Warm freshwater
and cold-blooded inhabitants
The Belgrano wetlowland hosts the most biodiverse aquatic biota. This single ecoregion includes at least 200 species of fish (including catadromous and anadromus taxa), thanks to a great habitat differentiation.
Most of them are cyprinids that arrived from the sea, but others were able to reach the continent by zoochorous dispersal, like the Vostok flyingcarp (Aerocyprinus diffusa). Despite its name, this species is not limited to the endorheic basin of Vostok, but it's just the place where this fish originated and then dispersed.
Like their ancestors, the eggs of these fish are extremely difficult to digest for aquatic birds and can pass untouched through their digestive system. This enables flyingcarps to disperse their eggs for even thousands of kilometers.
Percids are also very diffuse and have the largest anatomical differentiation. We can find large bottom-feeder species, like the Ultimo (Ultimo detritus), which live in lakes, swamps, and slow-flowing rivers.
They feed on everything they can find in the riverbed detritus, from algae to arthropods and small vertebrates. They are gregarious fish, that move in small shoals of no more than 30 individuals. They interact with each other with rapid movements of the fin, which creates waves that are perceived with their lateral line. This bizarre language is instinctively learned by the young, with no teachings from the adults, sharply reducing the energetic effort to raise fingerlings, which are born completely independent from their first day of life.
Ultimos often fall prey to a massive and voracious percid that resembles a mix of an alligator gar and a pike: the corefish (Exomimus defluxus). This species possesses a very elongated rostrum, which is perfect to increase the range of its mouth. When facing a large prey, like an adult ultimo, the corefish tear apart the skin of the victim, which often causes internal damage and subsequently death.
Their bodies are very elongated and robust compared to other predatory fish of its region, allowing juveniles to briefly crawl on the ground to move from swamp to swamp, similar to eels.
Inside the detritus
Riverbeds are rich of benthonic macroinvertebrates and meiofauna, which become regular food for a bizarre and unique group of hexapods: the freshwater giant springtails (Gigacollembulus sp.). While similar to insects, springtails are a separate group, due to the absence of wings and more importantly a different mouthpart structure.
They were already present in Antarctica during the Holocene, with extremophile species adapted to frosty environments.
While usually small in size, springtails of the Gigacollembulus genus can be as large as a human thumb. They are both detritivorous and predators that hunt underwater, paddling with their last pair of legs.
The middle pair of legs is completely disappeared, while the front limbs perform a grasping and manipulating function. They lay their eggs near the riverbank, where the rich vegetation protects larvae from strong currents.
Alongside giant springtails, a not-predatory invertebrate can be found grazing algae and aquatic plants in the warmest rivers and lakes of Belgrano: skubaflies (Panhydroptera sp.).
This strange hairy worm is none other than a caterpillar that has adapted to live and feed underwater. Its long hairs allow it to trap and store air, which is used gradually when submerged. After around 4-6 months of life, caterpillars pupate for several days, becoming completely terrestrial butterflies that feed on the flowering plants near rivers and swamps in search of a partner.
Adults are relatively long-lived and can survive for the entire spring and summer, eventually dying in late autumn when food becomes scarce.
Since water does not freeze in their habitat, caterpillars are able to survive even during the cold season by reducing their metabolism and entering a quiescent state; due to this, wintering skubaflies are usually smaller on average and need more time to become pupae.