By Stephen Dowling BBC News Online entertainment staff

Lee Unkrich and Andrew Stanton: Proud of their film

The computer-generated cartoon, about a father clownfish searching for his stolen son, has grossed more than $335m (£214m) in the US alone since May.

Co-director and writer Andrew Stanton said he thought the father-son story would mean smaller box office takings.

"I was convinced it was going to narrow the audience," he said. Finding Nemo has just been released in the UK.

The film has been produced by Pixar, which was also behind international hits Toy Story, A Bug's Life and Monsters, Inc.

"If somebody had told us before it was going to be so successful I'd have thought they were taking drugs," he told BBC News Online.

"It is the first time we had a movie come out in the summer in the US," said co-director Lee Unkrich.

Big-name stars

"We thought it was going to be between the Matrix Reloaded and The Hulk, as the Matrix Reloaded came out two weeks ahead of us, and The Hulk two weeks after us.

"We were just hoping we could carve out our own niche in that two weeks because we were going to be clobbered the rest of the summer," Mr Unkrich said.

How Pixar changed animation Read our review

The film stars the vocal talents of big-name actors such as Ellen Degeneres, Geoffrey Rush and Willem Dafoe, in keeping with Pixar's track-record of voicing its characters with household names.

"Nemo was the easiest movie to get everyone we wanted because of our past success," he said.

The two directors said the film had proven a test for its animators because water was so hard to capture on computer.

"The knowledge of how to do it has been around four or five years. What's been tough is the ability - the computing time and the money - for an entire movie.

Less edgy

"The thing we conquered the most was the cinematic trick of making you think you were underwater - something disappearing the in the murk, shafts of light, plankton in the water," Stanton said.

Both directors were aware of the criticism that Finding Nemo - Pixar's fifth feature - was deemed to be less edgy by movie reviewers.

Pixar also produced the film Monsters, Inc

"I can't be annoyed with that because it's a conscious choice - it's not like we are unaware of what our previous movies are."

Stanton and Unkrich are now looking forward to two new Pixar films.

Action-adventure The Incredibles "is about a family of superheroes in a world where there really are superheroes, but nobody wants them around anymore," Mr Unkrich said.

While the second new project - to be directed by Pixar co-founder John Lasseter - is called Cars, set on the USA's famous Route 66 with cars as the major characters.