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Originally Posted by ices

For those that wondering what accessories that Manaka wearing behind her ears,



Source: http://nagiasu.jp/special/07.html

I think those white wing-like things behind Manaka's ears are actually part of her physical make-up. They were pointed out to be gills in one of the early episodes, unless that was mistranslated. I didn't pay attention to what they were actually saying in Japanese as much, back then.