Because they don't swim in pairs. Are there two of those fish? Nahhh.
A tuna.
Damn! (This is my go-to joke that someone told me in highschool like 7 years ago. Felt like sharing it.)
They only have one pair of trunks.
Aqua-Vlad
Pair of medics.
To-mate-toes!
You can tuna piano, but you can't piano tuna.... What about the pot of glue I knew you'd get stuck.