It is quite a common error, even among some native speakers. Even though they sound the same, their meanings are different.
♦ Who's = Who is. Who's calling? Janet!
♦ Whose denotes possession: Whose pen is this? Mine!
Can I just put the "é" at the end of the sentence and say "De quem a tartaruga é"? It's a lot easier to say. I find switching from pronouncing "é" and "a" hard without either having to pause in between or messing up the word after.
And my goal at this moment is to understand and be understood. So I don't entirely care if I sound less than perfect. I can accomplish that later. It'll take to long for me to learn to speak Português Brasileiro perfectly from the start.