Q. Which one of the following is a filter feeder? (UPSC Prelims 2021)
Answer: Oyster
Notes: The correct answer is [C] Oyster. A filter feeder is an animal that obtains nutrients by straining suspended matter and food particles (such as phytoplankton or detritus) from water.

• Oyster (Correct): As bivalve mollusks, oysters pump large volumes of water through their gills. Cilia on the gills trap food particles, which are then transported to the mouth. A single oyster can filter up to 50 gallons of water per day, playing a crucial role in maintaining water clarity in marine ecosystems.
• Catfish (Incorrect): These are primarily bottom-feeders (detritivores) or predators. They use their barbels ("whiskers") to detect food in murky water but do not use a filtration mechanism to feed.
• Octopus (Incorrect): These are active cephalopod predators. They use their tentacles to catch prey (like crabs or fish) and a sharp beak to bite and consume them.
• Pelican (Incorrect): While pelicans use their large throat pouches to scoop up fish and drain out water, they are considered "gulp" or "scoop" predators. They target specific individual prey rather than straining microscopic nutrients from the water column.

Other common examples of filter feeders include sponges, clams, krill, and even large marine mammals like baleen whales (e.g., Blue whales).