Please enjoy this Enoshima Aquarium footage of leptocephalus, eels (and similar kinds of fish) in their transparent larval phase. This stage can last longer than most other larval phases, from three months to a year. From wikipedia

This is one of the most diverse groups of teleosts, containing 801 species over the span of 24 orders, 24 families, and 156 genera. It is supposed that this group arose in the Cretaceous period over 140 million years ago… 

Leptocephali (singular leptocephalus) all have laterally compressed bodies that contain transparent jelly-like substances on the inside of the body and a thin layer of muscle with visible myomeres on the outside. Their body organs are small and they possess only a simple tube for a gut. This combination of features results in them being very transparent when they are alive.

Related transparent viewing: the barreleye fish (an internet classic), as well as this fly-eating glass mantis. There are also lots of fish swimming in the archives. 

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