiotic relationships of vertical and horizontal transmission:

• Vertical symbiont transmission: when the symbionts are transmitted to their offspring. Can occur in the egg during germination, but can also occur after birth. This transmission method secures the symbiotic relationship for future generations.
  
  
• Horizontal symbiont transmission: when the host organism obtains its symbiont from their environment which reoccurs every new generation. This approach allows for greater genetic diversity among symbionts.
  
  
• Mixed symbiont transmission: when both above types occur at the same time. This method combines the advantages of both vertical and horizontal transmission.
  
  

Finally, there is another type of symbiosis we have not yet mentioned. This is known as cleaning symbiosis. This occurs when one of the symbionts cleans the body of the other, eating living organisms from the body of the host. An example may be when birds eat ticks from the bodies of larger mammals. There is debate as to whether this is a type of mutualism, commensalism or parasitism. It is possible different types of cleaning symbionts interact in different ways. Such relationships highlight the complexity and variability of ecological interactions.

As will become clearer through the subsequent examples, symbiotic relationships are very important in their larger ecosystems. This is because they enable many species to survive. It is for this reason we consider symbiosis as working to enhance the evolution of individual species, managing to improve their way of life by establishing relationships with other organisms and species.

Despite being aware of various types of symbiotic relationships in biology, we do not always understand them fully. It is possible not to be able to see the benefit of certain symbiotic species, but ecological and biological research has afforded us some answers. In the following examples, we can see clearer how these biological benefit or harm each species:

• Ants and aphids: some species of ants, such as the common black ant (Lasius niger), protect swarms of aphids. In return, the aphids provide them with food and a sugary waste known as honeydew. You can see this symbiosis in action with the title picture of this article. This mutualistic relationship highlights the intricate balance in nature.
  
  
• Ants and acacias: other ant species such as the acacia ant (Pseudomyrmex feruginea) protect acacias from other parasites or herbivores. In return, the tree provides shelter and food. This example demonstrates the mutual benefits that can arise in symbiotic associations.
  
  
• Crocodiles and plovers: everyone knows the great power of a crocodile's jaws. The 80 teeth found within their jaws can be replaced 2 or 3 times a year. Remains of food can cause serious problems such as infections. It is for this reason, a relationship with Egyptian plovers was created. These birds obtain their food by cleaning the remains they find between the crocodile's teeth, helping them to avoid oral problems by allowing them to move inside their mouths. This symbiosis exemplifies how different species can work together for mutual benefit.
  
  
• Sharks and remoras: this is the clearest example of commensalism in biology. You may have seen sharks swimming with small fish attached to their body. These fish are remoras which adhere to the sharks and obtain both protection and food. The latter comes from the remains of prey the sharks do not ingest. For sharks, they are practically indifferent to the presence of remoras. This interaction showcases the subtle balance found in ecological relationships.
  
  
• Goby fish and blind shrimp: despite their lack of vision, shrimps dig and maintain a burrow to which they allow goby fish to enter. In allowing the fish a place to live, the shrimp is able to be alerted to the presence of both food and potential danger. They do so by sensing vibrations from the fish, hiding in the burrow when necessary. This relationship demonstrates how species can compensate for each other's weaknesses.
  
  
• Clown fish and anemones: these fish carry out their entire lives in and around the anemones, the latter of which are very poisonous. They establish a mutualistic relationship in which the clownfish attracts other predatory fish. When they come into contact with the anemone, they are paralyzed and serve as food. The clownfish then feeds off their remains. This example highlights how species can thrive through strategic partnerships.
  
  
• Lichens: symbiotic associations between a fungus and an algae. The fungus protects the algae from drying out and provides a structure on which it can grow. The algae make carbohydrates which the fungus can use as food. There is a wide variety of lichens as they are very resistant and capable of colonizing very diverse environments. Lichens are a testament to the resilience and adaptability of symbiotic relationships.
  
  
• Mycorrhizae: mycorrhizae are fungi that establish symbiotic relationships with multiple species of vascular plants. They do so because the roots of these plants secrete useful substances for these fungi. In turn, they make materials found in the soil such as minerals and other decomposing materials more assimilable by plants. This relationship enhances nutrient exchange and plant growth.
  
  
• Intestinal flora and microbiota: there is a large number of bacteria and other microorganisms that live in the intestines and other body parts of humans, as well as many animal species. They are involved in a symbiosis with their cells. They are of great importance for human health. This is to such an extent that variations in this microbiota can cause alterations in our body, even making us ill. Understanding this symbiosis is crucial for advancements in medicine and health.
  
  

Now that you know more about the importance of symbiosis in biology, you may want to know other ways in which organisms interact. Understanding these interactions can provide deeper insights into the complex web of life on Earth.

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