Commensalism Interaction- Definition, Types and Examples

Commensalism Definition

Commensalism is a type of ecological interaction between two or more species where one of the species is benefitted without either harming or benefitting the other.

• Commensalism is a positive interaction but is quite different interactions like mutualism or parasitism.
• It is usually a long term relationship where some species remain in the interaction for their entire life.
• The interaction mostly exists between a larger host and a smaller commensal. The organisms that provide the benefit but are unaffected is the host and the organism which is benefitted from the interaction is the commensal.
• The commensal might even demonstrate different forms of structural and functional adaptation based on the relationship.
• The term commensalism is derived from the term ‘commensal’ which means eating at the same table. The term was popularized in ecology by Pierre-Joseph van Beneden in 1876.
• The benefits obtained by the commensal from the interaction can either be services like transportation, protection, or resources like nutrients.
• Commensalism, like most ecological interaction, has played essential roles in evolution as the interactions and adaptations accumulate over time.
• This form of interaction can exist between the organisms of the same species or different species.

Types of Commensalism

Commensalism is of different types depending on the strength and duration of the interaction and the purposes. 

1. Inquilinism

• Inquilinism is a type of commensalism where one of the species uses the body or a cavity with the body of the other organisms as a living.
• In this form of commensalism, one of the species is benefitted as it attains shelter while the other species remain unaffected.
• Inquilinism exhibits resource commensalism where the commensal attains a resource in the form of shelter.

Example- Scarab beetles and flies

• Scarab beetles roll balls of dung which they then bury underground in burrows as a food source for their immatures. When they roll the ball, small flies are attracted to the dung.
• The adult flies lay their eggs in the ball and escape from the burrow before the beetles seal the holes.
• The fly larvae in the ball share the dung with the beetles’ larvae without negatively impacting the host.
• The larvae beetles then develop into adult beetles and dig out of the burrow while releasing the adult flies.
• Thus, the species exist in a commensal relationship for a short period of time, and the interaction is essential for the lifecycle of the flies.

2. Metabiosis

• Metabiosis is a form of commensalism where the host species unintentionally creates a suitable environment for the commensal.
• In metabiosis, most commensal use the remains of some part of the remains of the host for their benefit.
• The commensal might use the remains of the host as tools for their survival or as a form of protection.
• The host species involved in this form of interaction are usually large, whereas the commensals are small.

Example- Hermit crabs and gastropod shell

• Hermit crabs live inside the empty gastropod shell to protect their soft abdominal exoskeleton.
• The shell protects the animals against predation, desiccation, thermal and osmotic stress. Hermits continuously evolve to rapidly access good quality shells through competitive interactions with other individuals and aggregation.
• Some of the hermit crabs also carry the shells around with them as portable homes which also protect the organisms against predation and other conditions.
• The ability of hermit crabs to utilize gastropod shells as a form of living space is due to the availability of different shapes and sizes of shells.

3. Phoresy

• Phoresy is a form of interaction between two species where the commensal or phoront latches onto the host animals for dispersal without inducing parasitism.
• The term phoresy is derived from the Greek word ‘phorein’ meaning ‘to carry’. 
• In a typical interaction, the phoront or phoretic organisms is an animal usually a nematode or mite, with the ability to travel limit distances and thus, requires aid in dispersal with the help of a mobile host.
• The benefit obtained by the phoront is measured in terms of dispersal instead of nutritional benefits, like in the case of parasitism.
• Phoresy is a temporary interaction that continues only until the phoront reaches the desired distance. The interaction can, however, turn parasitic over time.
• Phoretic interactions exist between species of the entire animal kingdom and thus are extremely diverse.

Example- Caenorhabditis remanei (soil nematode) and molluscs

• Caenorhabditis remanei is a soil nematode that has a limited dispersal ability on its own due to the small size and sensitivity to desiccation.
• The species thus depends on other animals like slugs, snails, and isopods for dispersal to longer distances.
• The interactions are strictly used for dispersal and to avoid harsh environmental conditions.
• Most of the species remain on the body of the slugs, but in some cases, the organisms can be found within the intestine of the slugs.

4. Microbiota

• Microbiota is a group of microorganisms that exist in a commensal interaction with the host surface as they colonize a particular tissue surface.
• The system formed is termed normal microbiota, and it mostly comprises bacteria with few fungi, protozoans, and viruses.
• These species exist in a commensal relationship with the host species and depend on the host for nutrition and living space.
•  Even though the population of a microbiota develops in close parallel with the host, they do not cause harm to the host body.
• However, in some cases, in individuals with reduced immunity and immune-compromised conditions, these species might cause different diseases.

Example- Gut microbiota in human

• In humans, a large group of microorganisms resides in different parts of the body, but the most diverse and the largest population of such organisms reside in the gut.
• These microbes exist on the surface of human tissue and depend on the host for nutrients and living space.
• Even though the microorganisms take up nutrients from the digestive system, it doesn’t cause any harm to the host unless the species is pathogenic.
• In some cases, the species might be opportunistic pathogens and cause diseases in immunocompromised individuals.

References and Sources

• (2008) Commensalism. In: Capinera J.L. (eds) Encyclopedia of Entomology. Springer, Dordrecht. https://doi.org/10.1007/978-1-4020-6359-6_775
• White, P Signe et al. “Phoresy.” Current biology: CB vol. 27,12 (2017): R578-R580. doi:10.1016/j.cub.2017.03.073
• Sourakov A. (2008) Inquilines and Cleptoparasites. In: Capinera J.L. (eds) Encyclopedia of Entomology. Springer, Dordrecht. https://doi.org/10.1007/978-1-4020-6359-6_1536
• Petersen, C., Hermann, R.J., Barg, MC. et al. Travelling at a slug’s pace: possible invertebrate vectors of Caenorhabditis nematodes. BMC Ecol 15, 19 (2015). https://doi.org/10.1186/s12898-015-0050-z
  
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