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Q50 (IAS/2021) Environment & Ecology › Ecology & Ecosystem Basics › Species interactions Official Key

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:

Result
Your answer: —  ·  Correct: D
Explanation

The correct answer is Option 4 (1, 2 and 3) because all three groups contain species capable of forming symbiotic relationships.

  • Cnidarians: Many cnidarians, most notably corals, host photosynthetic algae called zooxanthellae. This mutualistic relationship is vital for reef-building, where the algae provide nutrients and the coral provides protection.
  • Fungi: Fungi are renowned for symbiosis. They form lichens in partnership with algae or cyanobacteria and mycorrhizae with plant roots, assisting in nutrient uptake in exchange for carbohydrates.
  • Protozoa: These single-celled organisms frequently engage in symbiosis. For example, protozoa living in the guts of termites help digest cellulose, while others live within ruminants. Some protozoa also host endosymbiotic algae.

Since biological evidence confirms that species from all three categories exhibit various forms of symbiosis (mutualism, commensalism, or parasitism), the most comprehensive and accurate choice is 1, 2, and 3.

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Q. Which of the following have species that can establish symbiotic relationship with other organisms? 1. Cnidarians 2. Fungi 3. Protozoa Se…
At a glance
Origin: Mixed / unclear origin Fairness: Moderate fairness Books / CA: 6.7/10 · 0/10

This is a classic 'Science-Ecology Overlap' question. While Cnidarians (Corals) and Fungi (Lichens) are standard textbook examples found in Shankar/PMF, Protozoa requires biological intuition or the 'Possibility Heuristic'. The question tests the breadth of your understanding of 'Symbiosis'—it's not just mutual benefit, but any close long-term interaction.

How this question is built

This question can be broken into the following sub-statements. Tap a statement sentence to jump into its detailed analysis.

Statement 1
Do any species of Cnidaria (cnidarians) establish symbiotic relationships with other organisms?
Origin: Direct from books Fairness: Straightforward Book-answerable
From standard books
Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 4: Aquatic Ecosystem > 4.9. CORAL REEFS > p. 50
Presence: 5/5
“50 • Coral is actually a living animal. Coral has a symbiotic relationship (each gives something to the other and gets something back in return) with 'zooxanthellae' microscopic algae which live on coral [i.e. instead of living on the sea floor, the algae lives up on the coral which is closer to the ocean surface and so that the algae gets adequate light]. • Zooxanthellae assist the coral in nutrient production through its photosynthetic activities. These activities provide the coral with fixed carbon compounds for energy, enhance calcification , and mediate elemental nutrient flux. • The tissues of corals themselves are actually not the beautiful colors of the coral reef, but are instead clear (white)”
Why this source?
  • Explicitly states that coral (a living animal) has a symbiotic relationship with zooxanthellae algae.
  • Explains the mutual benefits: algae provide photosynthetic nutrients and enhance coral calcification.
Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 17: Major Landforms and Cycle of Erosion > Coral Reefs > p. 219
Presence: 4/5
“• Coral reefs are formed by the accumulation of calcareous skeletons of dead coral polyps. Polyps are related to anemones and jellyfish (phylum cnidaria).• They are shallow warm water organisms which have a soft body covered by a calcareous skeleton. The polyps extract calcium salts from seawater to form these hard skeletons. They occur in different forms and colours, depending upon the nature of the salts they are made of.• The polyps live in colonies fastened to the rocky seafloor. The tubular skeletons grow as a cemented calcareous rocky mass, collectively called corals. When the coral polyps die, they shed their skeleton (coral) on which new polyps grow.”
Why this source?
  • Identifies coral polyps as members related to anemones and jellyfish in phylum Cnidaria.
  • Links the organism (coral polyps) described in the symbiosis example to the cnidarian group.
Science ,Class VIII . NCERT(Revised ed 2025) > Chapter 12: How Nature Works in Harmony > 12.8 How Do Interactions Maintain Balance in Ecosystems? > p. 203
Presence: 3/5
“Chapter 12 — How Nature Works in Harmony 203 • z Mutualism: Both organisms benefit. For example: Honeybees and flowers.• z Commensalism: One organism benefits while the other is not affected. For example: Orchids on trees.• z Parasitism: One organism benefits while the other is harmed. For example: Ticks on the body of dogs. These interactions are all part of the complex web of life in an ecosystem. Fig. 12.16: Different types of interactions between organisms”
Why this source?
  • Defines mutualism as an interaction where both organisms benefit, framing symbiosis type relevant to coral–algae relationships.
  • Places mutualism among standard interaction types, helping classify coral–zooxanthellae association.
Statement 2
Do any species of Fungi establish symbiotic relationships with other organisms?
Origin: Direct from books Fairness: Straightforward Book-answerable
From standard books
Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 1: BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY > Niche > p. 12
Presence: 5/5
“Some species have symbiotic relationships, an arrangement that mutually benefts and sustains each organism. For example, lichen (pronounced "liken") is made up of algae and fungi living together. Te algae is the producer and food source, and the fungus provides structure and physical support. Teir mutually benefcial relationship (mutualism) allows the two to occupy a niche in which neither could survive alone. An ecological niche consists of: • (i) Habitat– where the species live.• (ii) Food niche what a species eats and decomposes and what species it competes with.• (iii) Reproductive niche how and when it reproduces.• (iv) Physical and chemical niche temperature, moisture, and landform.• (v) Geo-ecological niche– topography, terrain, slope, and soils etc.”
Why this source?
  • Gives a concrete example: lichen is formed by algae and fungi living together.
  • Explains the roles: algae provides food while fungus provides structure — a mutually beneficial association (mutualism).
  • Directly demonstrates that fungi can form symbiotic (mutualistic) relationships with other organisms.
Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 9: Indian Biodiversity Diverse Landscape > 2. Fungi: > p. 156
Presence: 4/5
“Non-green, non-differentiated plants characterised by total absence of chlorophyll are called Fungi. They grow either on dead, rotten organic matters as saprophytes or live as parasites on other living bodies, which are referred to as hosts. Moulds and mushrooms are the familiar examples of saprophytic fungi. The maximum diversity of fungi is in the Western Ghats, followed by the eastern Himalaya and the western Himalaya.”
Why this source?
  • States fungi may live as parasites on other living bodies (hosts).
  • Identifies parasitism as a fungal lifestyle distinct from saprophytism, showing fungi form close biological associations with other organisms.
  • Supports that fungal interactions with other species include symbiotic (parasitic) relationships.
Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 12: Major Crops and Cropping Patterns in India > Biotic: Living > p. 123
Presence: 4/5
“Swamp: A wetland that features temporary or permanent inundation of large areas of land by shallow bodies of water and covered by aquatic vegetation. Swell: Regular patterns of smooth, rounded waves in open water; can range from small ripples to very large waves. Symbiont: An organism that lives in symbiotic association with another. Symbiosis: A situation in which two organisms of diferent species live together and are closely associated. Te term generally implies some beneft to both, but is sometimes used to include parasitism. Synecology: It is the study of plant communities in relation to their habitats of a given ecosystem.”
Why this source?
  • Defines 'symbiont' and 'symbiosis' as close associations between two organisms.
  • Notes the term generally implies benefit to both but can include parasitism, connecting fungal mutualism and parasitism under symbiosis.
  • Provides the conceptual framework to classify fungal associations as symbiotic.
Statement 3
Do any species of protozoa establish symbiotic relationships with other organisms?
Origin: Weak / unclear Fairness: Borderline / guessy
Indirect textbook clues
Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 1: BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY > Niche > p. 12
Strength: 4/5
“Some species have symbiotic relationships, an arrangement that mutually benefts and sustains each organism. For example, lichen (pronounced "liken") is made up of algae and fungi living together. Te algae is the producer and food source, and the fungus provides structure and physical support. Teir mutually benefcial relationship (mutualism) allows the two to occupy a niche in which neither could survive alone. An ecological niche consists of: • (i) Habitat– where the species live.• (ii) Food niche what a species eats and decomposes and what species it competes with.• (iii) Reproductive niche how and when it reproduces.• (iv) Physical and chemical niche temperature, moisture, and landform.• (v) Geo-ecological niche– topography, terrain, slope, and soils etc.”
Why relevant

Defines 'symbiotic relationships' and gives a microbial example (lichen: algae + fungi) showing that single-celled organisms can form mutualistic partnerships.

How to extend

A student could note that if algae (microorganisms) form symbioses, other single-celled microbes (like protozoa) might also engage in similar relationships and then check host/microbe literature or habitat maps for protozoa co-occurrence.

Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 2: Functions of an Ecosystem > 2.6. BIOTIC INTERACTION > p. 16
Strength: 5/5
“Organisms living in this earth are interlinked to each other in one way or other. The interaction between the organisms is fundamental for its survival and functioning of ecosystem as a whole. S.No. | Type | Species a • 1. | Mutualism | (+) • 2. | Commensalism | (+) | • 3. | Amensalism | (o) • 4. | Competition | (-) • 5. | Predation | (+) | • 6. | Parasitism | (+) • (+) Benefited (*) Harmed • (o) Neither Benefited nor harmed.”
Why relevant

Lists biotic interaction types (mutualism, commensalism, parasitism) as common ecological relationships between species.

How to extend

One can place protozoa into these interaction categories (e.g., consider whether protozoa could be parasites or mutualists) and then search for examples or host associations accordingly.

Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 9: Indian Biodiversity Diverse Landscape > Protozoa > p. 155
Strength: 4/5
“• Protozoa are simple, single-celled animals. • They are the smallest of all animals. • Most protozoa are microscopic. • They do breathe, move and reproduce like multicellular animals. • E.g: amoebas, Flagellates, etc.”
Why relevant

States that protozoa are simple, single-celled, mostly microscopic animals (gives examples like amoeba, flagellates).

How to extend

Knowing protozoa are single-celled organisms, a student could compare them to other microbes known to form symbioses (from snippet 1) and look for ecological or host reports involving amoebae/flagellates.

Science ,Class VIII . NCERT(Revised ed 2025) > Chapter 2: The Invisible Living World: Beyond Our Naked Eye > Snapshots > p. 24
Strength: 3/5
“Plant, fungal, and bacterial cells have an extra covering, called a cell wall, around the cell membrane. Bacteria lack a well-defined nucleus.• Cells differ in shape and size. Their shape is related to the function performed by them.• Bacteria, fungi, and protozoa are different kinds of microorganisms.• Viruses are also small in size, but they are different from other microorganisms since they reproduce only inside the host organism.”
Why relevant

Groups protozoa with other microorganisms (bacteria, fungi, algae), implying they occupy similar microbial ecological niches where interspecies interactions occur.

How to extend

A student could use this grouping to infer protozoa are present in environments where microbial interactions/symbioses happen (ponds, soil) and then inspect such habitats for documented associations.

Science ,Class VIII . NCERT(Revised ed 2025) > Chapter 2: The Invisible Living World: Beyond Our Naked Eye > Activity 2.6: Let us study > p. 16
Strength: 3/5
“Fig. 2.8: Observation of soil suspension under the microscope A group of students studying in Grade 8 performed Activities 2.4 and 2.5. They also collected information from the library and internet. They recorded the data obtained after observing pond water in Table 2.1 and the data obtained after observing soil suspension in Table 2.2. They identified the microorganisms as protozoa, algae, fungi, and bacteria. You can record if you find any of these categories of organisms. S.No. | Diagram | Remarks • 1. | Amoeba (Protozoa) | Single cell, moving, irregular shape • 2. | Paramecium (Protozoa) | Single cell, moves from one place to another, movement takes place with the help of specialised structures • 3. | Algae | Single cell, looks green because of the presence of green pigment, movement takes place with the help of specialised structures”
Why relevant

Provides examples of observed protozoa in pond/soil samples (amoeba, paramecium), indicating protozoa commonly coexist with other microbes in natural communities.

How to extend

From co-occurrence in microhabitats a student could hypothesize potential symbiotic interactions and then look for specific behavioral/physiological evidence of association between protozoa and other organisms.

Pattern takeaway: The 'Existential Possibility' Pattern: When UPSC asks if a massive taxonomic group (like Fungi or Protozoa) *can* exhibit a fundamental biological trait (symbiosis, asexual reproduction, predation), the answer is almost always YES. Proving the negative is impossible.
How you should have studied
  1. [THE VERDICT]: Logical Sitter. Sources: Shankar IAS (Corals), NCERT Class 8/12 (Lichens/Microbes).
  2. [THE CONCEPTUAL TRIGGER]: Ecology > Biotic Interactions. You must move beyond definitions to memorizing 2-3 examples for each Kingdom.
  3. [THE HORIZONTAL EXPANSION]: 1. Cnidarians: Corals+Zooxanthellae (Mutualism), Sea Anemone+Clownfish (Commensalism), Sea Anemone+Hermit Crab. 2. Fungi: Lichens (Algae+Fungi), Mycorrhiza (Roots+Fungi), Leaf-cutter ants (Fungi farming). 3. Protozoa: Trichonympha in Termite guts (digests wood), Ruminant gut microbes, Plasmodium (Parasitism is also symbiosis).
  4. [THE STRATEGIC METACOGNITION]: Do not look for a single book line saying 'Protozoa have symbiosis.' Instead, ask: 'Is it possible that an entire Kingdom of life (Fungi/Protozoa) has ZERO species interacting with others?' The answer is statistically no. Broad groups + 'Can' = Usually All Correct.
Concept hooks from this question
📌 Adjacent topic to master
S1
👉 Coral polyps are cnidarians
💡 The insight

Coral polyps belong to phylum Cnidaria, so symbiotic relationships of corals are examples of cnidarian symbioses.

High-yield for ecology questions linking taxonomic groups to ecological roles; helps answer questions that ask for biological examples by connecting organism group (Cnidaria) to ecological phenomena (symbiosis). This links to topics on marine ecosystems, reef formation and organismal classification.

📚 Reading List :
  • Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 17: Major Landforms and Cycle of Erosion > Coral Reefs > p. 219
🔗 Anchor: "Do any species of Cnidaria (cnidarians) establish symbiotic relationships with o..."
📌 Adjacent topic to master
S1
👉 Mutualism as a form of symbiosis
💡 The insight

Mutualism describes interactions where both partners benefit and is the category under which coral–algae associations fall.

Essential for classifying biotic interactions in ecosystem questions; useful for distinguishing mutualism from commensalism and parasitism in mains and prelims. Mastery enables quick classification-based answers and explanation of ecological consequences.

📚 Reading List :
  • Science ,Class VIII . NCERT(Revised ed 2025) > Chapter 12: How Nature Works in Harmony > 12.8 How Do Interactions Maintain Balance in Ecosystems? > p. 203
  • Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 1: BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY > Niche > p. 12
🔗 Anchor: "Do any species of Cnidaria (cnidarians) establish symbiotic relationships with o..."
📌 Adjacent topic to master
S1
👉 Coral–zooxanthellae partnership and reef ecology
💡 The insight

The coral–zooxanthellae relationship demonstrates how cnidarian physiology and photosynthetic symbionts drive nutrient provision and calcification in reefs.

Directly relevant to questions on coral reef formation, marine productivity, and anthropogenic impacts on symbiotic systems. Understanding this micro-concept helps answer integrated questions on biodiversity, ecosystem services, and conservation.

📚 Reading List :
  • Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 4: Aquatic Ecosystem > 4.9. CORAL REEFS > p. 50
  • Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 17: Major Landforms and Cycle of Erosion > Coral Reefs > p. 219
🔗 Anchor: "Do any species of Cnidaria (cnidarians) establish symbiotic relationships with o..."
📌 Adjacent topic to master
S2
👉 Types of symbiotic relationships
💡 The insight

Symbiosis includes mutualism, commensalism and parasitism, all of which describe how fungi can associate with other organisms.

High-yield for ecology questions: distinguishing mutualism vs parasitism is frequently tested and links to ecosystem interactions, biodiversity and species roles. Mastery helps answer questions on organism interactions, ecological balance, and applied topics like agriculture and disease ecology.

📚 Reading List :
  • Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 12: Major Crops and Cropping Patterns in India > Biotic: Living > p. 123
  • Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 2: Functions of an Ecosystem > 2.6. BIOTIC INTERACTION > p. 16
  • Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 1: BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY > Niche > p. 12
🔗 Anchor: "Do any species of Fungi establish symbiotic relationships with other organisms?"
📌 Adjacent topic to master
S2
👉 Lichen as fungal–algal mutualism
💡 The insight

Lichen is a textbook example where fungi and algae form a mutually beneficial partnership.

Commonly asked specific example in UPSC prelims/mains for demonstrating mutualism and symbiotic adaptations; connects to topics on indicator species and adaptations to harsh environments.

📚 Reading List :
  • Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 1: BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY > Niche > p. 12
🔗 Anchor: "Do any species of Fungi establish symbiotic relationships with other organisms?"
📌 Adjacent topic to master
S2
👉 Fungal nutritional modes: saprophyte vs parasite
💡 The insight

Fungi exhibit different nutritional strategies, growing on dead organic matter as saprophytes or living on hosts as parasites.

Essential for questions on decomposition, nutrient cycling, plant pathology and agricultural impacts; helps classify fungal roles in ecosystems and their management implications.

📚 Reading List :
  • Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 9: Indian Biodiversity Diverse Landscape > 2. Fungi: > p. 156
  • Science , class X (NCERT 2025 ed.) > Chapter 5: Life Processes > 5.2.2 Heterotrophic Nutrition > p. 84
🔗 Anchor: "Do any species of Fungi establish symbiotic relationships with other organisms?"
📌 Adjacent topic to master
S3
👉 Types of biotic interactions (mutualism, commensalism, parasitism)
💡 The insight

Symbiotic relationships are classified as mutualism, commensalism, or parasitism, which determine benefit or harm to the partners.

High-yield for ecology questions: knowing these categories helps answer questions on organism relationships, ecological dynamics, and examples of interdependence. This concept links to ecosystem function, species interactions, and conservation implications, and frequently appears in both descriptive and application-style questions.

📚 Reading List :
  • Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 2: Functions of an Ecosystem > 2.6. BIOTIC INTERACTION > p. 16
  • Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 1: BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY > Niche > p. 12
🔗 Anchor: "Do any species of protozoa establish symbiotic relationships with other organism..."
🌑 The Hidden Trap

UPSC loves the specific examples in NCERT Ecology chapters. Next logical Q: 'Commensalism' examples. Memorize: Orchid on Mango branch, Barnacles on Whale, Cattle Egret + Grazing Cattle, Sea Anemone + Clownfish.

⚡ Elimination Cheat Code

The 'God-Mode' Reversal: To mark any option wrong (e.g., saying Protozoa CANNOT form symbiosis), you would need to know the behavior of every single protozoan species on Earth. Since you are not God, and the group is massive, assume the capability exists. Mark All Correct.

🔗 Mains Connection

Mains GS-3 (Agriculture & Environment): Link 'Fungal Symbiosis' (Mycorrhiza) to 'Biofertilizers'. Using symbiotic fungi reduces NPK dependency, bridging ecology with Sustainable Agriculture and Food Security.

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