Commensalism

1.0What is commensalism?

Commensalism is a type of symbiotic relationship between two species where one organism benefits, and the other is neither helped nor harmed. This interaction is often seen in nature and exemplifies a form of biological cooperation. In this interaction, the species that benefits is called the commensal, while the other species, which is unaffected, is referred to as the host.

2.0Types of commensalism

There are various forms of commensalism interaction found in nature, depending on the nature of the interaction between the two species involved. 

1. Inquilinism: 

• In inquilinism, one organism resides inside the habitat of another without causing harm. For instance, certain animals may inhabit tree hollows or burrows created by other species without adversely affecting the host. In this symbiotic relationship, the inquiline organism benefits from the shelter provided by the host's habitat, while the host is generally unharmed by the presence of the inquiline.

2. Microbiota Commensalism: 

• Microorganisms, including bacteria and fungi, frequently engage in commensal relationships with larger organisms.
• For example, bacteria inhabiting the human digestive system derive benefits from the environment and nutrients provided by the host without causing harm. In this symbiotic interaction, the microorganisms thrive within the host's system, receiving a conducive environment for growth and sustenance, while the host remains largely unaffected by their presence.

3. Phoresy:

• Phoresy is a form of commensalism where one organism hitches a ride on another for transportation.
• For example, mites may attach themselves to insects or larger animals, utilizing them as a mode of travel to new locations without causing harm to the host organism. In this relationship, the phoretic organism benefits from the transportation provided by the host, while the host is generally unaffected by the presence of the phoretic organism.

4. Epiphytism:

• Epiphytic plants, such as mosses, orchids, or ferns, grow non-invasively on the surface of other plants, often trees, using them for physical support. The host plant is typically unaffected by this relationship. In this commensal association, epiphytic plants benefit from the elevated position for sunlight access, while the host plant experiences minimal or no negative impact.

3.0Examples of commensalism

1. Barnacles on Whales:

• In this relationship, barnacles (commensals) attach themselves to the skin of whales (hosts), gaining a mobile substrate as the whale moves through nutrient-rich waters. Barnacles benefit from accessing food particles in the water, while the whales is typically unaffected by their presence. 

• This symbiotic association showcases how one species can utilize another for transportation and resources without causing harm to the host.
  

2. Cattle egrets birds and grazing animals:

• Another example of commensalism is cattle egrets feeding on insects flushed out of the grass by grazing bison, cattle, horses, and other herbivores. Because the birds increase their foraging rates when following the herbivores, they clearly benefit from the association. Much of the time, the herbivores are unaffected by the birds. 
• At other times, the herbivores also benefit; for instance, the birds remove and consume the ticks and other ectoparasites that attach to their skin, or they alert the herbivores to a predator's presence. This example shows another important point about ecological interactions: Their effects can shift through time.In this case, an interaction whose effects are typically +/0 (commensalism) may at times become +/+ (mutualism).

3. Epiphytic Plants on Trees:

• Epiphytes are the plants growing on other plants. They use other plants only as support and not for water or food supply. Epiphytes differ from the lianas in that they are not rooted into the soil. They may grow on trees, shrubs or larger submerged plants. They grow either on the trunks or leaves. Certain common examples of epiphytes are orchids, bromeliads, hanging "mosses", Usnea and Alectoria.
• Epiphytes are most common in tropical rainforests. They derive their supply of moisture and nutrients from the frequent rains and debris accumulated in bark crevices. Dust is also a source of the nutrients. Roots of epiphytes often store water in a special tissue, called velamen. The leaves of the epiphyte Dischidia fold to form a jug-like structure with a narrow mouth in which water collects; roots of the epiphyte enter and grow in these special cavities of the leaf. In many epiphytes, a thick network of roots develops on which windblown dust accumulates, providing the necessary edaphic environment.

4. Lianas:

• These plants are common in tropical rain forests where light at ground level is scarce because of the dense and multi storeyed growth of vegetation. Lianas are vascular plants rooted in the ground but which climb up with the support of other trees and reach almost to the top of the forest canopy.
• Lianas have a woody stem that is closely attached to a supporting tree, but this does not form any nutritional relationship. Thus, with much economy of mechanical tissues, lianas are able to get better light.
• Lianas are classified on the basis of the type of device used for climbing their supports into leaners, thorn lianas, twiners or tendril lianas. In North Indian tropical deciduous forests Bauhinia vahlii is a common liana. Other common lianas are species of Ficus and Tinospora.

5. Remoras and Sharks:

• Remoras (suckerfish) attach themselves to sharks (hosts) using a suction disc on their heads. Remoras benefit by gaining access to food scraps from the shark's meals, while the shark is minimally affected by their presence. This symbiotic relationship illustrates how one species can capitalize on the activities of another without causing substantial harm to the host.

6. Birds and Trees : 

• In this interaction, birds like sparrows and pigeons (commensals) construct nests on tree branches (hosts), utilizing the trees for shelter and support. The birds benefit by having a secure location to lay eggs, while the trees typically remain unharmed. 
• This symbiotic relationship exemplifies how one species can find refuge and support from another without causing significant harm to the host.

7. Epizoans:

• Some plants grow on the surfaces of animals. For example, some green algae grow on the long, grooved hair of sloth. Basicladia (Cladophoraceae) grows on the backs of freshwater turtle.

8. Epizoite:

• Similarly, ecto commensals or epizoite animals are attached to another animal for the purpose of anchorage and protection. Many small animals, for example, become attached to the outside of larger ones, such as the protozoans Trichodina and Kerona on Hydra, vorticellids (Vorticella, Epistylis) or various other aquatic organisms, branchiobdellida annelids on crayfish, the barnacles, which attach themselves to the backs of whales and shells of horse-shore crabs and so on. 
• It clings to the bellies of sharks, swordfish, tunny, barracuda, or sea turtle by means of a dorsal fin highly modified into a suction disk on top of the head. These fishes are of small size and unencumbering to the host. The remora gains from the host, however, for as the host feeds, the scraps of food floating back are swept up by the remora.

Commensals may also be internal:

• Many harmless protozoans occur in the intestinal tract of mammals, including man. Some microorganisms such as bacterium Escherchia coli are found in the human colon.
• Pinnotheres ostreum which is a pea crab lives as a commensal in the mantle cavity of certain sea mussels; crab steals food collected by the host mollusc but it does little or no other known injury.
• In hermit crab normal life is inside the gastropod shell.
• The pitcher of the pitcher plant found in bogs furnishes a breeding site or home for certain species of midge flies, mosquitoes, and tree toads.
• Many kinds of microorganisms, plants and animals like small crabs, shrimps, polychaetes, ophiuroids, ascid-ians, etc., live in the canal system of sponges.
• The nest of one species often furnishes shelter and protection for other species as well. Ant nests may contain guest species of various other insects; some birds place their nests close to wasps, bees or ants for the protection offered by these insects.
• Woodchuck burrows are used also by rabbits, skunks and racoons, especially in winter.

Commensalism-like associations:

• There are also associations similar to commensalism between a variety of microorganisms and higher plants.
• For example, the zone of soil around the roots of higher plants characterized by intense microbial activity, called the rhizosphere; the surface proper of the roots growing in soil—the rhizoplane; the boundary layer of air over the green leaves with active microbes—the phyllosphere; and the leaf surface—phylloplane constitute important ecological niches, where occur rich populations of microorganisms including fungi, actinomycetes and bacteria.
• The living roots and leaves of higher plants continuously exude various metabolic products. Most of such products are sugars and amino acids. These form sources of nutrition for the microorganisms present therein.