The crabs make their appearance in the film, trying to get scraps out of bubbles that are coming out of Sydney's sewage pipes. They have their own spot, and would order another crab to go and use a different spot. Shortly afterwards, Nemo comes out of the pipe, wondering where his father Marlin is. The crabs try to grab him, but he swims away before they could, causing them to fight each other.
Later, Nemo and Dory come to the crabs and ask them where Marlin is. The yellow crab tells them that he has seen him swimming past earlier, but refuses to tell them where Marlin is going, until Dory attempts to feed him to the seagulls if he does not talk.
During the credits, one of the crabs can be seen dangling from one of the filmmakers' names.
According to the Pixar Character Encyclopedia, their names are Bernie (yellow) and Baz (red).