Which one of the following is a filter feeder?

Answer: C. OYSTER is a filter feeder. FILTER FEEDERS are aquatic organisms that obtain food by FILTERING SMALL PARTICLES (plankton, organic detritus, bacteria) from the water passing through specialised feeding structures. They are ecologically important as water purifiers. OYSTER (C) — CORRECT. Oysters are BIVALVE MOLLUSCS that pump water across their gills using cilia. The gills (ctenidia) trap suspended particles in mucus, which is then transported to the mouth. A single adult oyster can filter 5-50 LITRES OF WATER PER HOUR. Oyster reefs are critical water-clarifying ecosystems (notably the Chesapeake Bay). Other bivalve filter feeders include MUSSELS, CLAMS, SCALLOPS. Distractors: (A) CATFISH — WRONG. Catfish are PREDATORY OR OMNIVOROUS BOTTOM-FEEDERS. They use barbels (whiskers) to detect prey and scavenge insects, smaller fish, plants, and detritus. Not filter feeders. (Some catfish species feed on plankton, but the prototypical catfish is not a filter feeder.) (B) OCTOPUS — WRONG. Octopuses are highly intelligent ACTIVE PREDATORS. They hunt crabs, shrimp, fish using their arms and beak. Definitely not filter feeders. (D) PELICAN — WRONG. Pelicans are LARGE PISCIVOROUS BIRDS that catch fish using their distinctive throat pouch. They scoop and gulp whole fish; the pouch DRAINS water but the pelican does not 'filter feed' on plankton/microparticles. (Flamingos are the famous filter-feeding bird; pelicans are not.) Source: NCERT Class 11 Biology — Animal Kingdom (Mollusca) / ecology references on filter-feeders.