The riddle
What Do You Call a Fish with No Eyes?
Eyes and i's
Exactly one letter i sits in the word "fish". Take it away and "fsh" is left. "Eyes" and "i's", the plural of the letter, sound the same, so "a fish with no eyes" can also be heard as "a fish with no i's". The joke needs both the ear and the page: the ear hears the pun, and the page shows the gap in the word.
Fish that truly have no eyes
Some fish really do live without working eyes. The Mexican tetra, Astyanax mexicanus, comes in two forms. One lives in rivers and has ordinary eyes. The other lives in caves in Mexico, in more than 30 separate populations, and the adults have no functional eyes. The cave fish start to form eyes as embryos, but the eyes stop developing and break down. Scientists still study why, and the energy cost of keeping eyes in permanent darkness is one idea they have tested.
A sister joke
The same sound trick sits behind a joke about a deer with no eyes, which is said to be called "no eye deer". Spoken quickly, it sounds like "no idea". Both jokes lean on the sound of the word "eye".
Try it in writing
This joke is best on paper. Write "fish" on a card, cover the i with a thumb and ask the question: the word that is left is the answer.