Q. 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. (UPSC Prelims 2021)
Answer: 1, 2 and 3
Notes: The correct answer is [D] 1, 2 and 3. Symbiosis refers to a close and long-term biological interaction between two different biological organisms, which can be mutualistic, commensalistic, or parasitic.

• Cnidarians (Statement 1 – Correct): Many cnidarians establish symbiotic relationships. The most famous example is the mutualism between reef-building corals (anthozoans) and zooxanthellae (dinoflagellate algae). The algae provide food via photosynthesis, while the coral provides protection and nutrients. Another example is the relationship between sea anemones and clownfish.
• Fungi (Statement 2 – Correct): Fungi are prolific symbiotic partners. Lichens are a symbiotic association between a fungus and an alga or cyanobacteria. Additionally, Mycorrhizae are symbiotic associations between fungi and the roots of vascular plants, where the fungus helps in nutrient absorption (especially phosphorus) in exchange for carbohydrates.
• Protozoa (Statement 3 – Correct): Many protozoans live symbiotically within other organisms. For example, Trichonympha is a protozoan that lives in the gut of termites; it digests cellulose for the termite, while the termite provides a stable environment and food source. Ciliates in the rumen of cattle also help in the breakdown of plant material.

All three groups contain diverse species that rely on or provide benefits through interspecies relationships to survive and thrive in their respective niches.