Learning Objectives
After studying or reading this, you should be ble to:
1. Explain the concept of symbiosis.

2. Identify the different symbiotic relationships.

Organisms may form very close associations or relationships with one another for many reasons. These associations may be between organisms of different species (interspeciflc associations) or between individuals of the same species (intraspecific associations).

Symbiosis: It is a close, long lasting, physical relationship between two different species of organisms. The two species are usually in physical contact and at least one of them derive some sort of benefit from this contact. There are three different categories of symbiotic relationships: parasitism, commensalisms and mutualism.

This is an association between two living organisms 0f different species in which one organism benefits while the other (host) is not Slgmficanuy affected in any way. For example, the Remora fish uses a sucker on its head to cling to the belly of the shark.

This provides the Remora with protection and  with scraps of food scattered by the host, shark during its own feeding.

In commensalism, it is only the commensal who benefits but the host neither benefits nor loses.

   Some symbiotic relationships are actually beneficial to both species involved. This kind of relationship is called mutualism.  In many mutualistic relationships, the assoc1ation lS obligatory. Flowers of the yucca plant are Pollinated by a specific insect known as the yucca moth. The moth in turn, lays it eggs in the flower, where the immature moth larvae feed on the developing seeds, the yucca is dependent on the moth for pollination and the moth is dependent on the seeds as a source of food for its larvae. Other examples include the associations between the nitrogen fixing bacteria receives shelter and nutrients from the roots and it 'in turn provides the plant with nitrogen containing molecules that the plant can use for growth. Mutualism is therefore a close association between two organisms of different species in which both organisms benefit.

     A parasite derives its nourishment from the host and so the relation is to the detriment of the host. Ectoparasites, plasmodium attack the red blood cells and cause a characteristic disease. Tapeworm is another example of parasite in man. Can you mention any parasite and its host in your environment. The parasite causes a great harm to the host as it derives nourishment.

    Saprophytes are certain species of bacteria or fungi such as moulds yeast, toadstools and mushrooms that _feed on dead organic matter. Saprophytes obtain nourishment by releasing digestive enzymes into dead tissues and absorb the resulting substances. They act as decomposers, thereby helping in the recycling of essential nutrients.

Saprophytism therefore refers to the mode of feeding or nutrition where fungi and some bacteria secrete digestive enzymes onto dead organic materials and absorb the products that results from the reaction (between digestive enzymes and dead organic material).
An example is a mould growing on deserted pieces of bread.

     This is a kind of relationship in which a much smaller tree (epiphyte) grows on the branches of another tree in ,order to get enough sunlight for photosynthesis. Epiphytism normally occurs in the forest where the leaves of the big trees form a canopy or a layer over the smaller trees. In epiphytism, the epiphytes evolves a root system capable of absorbing nutrients from the water that drips down the stern of the host. In addition, the epiphyte competes with its host for sunlight. The epiphytes normally grow in the rotting organic materials that forms in the angles between big branches.
Examples of epiphytic plants include the orchids and ferns.

The platycerium (staghorn fern) may grow on the angle between trees and their large branches in the forest. Some ferns may also grow within the angles between palm fronds and the palm nut tree.

Have you UNDERSTOOD what you have just studied or read?

To make sure you have UNDERSTOOD, ANSWER the QUESTIONS below:


1. Define eaCh of the following:
i. Mutualism
ii. Parasitism
iii. Commensalism

2. a. Define symbiosis

b. Name and describe the type of relationship that exists between the estrogen fixing bacterium, Rhizobium and leguminous plants.

3. a. What is parasitism?

b. Describe how a named endoparasite is adapted to survive in its habitat.

4. a. Define Epiphytism.

b. Describe how a named epiphyte is able to survive on its host.

5. Mention any four symbiotic associations and give one example of each.

6. a. What is commensalism .

     b. Describe the symbiotic association that exists between the remora fish and the shark.

7. a. What is symbiosis?

       b. Describe with examples the relationship of saprophytism.

8. a. What are ectoparasites?

      b. Describe how the dodder plant is able to obtain nutrients from its host.