I first thought it could be an Alocasia specie, but then the genus Alocasia is not listed in the Flora do Brasil
http://floradobrasil.jbrj.gov....

What else could it be?
I found them growing in the rainforest by a river

This one was smaller and had softer leaves..

Hi Myriam, Your first plant looks a lot like a Colocasia, with peltate leaves perpendicular to the petiole. I checked distribution on Tropicos and apparently C. esculenta and other Colocasia have naturalized in parts of South and central America. http://www.tropicos.org/Name/2...
The 2nd plant looks like a Xanthosoma with the veins outlining the leaf margin and the sinus(?) where the petiole meets the leaf.
http://www.tropicos.org/Name/2...

I'm just comparing general characteristics with plants I have. Hopefully Lari Ann and Zig will weigh in.

Thank you Evan, I'm sure you're right with the first one, certainly a Colocasia and I think Colocasia esculenta would make sense, thinking of it, I often found the bulbs (or tubers) for sale in Brazilian food shops, it probably naturalizes that way in places where it is not native...

I agree with Evan, Colocasia does spread rapidly and the Xanthosoma is close to home.

Thanks Zig for the confirmation!

I found the species name for the Xanthosoma
There are only 3 species native in the state of Rio de Janeiro
http://floradobrasil.jbrj.gov....
Xanthosoma maximiliani is a perfect match!

Thanks so much for your help Evan and Zig!

Myriam, that Flora do Brasil site is fantastic!

It is Evan! Would be even better if it had pictures for each species!