The death of the rather docile and gentle Rainbow Squid, dying helplessly as it gets torn in two by several vicious Sharkopaths.
The mass extinction at the end of the second segment. Especially the way the book describes it.
The Great Blue Windrunners fleeing the cataclysmic eruption, with their chick, still unable to fly, desperately calling out to its parents as they fly away.
The way the bumblebeetle perishes at the end of her 24-hour lifespan, using the last of her energy resources to drag herself onto the dead flish, where she bursts open to release her young and dies, having completed her life cycle.
A bit of Harsher in Hindsight, but the prediction that sharks will continue to rule the seas 200 million years in the future...even though in the present day, they're in serious risk of extinction from human activity.
Fridge Tearjerker—200 million years into the future and all remaining birds have gone extinct. The once proud, Defiant to the End lineage of the dinosaurs is finally gone for good.
From the cartoon
The episode "Be True to Your Crew" is full of this, since this is the episode where C.G.'s father tells C.G. to take Ethan, Emily, and Luis back to the 21st century.
Ethan, Emily, and Luis seem heartbroken at the fact that C.G. is taking them back. C.G. seems heartbroken about it to, but wants what's best for her friends.
C.G. tells her father that she had never fit in with other people her age. Her father's reaction shows that this is news to him.
For a brief period in the episode "Around the World in 80 minutes, Luis believes that the others left him on purpose, since he messed up. The audience knows that the others have a plan, but Luis' reaction is still heartbreaking.
Luis: They left me? I know I sort of...kind of...violated our mission directive but I'm really really sorry! Come back, please! Don't leave me behind!
A similar thing happens in "Queen of the Squibbons" when C.G., Luis, and Ethan go back to pick up Emily after accidentally leaving her in the Northern forest, but due to a time flyer malfunction, they immediately go to another location, with Emily watching.
