Many species of jellyfish are transparent or partially transparent. The bell-shaped body of the jellyfish is filled with a gelatinous substance that is mostly water. With no skeleton and few specialized internal organs, there is very little to prevent light from passing through a jellyfish's body.
Scientists often use the term "jellies" or "sea jellies" because the animal is not a fish at all but an invertebrate, more closely related to sea anemones or corals than any true fish. There are many varieties of sea jelly. The smallest are almost too small to see with the naked eye, while the largest can grow to well over 6 feet in diameter.