Most rivers in Brazil's Pantanal have zero visibility, and so photographing yacare caiman under water isn't easy, even though numbers have increased, following legal protection from hunting. In fact, photographing anything under water is difficult. When Marcelo entered the Vazante do Castelo river, it was the start of the dry season, when visibility is still reasonable. His intention was to photograph piranhas, but when he put out bait to attract them, several large yacare also arrived, probably attracted by all the movement and finding plenty of piranhas to eat. 'This huge one warned me off by opening its mouth,' says Marcelo. 'Then some of them started to bite my camera housing - time to get back in the boat.'