#spiky seashell

The spikes could be very smooth turritella shells. Or, the entire tiara might be cut from a single huge shell with exceptionally long spikes (a spider conch perhaps). The longer versions of her crown also resemble the spikes of a Crown of Thorns starfish.

Since Attina has more than one tiara, it could be "D, All of the above."

Harder to identify is the lifeform that she employs as a hat during the opening concert.

It seems to be some kind of ray. If so, it's either de-stingered, or Ariel isn't the only sister who likes to live dangerously.