Which one of the following is a filter feeder? ​

Oyster is the right answer. Oysters are bivalve molluscs that feed by filtration: they pump seawater through their gills, where cilia trap plankton, detritus and other suspended particles before the water is expelled. A single adult oyster can filter many litres of water a day, which is why oyster and mussel beds are used as natural water purifiers.

Why the others are wrong:

• Catfish is a bottom-dwelling fish that feeds on worms, insects, small fish and detritus, locating food with its barbels. It does not strain water for food.
• Octopus is a cephalopod mollusc and an active predator that hunts crabs and fish with its arms and beak. It is a carnivore, not a filter feeder.
• Pelican is a bird that scoops individual fish into its throat pouch. It catches prey one at a time, so it only looks like a filter feeder.

Core concept: filter feeding is a strategy where an organism strains small food particles out of water. Sponges, barnacles, baleen whales and many copepods also use it. For Mains, this links to aquaculture, bivalve-based bioremediation of coastal waters and control of eutrophication.

Trap: Students pick pelican because it visibly takes water into its pouch. Filter feeding means continuously straining particles from water, which the oyster does and the pelican does not.