Atún vs. tuna
In Spanish, the words atún and tuna are a classic example of false friends for English speakers. While they sound very similar to the English word tuna, they refer to completely different things: one is a fish and the other is a fruit.
Atún
A2The word atún refers to the fish known in English as tuna. It can describe the living animal or the meat, which is commonly found canned or served as a fresh steak.
Voy a preparar una ensalada de pasta con atún y aceitunas.
(I am going to prepare a pasta salad with tuna and olives.)
El chef selló el filete de atún a la perfección.
(The chef seared the tuna steak to perfection.)
En el documental marino, vimos un enorme banco de atún.
(In the marine documentary, we saw a huge school of tuna.)
Mi sándwich favorito para el almuerzo es el de atún con mayonesa.
(My favorite sandwich for lunch is tuna with mayonnaise.)
Tuna
B1The word tuna refers to the fruit of the nopal cactus. In English, this fruit is commonly known as prickly pear. It is sweet, juicy, and can be red, green, or yellow-orange.
En México, el agua de tuna es una bebida muy refrescante.
(In Mexico, prickly pear water is a very refreshing drink.)
Hay que tener cuidado al pelar una tuna por sus pequeñas espinas.
(You have to be careful when peeling a prickly pear because of its small thorns.)
Compré unas tunas verdes muy dulces en el mercado.
(I bought some very sweet green prickly pears at the market.)
La mermelada de tuna tiene un sabor exótico y delicioso.
(Prickly pear jam has an exotic and delicious flavor.)
Summary
To summarize the key difference: atún is the fish (tuna), which you eat in sandwiches or as a steak. On the other hand, tuna is the fruit from a cactus (prickly pear), which you might enjoy fresh or as a juice. They are never interchangeable in Spanish.