Barbatana vs. nadadeira
While both barbatana and nadadeira translate to fin in English, their usage varies based on the type of creature or object described. Generally, barbatana refers to the anatomical structure of fish, whereas nadadeira emphasizes the function of swimming and applies to a broader range of animals and equipment.
Barbatana
B1This word usually describes the membranous appendages of fish and sharks used for stability and steering. It is also the specific term used for the rigid strips usually made of plastic or metal found in shirt collars (collar stays) or corsets.
O tubarão cortou a água com sua barbatana dorsal.
(The shark cut through the water with its dorsal fin.)
Muitas pessoas são contra a sopa de barbatana de tubarão.
(Many people are against shark fin soup.)
Você precisa tirar a barbatana da camisa antes de lavá-la.
(You need to take the collar stay out of the shirt before washing it.)
O peixe tinha uma barbatana colorida e brilhante.
(The fish had a colorful and shiny fin.)
Nadadeira
B1Derived from the verb nadar (to swim), this term describes the limb or organ used for swimming. It applies to fish, but is the preferred term for marine mammals (flippers of whales, seals, dolphins) and diving equipment (swim fins).
O mergulhador colocou as nadadeiras para nadar mais rápido.
(The diver put on his flippers to swim faster.)
A baleia jubarte tem uma nadadeira peitoral muito longa.
(The humpback whale has a very long pectoral flipper.)
O golfinho machucou a nadadeira direita.
(The dolphin injured its right flipper.)
Comprei um par de nadadeiras para usar na praia.
(I bought a pair of swim fins to use at the beach.)
Summary
In summary, use barbatana or nadadeira interchangeably for fish anatomy, though barbatana is biologically more precise. Use nadadeira exclusively for marine mammals (whales, dolphins) and diving gear (flippers). Use barbatana exclusively for stiffeners in clothing like shirt collars and corsets.