Board Thread:Questions?/@comment-5246571-20130910183755/@comment-3448772-20130910213357

According to the Super Mario Wiki article:

"Like Birdo and Vivian, Naval Piranha has had some controversy surrounding her gender in the past. In the original Super Mario World 2: Yoshi's Island, Naval Piranha's gender was not mentioned, and since there was no visible evidence that she was a female, most players referred to Naval Piranha as a "he". Later appearances, however, have stated that Naval Piranha's gender is female."