All Myths Are True: The World Tree does exist and is located in Antarctica. Given Atlantean influence on ancient Earth, it's likely their global empire is why the concept exists in multiple Indo-Euorpean mythologies.
Amusing Injuries: The Grandis' Gang's exposure to the frigid environment of the South Pole is played for laughs.
Animals Not to Scale: Irion is gigantic even by whale standards. It's likely due to the genetic engineering employed by the ancient Atlanteans during their experiments to find a suitable servant race.
Brick Joke: Nadia complaining about the South Pole not actually being cold (due Nemo's base being heated)...and then goes back to complaining about it actually being cold when Nemo takes her and Jean to the surface at the end of the episode.
Covert Pervert: Nadia, due to Nemo's base being heated, discards her cold gear and adjusts her tube top. Jean once again can't help trying to sneak a peak...and once again gets slapped in the face (by both Nadia and an equally disgusted Marie).
Dramatic Irony: Irion tells Nadia she will find her Father soon. Nadia is unaware her Father is standing right there.
"Everybody Laughs" Ending: Intentionally invoked, as this episode is the last bout of levity before all hell begins breaking loose for the Nautilus beginning next episode.
Irion's sentience and advanced age are setup for the reveals of Ancient Atlantis' genetic engineering experiments. Likewise, being able to communicate through the Blue Water is another clue tying Irion to Atlantis.
Similarly, as with Episode 14, Nemo's knowledge of prehistoric history and evolution. This was only a glean in the eye of Late 19th Century paleontologists and scientists, so it further hints at Nemo being both educated and having had access to the cumulative knowledge of ancient Atlantis.
The first mention of Nadia's brother. If he's still alive, then where is he and how were they separated?
Speaks Fluent Animal: Nadia is able to speak with Irion by means of the Blue Water. This also retroactively means she could understand King thanks to the jewel.
Surprisingly Realistic Outcome: Grandis trying to follow Nemo out onto the surface of the South Pole (with Hanson and Sanson in tow) and without any protective gear or clothing. This goes exactly as you'd expect (frankly, they're lucky they didn't get frostbite).
Wham Line: Nadia learns from Irion that she has a brother and he (and her Father) are still alive.
