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Warning: preg_replace(): The /e modifier is deprecated, use preg_replace_callback instead in ..../includes/class_bbcode.php on line 2958 What's this BIZARRE animal?
An exact ID is way beyond my ken, but I'd bet on a opisthobranch in the suborder Gymnosomata. That'd make it a "Sea angel", also known as a clione, and previously known as a pteropod, which are a group of small swimming sea slugs.
On the other hand it could be a jelly that I'm unfamiliar with, but I doubt it due to the way it swims.