Stingrays, just like sharks and chimeras, belong to cartilage
Chondrichthyes. Different to other type of fish, they have a
cartilaginous skeleton and do not have a gas bladder. Buoyancy
is provided by large, high in fat content liver.
Most species of cartilaginous fish live in sea/salt waters. Yet,
approximately 30 species are known to inhabit fresh and brackish
waters. The vast majority of those can be found in South America,
several species come from Asia and one species lives in Africa.
All South American freshwater stingrays belong to the Freshwater
Stingray family (Potamotrygonidae), Asian and African species to
the Stingray family (Dasyatidae)(Ross et Schäfer, 2000; Hanel
2009, pers. comm.).
Systematics of the South American genus Potamotrygon is relatively
complex and unsettled due to the large area which individual
species inhabite as well as high variability within the individual
populations. The fact that there is a large number of cross-species
hybrids/breeds both in nature and aquariums makes the whole
situation even more misleading (Ross, 2000).
Interestingly enough the Potamotrygonidae family had evolved
from marine family before the Andes were formed 100 milion years
ago and at time the Amazon river had run in an opposite direction
into the Pacific ocean. As a result, the South American stingrays
are the closest relatives of the Pacific family Urolophidae despite
the fact the Amazon flows into the Atlantic ocean today. This
theory is supported by 50-million-year old fossils of marine rays
found in South Dakota a few years ago. These fossils differ only
slightly from freshwater stingrays found in the Amazon today.
The theory of the opposite flow of the Amazon in pre-Andes
formation era was already highlighted earlier by parasitologists
who pointed out the close relation between freshwater stingrays´
parasites and parasites of the Pacific species. This was also
proven by molecular biologists (Lovejoy, 1997).
Distribution of the family Potamotrygonidae
All Potamotrygonidae species are endemic/present in all major
South America river systems – in Venezuela (Rio Orinoci and
Maracaibo), Guyana, French Guyana, Suriname, Colombia (Rio
Atrato and Magdalena), Peru (Rio Negro), Bolivia, Paraquay,
Argentina, Uruguay and Brazil (Rio Paraquay, Paraná, Tocantins
and Amazon) (Achenbach et Achenbach, 1976; Rosa, 1985;
Carvalho et al., 2003). Freshwater stingrays cannot be found
in Chile (Deynat, 2006).
Most species live in one river basin only. P. motoro and P. orbignyi
are exceptions that inhabit more basins, though they are considered
to be two species complexes rather than species due to their large
area of occurence and high variability.
P. signata basin also has a very large occurence area as it lives in
the Amazon river from the north of Brazil to its southern part.