having just been reading about sexing apples... it sounds like you've got a rare Pomacea bridgesii bridgesii
to quote www.applesnail.net
"Pomacea bridgesii is known in two varieties in the wild, which could turn out to be two different species. The rare Pomacea bridgesii bridgesii subspecies (Reeve, 1856), which is only known from the Rio Grande, Reyes River at Beni (bridges) and the Pomacea bridgesii diffusa subspecies (Blume, 1957), which inhabits almost the entire Amazon river system.
Pomacea bridgesii bridgesii is rather large (65mm shell height and more), while Pomacea bridgesii effusa is much smaller, has a much more pronounced spire and a darker shell colour"