Amazonian Fish Diversity
There are more than 2,400 species of fish known to
inhabit the waters of the Amazon and its tributaries,
and additional species may remain yet to be formally
described. Approximately 40% of the species thus far
described are members of two groups, the characins
(plate 12- 62) and the catfish. These multitudes include
many favorites of the aquarist such as the Neon Tetra
(Paracheirodon innesi), Cardinal Tetra (P. axelrodi),
Pearl Headstander (Chilodus punctatus), Silver
Hatchetfish (Gasteropelecus levis), Oscar (Astronotus
ocellatus), and various species of catfish such as the
many Corydoras species.
Many Amazonian fish species are small. It has been
hypothesized that small size evolved in response to a
diet of tiny arthropods obtained during the flood cycle
from within the flooded forest. Thus, when the tiny
Neon Tetras gather at the surface to grab up miniscule
morsels of tropical fish food, they may be exhibiting
a behavior originally evolved as they massed around
flooded forest trees, gathering up the displaced insects
and spiders.
The Amazon Basin is home for the infamous Red
Piranha (Pygocentrus nattereri; plate 10- 25) and its
relatives, a group of fish whose reputation for collective
ferocity is rarely merited. Though widespread and
abundant, piranhas, which reach 35 cm (approx. 14 in)
in length, are potentially dangerous only when water
levels are low and food supply is poor, concentrating
the already hungry predatory fish and putting any
potential protein source at risk of attack. Piranha do
sometimes increase to such abundance that their
collective predatory habits may act to deplete some
local fish species. Along parts of the Brazilian Amazon,
a plant known locally as Timbo (Lonchocarpus utilis)
is used to reduce numbers of piranha and piranha
eggs. The plant contains rotenone, a toxin often
used throughout Amazonia by indigenous people to
temporarily paralyze fish and thus make them easy to
catch. The rotenone, at the concentration used, is lethal
to piranha and their eggs but does little if any harm to
other fish.
Electric Eels (Electrophorus electris) attain lengths
of up to 1.8 m (6 ft). With their uniquely specialized
skeletal muscles, they are capable of emitting a jolt of 650
volts. To make matters worse, they are fairly common
in Amazon waters. They are generally considered to be
far more dangerous than piranhas, as accidental contact
with an Electric Eel can lead to a debilitating shock.
The huge Arapaima, also called Pirarucu (Arapaima
arapaima; plate 12- 63), is an important protein source
for people who live along the Amazon, and in many
areas it is in decline from overfishing. It reaches weights
of 150 kg (about 335 lb) and lengths of up to 3 m (9.8
ft), but such colossal sizes are now rarely seen, due to
intense fishing pressure from people. The Arapaima is
in a group called the bony- tongue fishes. It occurs in
quiet lakes throughout Amazonia, where it preys on
many other fish species. A relative of the Arapaima,
the smaller Arowana (Osteoglossum bicirrhosum), is
one of many Amazonian fish species that commonly
show up in the tanks of aquarium fanciers, but it is also
an important food fish for Amazonian people.Plate 12- 62. The characins, a group to which this fish belongs,
are among the most species- rich fish groups of Amazonia.
Photo by John Kricher.
Plate 12- 63. Not a particularly large specimen, this 1 m (39 in)
long Arapaima is readily visible in clear water. Photo by John
Kricher.228 chapter 12 cruising the rivers to the sea