User blog comment:Miskos3/Fairy Tail Episode 237: Tartaros arc Prologue: Natsu vs. Jackal, Review/@comment-5303512-20150614030216/@comment-7418318-20150614052912

It was only to be expected.

Minerva was hanging there more or less motionless and when she moved, they showed only her face and shoulders, so it's easy to hide the privates behind the tentacles and bubbles.

But the scene with Erza and Kyouka will be longer and contain more movement. There was no way the usual mist-method would work on such scenes, so they had to find another way. A shirt is the most convenient, easiest and also most logical choice. How else would you propose to hide her nipples and privates in an animated scene like that?