Fish in the Amazon River: What Lives Beneath the Surface?

The Amazon River is one of the planet’s most significant freshwater ecosystems, draining a basin of over seven million square kilometers. Accounting for roughly one-fifth of the world’s total river flow, it carves a path through South America’s vast rainforest. The river and its thousands of tributaries create a complex mosaic of conditions. This system supports a collection of fish life found nowhere else on Earth.

The Amazon’s Diverse Aquatic Environments

The variety of fish in the Amazon is supported by an equally varied range of aquatic habitats, which are classified into three types. Whitewater rivers, like the main stem of the Amazon, originate in the Andes and carry nutrient-rich sediments, giving them a turbid color and near-neutral pH. Blackwater rivers, such as the Rio Negro, are stained dark by organic compounds from decaying vegetation, resulting in acidic, nutrient-poor water. Clearwater rivers, like the Tapajós, are chemically intermediate with low sediment and highly transparent water.

This chemical mosaic is animated by an annual flood pulse, the engine of ecological life in the basin. During the wet season, river levels can rise by more than nine meters, inundating vast tracts of the surrounding landscape. This seasonal event creates two types of floodplain forests: the várzea, associated with whitewater rivers, and the igapó, formed by blackwater and clearwater rivers.

This massive expansion of the aquatic realm unlocks enormous areas for fish to feed and reproduce. It creates a dynamic world of lakes, channels, and flooded forests that offers a multitude of niches for species to exploit.

An Unmatched Spectacle of Fish Biodiversity

The Amazon basin is home to the most diverse freshwater fish fauna on Earth, with at least 2,700 scientifically described species and potentially thousands more awaiting discovery. This represents about 15% of all known freshwater fish species. The fish population is dominated by several major taxonomic orders.

• Characiformes, which include piranhas and their fruit-eating relatives, the pacus.
• Siluriformes, a varied order of catfishes that range from giant predators to tiny parasitic forms.
• Cichliformes, known for complex parental care.
• Gymnotiformes, or knifefishes, which navigate and communicate using weak electric fields.

This speciation results from millions of years of evolutionary history shaped by the region’s geology and habitats. The uplift of the Andes Mountains created the modern basin and its gradient of environmental conditions. The basin’s size, combined with the separation of water types and the cyclical isolation of populations in floodplain lakes, has created many opportunities for evolutionary divergence and adaptation.

Profiles of Remarkable Amazonian Fish

Among the thousands of species, some stand out for their biology and reputations. The piranhas, belonging to the family Serrasalmidae, are perhaps the most famous. Species like the Red-bellied Piranha (Pygocentrus nattereri) are armed with a single row of interlocking, razor-sharp teeth. While their diet is omnivorous, including fish, seeds, and fruit, they are known to engage in feeding frenzies in shrinking pools during the dry season.

The Arapaima (Arapaima gigas) is one of the world’s largest freshwater fish, reaching lengths of up to 3 meters and weights of 200 kilograms. To survive in oxygen-poor waters, the Arapaima is an obligate air-breather, surfacing every 10 to 20 minutes to gulp air into a modified swim bladder. These predators primarily feed on other fish and crustaceans, using their mouths to create a vacuum to suck in prey.

The Electric Eel (Electrophorus electricus), a type of knifefish, possesses three electric organs that make up most of its body. It generates two types of electric discharges. Low-voltage pulses are used for navigation and finding prey in murky water, while high-voltage shocks of up to 860 volts can stun prey or defend against predators.

The Candiru (Vandellia cirrhosa) is a tiny, translucent parasitic catfish that feeds on the blood of larger fish. It uses chemical and visual cues to locate a host and swims into its gill cavity. Once inside, it latches onto the gill arteries with backward-pointing spines and feeds on the blood.

Survival Strategies in a Complex Water World

To thrive in the Amazon, fish have evolved an array of survival strategies. Respiratory adaptations are common due to the prevalence of low-oxygen waters in floodplain lakes. Many species are air-breathers, while others are facultative, breathing air only when dissolved oxygen is low. Another strategy is aquatic surface respiration, where fish like the Tambaqui (Colossoma macropomum) use modified lips to skim the oxygen-rich micro-layer at the water’s surface.

In the turbid and dark waters, sensory adaptations beyond sight are common. The Gymnotiformes, or knifefishes, use electroreception by generating a weak electric field around their bodies. They detect distortions caused by objects, prey, or predators, allowing them to navigate and hunt in complete darkness and communicate with other knifefish.

Feeding strategies are incredibly diverse. While many fish are predators or detritivores, a significant number have specialized in fruit-eating. During floods, species like the Tambaqui move into inundated forests to feed on fruits and seeds that fall into the water. These fish are effective seed dispersers, a process important to the health of the floodplain forests. Reproductive strategies are also linked to the environment’s rhythm, with many species undertaking long migrations to spawn with the rising waters.

Amazon Fish: Human Connections and Conservation Needs

The fish of the Amazon are intertwined with human life, serving as a primary source of protein and income for millions of people. Commercial and subsistence fishing operations harvest hundreds of thousands of tons of fish annually. The basin is also a supplier for the global aquarium trade, which provides income for thousands of families in remote areas. When managed correctly, this trade can offer a sustainable livelihood. Sport fishing for species like the Peacock Bass (Cichla spp.) also attracts tourism and generates economic benefits.

Despite their abundance, Amazonian fish populations face threats. Widespread deforestation leads to soil erosion and sedimentation of rivers, degrading habitats. Pollution from agriculture and illegal gold mining introduces toxic substances like mercury into the water. Mercury bioaccumulates up the food chain, reaching high concentrations in predatory fish and posing a health risk to communities that rely on them for food.

Large-scale infrastructure projects, especially hydroelectric dams, also present a danger. These projects permanently alter river flows and block migratory routes for spawning fish. This disruption of the natural flood pulse threatens the productivity of the entire ecosystem.