Some fish go through a larval stage called the leptocephalus. During this stage their bodies look completely transparent. This stage is very common in marine eels. Their bodies contain transparent jelly-like substance s on the inside. And a thin layer muscle with visible myomeres on the outside.
Their body organs are extremely small. And they process only a simple tube for a gut. They also don't have any red blood cells. Until they begin to metamorphose into the juvenile glass eel stage. This combination of features results in them being very transparent. This group is quite diverse. It contains 801 species in 24 orders, 24 families and 156 genera