This is the larva of an eel. (The name translates to "thin head.") Additional information at the Wired Science source and at Wikipedia.
The leptocephali of the common Japanese conger, Conger myriaster, are called Noresore·のれそれ in Kochi Prefecture, Japan, and are often served un-cooked to the table, and are eaten after dipping in Tosazu mixed vinegar. It is a spring seasonal specialty.I bet they're not very filling.
