Rathbun’s tetra in the wild


In the Pantanal wetlands of Brazil, the wet season is the time of plenty. As rivers rise and spill into the surrounding grasslands and forests, fish follow the waters and move into the newly flooded landscape.

Here they may find great opportunities: a diverse range of food items, including seeds, fruits, nuts, insects and molluscs, as well as algae, detritus and vegetable matter give them the boost they need to achieve breeding condition.

Newly submersed leaf litter and other decaying matter provides the basis for a new food chain, with tiny invertebrates providing protein to many small fishes, including some of our aquarium favourites such as Hyphessobrycon eques and Hemigrammus ulreyi.

The temporary underwater world is filled with new habitat: submerged grasses and shrubs, logs, branches, entire forest floors which fish can exploit for cover and safety from predators and as places to breed and raise young. The flooded meadows over open grassland which sit between fragments of forest are vital nurseries for many fishes. Here, the Pike cichlid, Crenicichla lepidota, can be encountered, fiercely guarding its young as it shepherds them through the grasses. Tiny juvenile tetras may be spotted darting into cover, and Apistogramma commbrae keep a wary eye out while supervising small broods of fry as they forage over the substrate.

One of the most conspicuous species to be seen here is the brightly coloured Aphyocharax rathbuni, often known as the ‘Rathbun’s tetra’ in the hobby. These fish move through the submerged vegetation, grazing on algae which forms soon after the meadow is flooded.

The rising waters are almost stagnant, with the flow from the river dispersed. This means that the sediment and particles in the water begin to sink and therefore the water becomes clear, allowing lots of sunlight penetration. The power of the tropical sun, beating down through the clear water, combined with the vast volume of nutrients entering the water column from the flooded landscape provide an initial boom to algae, which covers the leaves of many flooded terrestrial plants.

Many plant species in the region can survive for months underwater, even if they are not aquatic or semi-aquatic. Fuelled by the nutrients in the settling sediment, they remain lush until the waters fall once more. The algae however may burn itself out and so the small Aphyocharax race into newly flooded areas to take advantage of it as quickly as possible. There constant nibbling on the algae may be the clue to their intense green colouration in the wild.

You can see a video filmed by myself of these fish in their native habitat right here:


Words: Tai Strietman

Species which appear in this video:

Salminus brasiliensis

Aphyocharax rathbuni

Hyphessobrycon eques

Characidium laterale

Steindachnerina brevipinna

Cyphocharax gilli

Serrapinnus kriegi

Hemigrammus neptunus


Other species encountered in this habitat location:

Hoplias cf. malabaricus

Crenicichla lepidota

Poptella paraguayensis

Moenkhausia bonita

Moenkhausia dichroura

Apistogramma commbrae