Mutation (Cryptid Hunters #4)

the long version

Marty and his best friend, Luther, have managed to rescue Marty's cousin Grace from the clutches of the nefarious pseudo-naturalist Noah Blackwood, but their most dangerous mission lies ahead of them. Marty's parents have been missing in Brazil for months and their trail has all but run cold. With time running out, Marty and the Cryptos Island crew race off for Brazil -- where they discover that Noah Blackwood has twisted the natural order of things beyond their wildest, most terrifying dreams.