search google and this website:

NEOTROPICAL
Tropical Freshwater
(Flooded Grassland & Savanna, and Rivers)

The tropical freshwater ecosystems of flooded grasslands and savanna, and rivers and lakes, of South and Central America...

Class Actinopterygii : 'Ray-finned Fish'

Family Serrasalmidae: including Piranha

There are somewhere between 30 and 60 species of Piranha over several different genera. They are a group of freshwater fish that live in the Amazon river and other waterways across tropical South America. 

Many species of piranha are vegetarian, eating fruits that plop into the water. Some do attack and eat other animals. They can be attracted by blood or erratic movement in the water, and can eat the flesh off a dead animal in the water with their small sharp teeth and powerful bites. The most threatening species to humans are the 'Black Piranha' and especially the 'Red-bellied Piranha'. However, it is thought the many of the humans that have been found eaten by these fish, were people that had drowned already. While bites to fingers and body parts occur, fatal attacks by school of piranha are hard to prove. Piranha are more commonly eat by people rather than the other way around. They are sometimes easy to catch, with just a hook and no bait, as I have experienced in Brazil. They are the only fish I have eaten and enjoyed (it helps that they were freshly cooked). 

Class Reptilia

Order Crocodilia

Family Alligatoridae

This family includes the alligators and caimans.

Subfamily Caimaninae: Caimans

A young Caiman. These crocodilians are generally much safer to approach than their  Australasian or African relatives, as they (usually) don't see us people as prey. 

Caiman yacare, 'Jacaré' (Portuguese) 'Yacare Caiman'.

Melanosuchus niger, 'Black Caiman'. The largest of the Caimans, and the apex predator of Neotropical freshwater ecosystems. 

Order Squamata: Lizards and Snakes

Family Boidae

Eunectes spp., 'Anacondas'

These boas are found around freshwater habitats in the tropical Americas. They include some of the biggest snakes in the world.

Eunectes murinus, 'Green Anaconda'. The largest of the anacondas. This is the heaviest snake in the world, and the second longest (after the reticulated python of tropical Asia). They have also been recorded to live the longest, a captive specimen dying just before it's 38th birthday. They are found in freshwater across tropical South America east of the Andes.

Family Colubridae

perhaps Philodryas?, (South American) 'Green Snakes'.

For the page on the birds of the Neotropical freshwater wetlands

Class Mammalia

Order Rodentia

Family Caviidae

Hydrochoerus hydrochaeris, 'Capybara', 'Capivara' (in Brazil), Chigüire (in Colombia). The world's largest rodents. Found wherever there is freshwater over much of tropical South America.

Places to experience the Neotropical freshwater wetlands

The largest freshwater wetland in the world is the Pantanal, mostly in Brazil. Places I have stayed there include: Hotel Pantanal Mato Grosso (lodge),  and Pousa Alegre.