straight fin

例句：

1. 1.The fish swims with its straight fins.
   这条鱼用它的直鳍游动。
2. 2. The shark has sharp, straight fins.
   鲨鱼有锋利的、直的鳍。
3. 3. The dolphin has a long, straight fin on its tail.
   海豚在尾巴上有一根长长的直鳍。