The joke in "Finding Nemo"

What was the joke that Marlin couldn’t get tell correctly until the very end of the movie? It had something to do with mollusks and sea cucumbers or something…

Thanks.

I don’t know the whole joke (which never gets told) but the punchline is

“…with fronds like these, who needs anemones?”

The joke was:

He is a clown fish (“I know funny!”) who cannot even tell a simple joke

I entirely accept Casey1505’s explanation, but I have to say, yay for Google, jokes a-plenty:

See here:

Or even better but slightly off-topic, I found in a few places:

I’m so sorry :wink:

I’m glad I’m not the butt of that joke!