UPSC Prelims 2021 Q26: Environment & Ecology – Ecosystems & Ecology Concepts
- (a) Catfish
- (b) Octopus
- (c) Oyster
- (d) Pelican
Answer: (c) Oyster
Explanation
Filter Feeders in Aquatic Ecosystems
| Animal | Feeding Style | Filter Feeder? |
|---|---|---|
| Catfish | Active predator/omnivore | NO |
| Octopus | Active predator with beak/tentacles | NO |
| Oyster | Filters water through gills, extracts plankton/detritus | YES |
| Pelican | Scoops fish in throat pouch | NO (piscivore) |
Oysters (bivalve molluscs) are filter feeders - they pump water through their gills and trap microscopic plankton, algae and organic detritus. A single oyster filters ~50 gallons of water per day, playing a critical role in water clarification. Other filter feeders: mussels, clams, sponges, baleen whales, flamingos, whale sharks.
Answer: (c) Oyster.
Why each option is right or wrong
- (a) Incorrect. Catfish are active predators/omnivores with barbels for finding food, not filter feeders.
- (b) Incorrect. Octopus is an active predator using beak and tentacles to catch prey.
- (c) CORRECT. Oysters (bivalve molluscs) filter water through gills to extract plankton, algae and organic detritus. One oyster filters ~50 gallons of water daily.
- (d) Incorrect. Pelicans are piscivorous birds that scoop fish in their throat pouch - not filter feeders.
Key point
Filter feeders = extract food from water by filtering. Oysters, mussels, clams, sponges, baleen whales, whale sharks, flamingos. Oysters filter 50 gallons/day and clarify water. Catfish, octopus, pelican = active predators, NOT filter feeders.
Remember
Oysters (bivalve molluscs) are filter feeders - they pump water through their gills to extract plankton and organic matter. One oyster filters about 50 gallons of water daily. Catfish, octopus and pelican are active predators. Correct answer: (c).
Common mistakes
- Confusing filter feeding (passive) with active predation. Filter feeders sit and process water; predators actively hunt.
- Not knowing oysters are ecosystem engineers - Chesapeake Bay oyster restoration is a major US programme for water quality.
- Missing that even whales (baleen whales, whale sharks) are filter feeders.
Exam tip
Filter feeders scale: microscopic (sponges, clams) to gigantic (blue whale, whale shark). They are ecosystem engineers - clarifying water and cycling nutrients. Oysters, mussels and clams are the classic examples in Indian coastal ecology.
Sources
- NCERT Class IX Biology - The Fundamental Unit of Life
- IUCN - Bivalve Ecology and Ecosystem Services