Iguassu

The Rio Iguaçu drains a corner of subtropical southern Brazil and a sliver of Argentine Misiones, running roughly 1,199 mi westward off the Serra do Mar to meet the Paraná at the triple border. Near its mouth it pours over the Iguaçu Falls — some 275 linked cascades, the deepest of them the 82-m Devil's Throat — and that wall of water is the whole story for its fishes. The falls cut the upper river off from the Paraná about 22 million years ago, and in that long isolation a fauna evolved that lives nowhere else: around 39 endemic species, more than 60% of the basin's fishes, including a small flock of pike cichlids that split into feeding specialists within one river. Today a staircase of large dams has drowned most of the rapids that fauna needs.

Geography & hydrology

The Iguaçu rises in the Serra do Mar near Curitiba, where headstreams draining the coastal range gather and turn inland, and from there the river runs generally west across the Paraná Plateau before dropping into the Paraná River at the point where Argentina, Brazil and Paraguay meet. FEOW treats the whole catchment above the great falls as a single freshwater ecoregion (Iguassu, Ecoregion 346), bounded below by the cataract and taking in the Rio Iguaçu itself together with its larger tributaries — the Rio Negro, Rio Chopim, Rio das Várzeas and Rio Jordão. Elevation across the ecoregion runs from about 446 ft up to 1,1148 ft, averaging near 2625 ft, so this is a cool upland river, not a lowland one (FEOW Ecoregion 346; Britannica).

The plateau the river crosses is built of flood basalt — the vast Serra Geral lavas that cap the Paraná Basin — and the Iguaçu has spent millions of years cutting into that hard, layered rock. The falls themselves sit where the river reaches a basalt escarpment near its confluence with the Paraná: the water fans out across a broad lip and breaks into roughly 275 individual cascades strung along a horseshoe nearly three kilometers wide, the most violent of which is the Garganta del Diablo, the Devil's Throat, plunging about 269 ft. The cataracts are not just scenery. For the fishes of the upper basin they are an absolute one-way barrier, and they have been in place long enough — formed in the Oligo-Miocene, on the order of 22 million years ago — to drive the river's high endemism. Almost nothing crosses; almost nothing recolonizes from the Paraná below (FEOW Ecoregion 346; Baumgartner et al., ZooKeys, 2021).

One consequence of that long isolation shows up as an absence. The Iguaçu has essentially no large migratory fishes of the kind that dominate the open Paraná — no big characin or catfish runs surging up from the lowlands — because the falls block them. The single exception is the surubim Steindachneridion melanodermatum, a large endemic catfish confined to the lower river. The other consequence is the modern shape of the system: above the falls the Iguaçu is now a chain of reservoirs. A staircase of five major hydroelectric dams — Foz do Areia, Segredo, Salto Santiago, Salto Osório and Salto Caxias — converts much of the mainstem into still water, and a sixth, the Baixo Iguaçu plant completed in 2019, sits below Salto Caxias. Between that last dam and the falls runs the river's final free-flowing reach, about 118 mi long, threading through Iguaçu National Park (FEOW Ecoregion 346; Baumgartner et al., ZooKeys, 2021).

Water & habitat

The Iguaçu is a subtropical river, and that single word separates it from most of the cichlid waters people picture. It sits in the humid temperate belt of southern Brazil, far enough south and high enough on the plateau that winters are genuinely cool. Water temperature swings hard with the season rather than holding steady the way it does in the equatorial Amazon: measurements in the river have ranged from about 60 °F in the cold months to roughly 84 °F at the height of summer. The fishes here are built for that annual cold snap, which is part of why so few of them turn up in the warmer Paraná below the falls and vice versa.

Underneath, the river is basalt. The bed is hard volcanic rock — ledges, boulders, cobble and bedrock pavement — rather than the deep sand and mud of a lowland floodplain, and where the river still runs free it does so as a fast, broken sequence of riffles, rapids and deep rock-walled pools. That combination of swift, well-oxygenated water over stone is the habitat the endemic fauna is shaped around. The pike cichlids of the upper basin are rheophilic specialists that wedge into rocky crevices in the current; the big endemic surubim shelters in the deep pools scoured between rapids. The mean annual discharge of the Iguaçu is large — on the order of 19,2297 ft³/s near the mouth, among the bigger rivers of South America — but flow is seasonal, building through the wet months and falling in the dry, and the basalt channel concentrates that water into chutes and races rather than spreading it across floodplain lakes.

The river's chemistry and clarity have been heavily reworked by people. The Iguaçu drains a densely settled, intensively farmed part of Paraná state, and it passes through the Curitiba metropolitan region near its source; the upper river is among the most polluted in Brazil, carrying urban and agricultural loads that the dam reservoirs then trap and concentrate. The cascade of impoundments has also changed the physical river profoundly — slack reservoir water in place of rapids, altered temperature and oxygen regimes, and sediment settling where current once kept the rock swept clean. The last unimpounded stretch below Salto Caxias, inside Iguaçu National Park, is where something close to the original river survives: cool, fast, basalt-bedded water still doing what it did before the dams (Baumgartner et al., ZooKeys, 2021; FEOW Ecoregion 346).

The cichlid fauna

The Iguaçu carries roughly 127 known fish species, and endemism in the basin has been put at around 70%, with FEOW recording 39 endemic species — more than 60% of the fauna — and one endemic monotypic genus. That is an extraordinary figure for a river of this size, and it is the direct product of the falls: cut off for 22 million years, the upper Iguaçu became an evolutionary island, and its lineages diverged in isolation from their Paraná relatives. The cichlids are among the clearest examples of what that isolation produced (FEOW Ecoregion 346; Baumgartner et al., ZooKeys, 2021).

The headline is the pike cichlid flock. Four endemic Crenicichla live above the falls in sympatry and syntopy — sharing the same stretches of river — and together they form a candidate species flock within the wider C. mandelburgeri complex of the Middle Paraná. What makes them more than four similar predators is that each has evolved into a different feeding specialist. Crenicichla iguassuensis (Haseman, 1911) is the piscivore, the fish-eater of the group. Crenicichla tesay (Casciotta & Almirón, 2009) is a molluscivore, built to crush snails. Crenicichla tapii (Piálek, Dragová, Casciotta, Almirón & Říčan, 2015) is a gregarious periphyton grazer that rasps algae and biofilm off the rocks. And Crenicichla tuca (Piálek, Dragová, Casciotta, Almirón & Říčan, 2015) is a thick-lipped, crevice-feeding invertivore, its fleshy lips suited to picking prey out of cracks in the basalt. Phylogenomic work using ddRAD sequencing supports the Iguazú group as monophyletic and shows it diversified rapidly, the populations now completely isolated — a textbook case of a flock arising through ecological speciation, splitting along feeding niches inside a single river (Piálek, Burress, Dragová, Almirón, Casciotta & Říčan, Hydrobiologia, 2018).

The other endemic cichlids on the roster sit alongside that flock. Australoheros angiru (Říčan & Kullander) is a deep-bodied acara of the upper Iguaçu, one of the more compressed members of its genus, part of the Australoheros radiation that occupies slower margins and pools across southern South America. Gymnogeophagus taroba (an eartheater of the G. setequedas group, described in 2017) is another Iguaçu endemic, separated from its nearest relatives by the falls — its name comes from Tarobá, the warrior of Kaingang legend who, in the story, defied the elders for the maiden Naipí and was turned to a palm tree on the bank when the serpent Mboi tore the river open to make the cataract. The list closes with Apistogramma borellii, the umbrella dwarf cichlid, which differs from the rest: it is a widespread, hardy little fish of the broader Paraná–La Plata system rather than an Iguaçu endemic, holding to vegetated, slacker water. Set against the endemic pike-cichlid flock and the endemic eartheaters, it is a reminder that the Iguaçu fauna mixes a few widespread newcomers with a core of species the falls built in place (FishBase; novataxa/Gymnogeophagus taroba description, 2017; Baumgartner et al., ZooKeys, 2021).

Conservation

The thing that makes the Iguaçu cichlids special — life in cool, fast, rock-bedded water above an impassable falls — is exactly what the dams take away. Six large hydroelectric plants now occupy the river above the cataract: Foz do Areia, Segredo, Salto Santiago, Salto Osório, Salto Caxias and, since 2019, Baixo Iguaçu. Each turns a reach of broken, oxygen-rich rapids into a long stretch of still reservoir, and each is a barrier that chops the river into isolated segments. For fishes adapted to current and connected rocky habitat, slackwater reservoirs are not a substitute; they are a different river. Sediment that the current once swept off the basalt settles into the crevices that crevice-feeders and rock-grazers depend on, and the staircase of impoundments fragments populations that were already confined to a single basin with nowhere to go (Baumgartner et al., ZooKeys, 2021).

The endemic pike-cichlid flock is the fauna most exposed to this. Because the four Crenicichla live together in the flowing upper river and are found nowhere on Earth except above the Iguaçu Falls, drowning their rapids does not just thin a population — it removes habitat from species with no second home and no upstream colonists to replace them. The same logic applies to the endemic surubim Steindachneridion melanodermatum, the largest catfish of the lower basin and an endangered species that shelters in the deep pools between rapids; dams alter the flow and the pool-and-riffle structure those fish need, on top of fishing pressure. With around 39 endemic species packed into one isolated catchment, the Iguaçu is the kind of place where a single badly sited reservoir can erase a lineage outright (Baumgartner et al., ZooKeys, 2021; FEOW Ecoregion 346).

Two other pressures compound the dams. The upper Iguaçu is one of Brazil's most polluted rivers, loaded with urban and agricultural runoff from the Curitiba region and the farmland of Paraná state, and reservoirs concentrate those loads rather than flushing them. Introduced and non-native fishes are a further threat to an endemic-heavy fauna with no evolutionary experience of them. Against all this, the one real refuge is the river's last free-flowing reach: the roughly 118 mi below Salto Caxias dam, running through Iguaçu National Park, a UNESCO World Heritage site shared with Argentina's Parque Nacional Iguazú. That dam-free stretch still holds rapids, deep pools and a rich endemic fauna, and it is increasingly the last place where the Iguaçu's distinctive cichlids can live in the kind of river that produced them. Protecting it — and resisting the next dam — is most of what conservation of this fauna now means (Baumgartner et al., ZooKeys, 2021; FEOW Ecoregion 346).

Where these cichlids live

Geography & hydrology

Water & habitat

The cichlid fauna

Conservation

Sources

Cichlids recorded here

Apistogramma · 1

Australoheros · 1

Crenicichla · 4

Gymnogeophagus · 1