The creature in the GIF below looks like something from another planet, but it's actually an eel larva. Also known as leptocephali, eel larvae are transparent, thin, and contain a large amount of clear energy storage compounds. To eat, they rely on their simple tube-like gut. They live near the ocean surface and feed on particulate material like marine snow. While still in their larval phases, they lack red blood cells, which likely evolved to avoid predation by maintaining their transparency.
According to a review in TERRAPUB, leptocephali are poorly understood because they grow so large and are such good swimmers that they're rarely collected in standard-sized plankton nets used by scientists and oceanographers.
Eel larva appear so different from their adult forms that for about a century, they were classified as separate species!
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