Which one of the following is a filter feeder?

Answer: (c) Oyster

Oysters pump water through their gills and filter out plankton — up to ~190 litres a day, cleaning the water.

The idea

A filter feeder does not chase food. It passes large amounts of water through a sieve in its body and keeps the tiny food particles. The test is whether the animal strains small particles from the water.

Oyster draws water in → Water passes over its gills → Plankton sticks to mucus → Food goes to the mouth → Clean water goes out

Why the others are wrong

(a) Catfish feel for food on the bottom with their barbels.

(b) The octopus hunts crabs and fish with its arms.

(d) Pelicans scoop fish in their pouch, then drain the water — that is catching prey, not filter feeding.

The trap

The pelican drains water from its pouch, which looks like filtering. It is hunting fish.

Also know

Other filter feeders: mussels, sponges, baleen whales (krill), the whale shark and flamingos. Because they clean the water, oyster beds are valuable to an estuary. They also collect pollutants in their bodies, so scientists test mussels and oysters to measure pollution.