What you've heard is true. Nemo and his family are anemone clownfish (Amphiprion ocellaris), also known as the common clownfish. These stripy orange fish live in coral reefs in tropical waters around Asia and Australia, and they develop a symbiotic relationship with the sea anemones Heteractis magnifica, Stichodactyla gigantean, and Stichodactyla mertensii. The fish depend on the anemone for shelter, and in return, the fish protect their home, sometimes ferociously. Their mucus helps keep clownfish immune to the anemone sting. And yep, they all start off as males.
There are at least 28 species of clownfish and anemonefish in the genus Amphiprion, and they are all protandrous hermaphrodites. That means everybody first develops into a male (complete with male reproductive organs), and then possibly into a female later. Females use aggressive dominance to control males and prevent the creation of other females in the social group. That’s why whenever you see a breeding pair, the female is always the bigger one.
An adult female and male can live together in an anemone with their non-reproductive juvenile fishlets. But if the female dies (or is experimentally removed), the largest male would then transform into a female. The larger of the pre-pubescent fish develops into a male, and he then dominates all the juveniles, preventing them from spawning. (They're called neuters, and they don't have fully functioning sex organs of either gender.) This life history strategy is known as sequential hermaphroditism.
That clownfish are hermaphrodites isn’t news -- we’ve known for over a decade -- but it’s definitely a shocker to hardcore Finding Nemo fans. After all, the implications for Marlin, Coral, and Nemo are too horrific to think about. Here's a video from Destin at Smarter Every Day:
Finding Nemo isn't the only children's movie that left out some less adorable biological facts. In a lion pride, only the dominant male mates. He will actually kill any offspring fathered by another male, in order to bring the female back into heat. Therefore, in the Lion King, Nala is Simba's sister.
[Via Smarter Every Day]