UPSC Prelims 2021 Q30: Symbiosis in Cnidarians, Fungi, Protozoa
Which of the following have species that can establish symbiotic relationship with other organisms?
1. Cnidarians
2. Fungi
3. Protozoa
Select the correct answer using the code given below.
- 1 and 2 only
- 2 and 3 only
- 1 and 3 only
- 1, 2 and 3
Answer: (d) 1, 2 and 3
Statement-wise explanation
- 1: correct — Cnidarians include corals, sea anemones, hydras and jellyfish. Reef-building corals keep single-celled dinoflagellate algae called zooxanthellae inside their cells. The algae photosynthesise and pass glucose and oxygen to the coral, and the coral gives them shelter and carbon dioxide. Sea anemones also live in mutualism with clownfish.
- 2: correct — Fungi form two textbook symbioses. Mycorrhiza is a fungus living with plant roots, and the roots of about 90% of vascular land plants carry one; the fungus brings in phosphorus and water and gets sugars back. A lichen is a fungus living with an alga or cyanobacterium that makes its food by photosynthesis.
- 3: correct — Flagellate protozoa such as Trichonympha live in the gut of wood-eating termites and break down the cellulose the termite cannot digest on its own. The protozoa get food and a safe, oxygen-free home. Neither partner survives well without the other, and NCERT cites this as an example of mutualism.
How to eliminate
The stem asks only whether these groups 'have species that can' form a symbiosis. That is a weak claim: one example per group proves it. Most aspirants know two examples from NCERT: corals with zooxanthellae (cnidarians) and mycorrhiza or lichens (fungi). With statements 1 and 2 settled, only the two options that keep both are left. For statement 3, remember that protozoa are a very large group, and some of them almost certainly live in partnership with another organism. The fact to remember is termites and Trichonympha. Rule of thumb: when a statement says a broad group of organisms has 'some species' that do something common in nature, it is usually true.
Know more
Symbiosis means two different species living in close, long-term association. Biologists use it as an umbrella term for three types of interaction:
| Type | Partner A | Partner B | Example |
|---|---|---|---|
| Mutualism | + | + | Coral and zooxanthellae; termite and Trichonympha |
| Commensalism | + | 0 | Orchid on a mango branch; barnacles on a whale |
| Parasitism | + | − | Tapeworm in humans; Cuscuta on hedge plants |
Classic mutualisms for Prelims
- Coral–zooxanthellae: the algae make most of the coral's food and give it colour. When sea water stays too warm, the coral pushes the algae out and turns white. This is called coral bleaching.
- Mycorrhiza: fungal threads grow into or around roots and take up phosphorus. Glomus is a mycorrhizal fungus used as a biofertiliser.
- Lichens: a fungus with an alga or cyanobacterium. Lichens are sensitive to sulphur dioxide, so they are used as indicators of air pollution.
- Rhizobium–legume root nodules: bacteria in the nodules fix nitrogen for the plant.
- Termite–gut flagellates and rumen protozoa in cattle: both digest cellulose for the host.
- Fig–fig wasp and clownfish–sea anemone: textbook pairs from NCERT Class XII.
Trap to watch: zooxanthellae are dinoflagellates, which are protists. So the coral partnership is also an example of a protist in a symbiosis.
NCERT Concepts to Revise
- Coral bleaching: Coral bleaching happens when a coral loses its zooxanthellae, the partner algae that statement 1 is about.