The barracuda is seen at the beginning of the film as it looks at Marlin and Coral. Coral sees her eggs and rushes to protect them. Marlin tries to help Coral, but the barracuda knocks Marlin unconscious and eats Coral and her eggs, except one that later becomes Nemo.
In the original storyboard of the movie, there was going to be a scene where Coral and the eggs were seen inside the barracuda's mouth, but it later cut from the final film because it would have scared little kids watching the movie.
The Barracuda can be heard roaring as it attacks Marlin and Coral. In a similar vein to Glut the Shark from The Little Mermaid, this may have been done merely for dramatic purposes.
In real life, barracudas do not eat clownfish (or any other kind of fish) eggs. Instead, they eat jacks, grunts, groupers, snappers, small tunas, mullets, killifishes, herrings, and anchovies.
In the 3-D re-release, when the barracuda closes its teeth, its bottom lip is shown.
The barracuda appears
Barracuda stares at clownfish angrily
The barracuda attacks the moment Coral moves
The barracuda opens its jaws
The barracuda gets into a fight with Marlin
Marlin vs. the barracuda
The barracuda slaps Marlin with its tail, knocking him out cold