Exploring South America’s Xingu River
Author: Oliver Lucanus
After 30 years of traveling to photograph fish in nature, it is normal to be drawn to one favorite, most spectacular place. For me, this place is the Xingu (pronounced shin-goo), the most incredible, diverse, dynamic river in South America. Yet, it is threatened by the never-ending destruction of the Mato Grosso by industrial agriculture, deforestation, gold mining, and one of the world’s largest hydroelectric projects.
Nevertheless, the Xingu River captivates the imagination of aquarists and adventurers alike with all it has to offer. For the past seven years, Dr. Leandro Sousa and I have had the privilege to spend countless hours diving, snorkeling, wading, and exploring the Xingu, along with its tributaries and headwaters, to produce a book showcasing the river’s fish community.
At least in dry season, the Amazon lowland is somewhat predictable: the larger rivers look alike, appearing as a high wall of mud or sand topped by dense forest. The Xingu is so different in comparison.
The Brazilian Shield’s slight northward drop in altitude causes the major rivers dewatering the region to form a long stretch of wild rapids. The middle Xingu is a wild ride of endless torrents and waterfalls. It is also dotted with thousands of islands, some of which emerge in dry season and are only slightly bigger than a turtle’s back. Others are formed in solid rock and range from the size of a small parking lot to that of a shopping mall.
But what happens there that makes the aquatic fauna so unique? Many factors play into the evolution of the Xingu’s incredible fish population, but one set of special circumstances sets the river apart from others in the Amazon.
While the Xingu is large, it is also shallow and clear. This means sunlight penetrates the water, allowing nutrients for an incredible amount of algae, and with it, microinvertebrates sustained by the biomass on the substrates. With the rapids supplying oxygen, it has led to the evolution of specialized animals adapted to life in fast-flowing water. It is this web of ideal conditions that gives us the biodiversity and spectacular shapes and colors of the Xingu’s fishes.
The fishes of the Xingu were largely unknown until the 1990s, a time when “aquarium catfish” meant a brown pleco cleaning the glass in a mixed community tank. The discovery of the new and colorful catfish of the Xingu caused a sensation in the hobby.
No doubt, the river’s most iconic fish is the zebra pleco (Hypancistrus zebra), first sent to Japan by fisherman Grande Ogawa, who still lives in Altamira today, a stone’s throw from the Xingu. But other fishes of the Xingu also helped redefine the aquarium hobby. Suddenly, the diversity of the river’s plecos started the L-number craze, with new catfish from the Xingu, Tapajos, Tocantins, and eventually the Orinoco appearing nearly every month.
Some of the most beautiful and sought-after species from the Xingu were known then only by their L-numbers but have names today: the gold nugget pleco (L018/081/177, now Baryancistrus xanthellus), the red cactus pleco (L025, now Pseudacanthicus pirarara), and the goldy pleco (L014/L253, now Scobinancistrus aureatus), to name a few.
But, the variety goes well beyond color. Suddenly, the glass cleaning catfish were center stage. This was because food specialists, such as Hypancistrus, Pseudacanthicus, Leporacanthicus, and Scobinancistrus, have little use for algae, and their specialized diet meant that, for the first time, aquarists set up aquariums centered on cryptic catfish to perfect their environmental requirements and diet in early attempts to breed them.
The gold nugget (B. xanthellus) is the most common pleco in the river, feeding on algae in the shallow water, like huge herds of buffalo grazing on the plains. Below the rapids at the end of the Xingu (at Belo Monte), the fish is green with no spots, while in the upstream portions of the Rio Iriri and Rio Curua (the Xingu’s main western tributaries), it features spectacular wide yellow margins and bright yellow spots all over the body.
Plecos in Peril: The Belo Monte Project
The story of Hypancistrus zebra is tragic, because the most iconic species of the river is under threat today. For one, the world’s fourth largest hydroelectric dam, the Belo Monte project, threatens the survival of many of the river’s endemic species. The entire range of the zebra pleco is within the impact zone of the dam, and the species is also heavily overfished for the aquarium trade.
The zebra pleco is an incredibly cryptic fish, found only in horizontal crevices of large boulders in the slack water behind major rapids, where they occupy the narrowest portion of the cracks, and never venture out of this zone. This unique requirement makes their distribution predictable and vulnerable to overfishing by skilled fishermen.
Since its discovery, the zebra pleco went from being expensive, then inexpensive, and then slowly increasing in price again, until the Brazilian authorities finally decided to ban export of the species. When the measure did not stop illegal exports via Colombia and Peru, it was listed on CITES, affording at least some protection from further damage to the fragile population.
The dam, with an intake dewatering the Xingu’s most spectacular rapids, the Volta Grande (big curve), and dropping the water into the Amazon lowland at Belo Monte has been plagued by scandal and failed to deliver on its projected energy production. The greatest danger posed to the fishes stems from the controlled flow, and the now absent flood pulse of the rainy season.
In the Amazon, and elsewhere in the tropics, the gigantic flood pulse and rise in water levels signal the onset of breeding season, the same way the snowmelt and spring do in the northern hemisphere. This principal change in dynamics will cause permanent harm to the Xingu’s ecosystem, just as major dams have damaged those in Europe and North America and are now targeted for removal, as governments slowly recognize that hydroelectricity is by no means green energy.
Fabulous Freshwater Stingrays
The Xingu River is also home to some of the most beautiful fishes from several other families. Perhaps the black stingray (Potamotrygon leopoldi) is chief among them. Arguably the most striking of the freshwater stingrays, P. leopoldi—at least in the early 2000s—was also one of the most expensive aquarium fishes in the hobby.
When the first photos of the ridiculously patterned Xingu River rays were published in magazines, it was hard to believe that such a totally different and bold-colored ray could occur in freshwater. But to anyone who has visited the Xingu and seen its equally alien-looking substrates, their camouflage will seem a logical evolution.
The rapids flow over a lot of exposed bedrock substrates of granites and gneiss, with orange and red iron ore pebbles, white silica pebbles, and totally obsidian-colored sectors covered in goethite, a black, shiny material, covering softer conglomerate substrates. Over these brightly colored substrates, the Xingu River rays’ pattern acts as a great camouflage.
Aquarists have known Leporinus as black-and-yellow striped fish for more than a century. But in the Xingu, the diversity of the anostomids is on another level, with crazy-colored orange-and-black ringed Synaptolaemus cingulatus, boldly spotted Hypomasticus julii, bizarrely shaped Sartor respectus, and dozens of other species.
Anostomids are everywhere in the river, and their cylindrical shape and ability to thrive in fast-moving water make them one of the dominant families in the Xingu. Likewise, the piranhas and pacus are wildly diverse, with a handful of endemic species, including the unusual parrot pacu (Ossubtus xinguense), which is hard to see in the river because it prefers habitats with such high flow that the current pulls the mask off your face.
Not to be outdone, the cichlids of the Xingu are unique. Several endemic Geophagus species occur here, as well as one of the most basal of all new world cichlids, Retroculus xinguensis. It is oddly shaped and rheophilic, and it can reach a size of 15 inches (38 cm).
This huge fish can often be heard by snorkelers before it is seen. They chew through the substrate looking for invertebrates of all kinds, pushing with their large mouths until they are up to their eyes in the gravel. They then spit out pebbles while sifting the sand through their gills. The sound becomes familiar with time, announcing the presence of one of the river’s most beautiful and endearing fishes.
Retroculus have an impressively complex strategy for breeding. They lay eggs in a high-flow, shallow area and cover the nest with larger pebbles. Once the eggs hatch, the nest is moved frequently to new locations with the right conditions.
The pike cichlid group is the one that has been the most successful in the river. It features an impressive list of species found only in the rapids of the middle Xingu. Crenicichla dandara, C. anamiri, C. percna, C. phaiospilus, C. sp. “Xingu 1” (the orange pike cichlid), and C. sp. “Xingu 2” are all endemic to the rapids of the river. Several other species occur in the upper tributaries. Added to this are no fewer than five Teleocichla species: T. preta, T. monogramma, T. gephyrogramma, T. centrarchus, and T. centisquama.
Teleocichla are a constant presence in the shallows near almost any of the rapids. One cannot move a rock without the inquisitive little darter-like pike cichlids coming to investigate if a morsel or micro invertebrate has been made available. These endearing fish are challenging to transport, and difficult to keep in the aquarium, but they are one of the most interesting cichlids to watch in nature.
By contrast, the larger Crenicichla are an ever-present danger to the other fishes. The beautiful fry of the orange Crenicichla sp. “Xingu 1” are often seen in the shallows, where they approach people to see if the movement affords a feeding opportunity. The parents remain at a safe distance but stay with the young until they are almost 5 inches (12.5 cm) long. Perhaps no other fish guards its young longer than the large Crenicichla.
My favorite fish in the river is Crenicichla dandara. The nearly completely black fish is found in deeper water, and it is like an ominous presence from a horror movie, lurking in the shadows, stalking fish in the shimmering light at the edge of the substrate’s many caves and crevices. The huge mouth allows it to feed on the spiny plecos of the Xingu. In breeding color, females develop a bright red stomach, as if they had swallowed a gaudy LED Christmas ornament, to attract males in a brief courtship.
Beyond the Rapids
With the limited scope of this article, it is not possible to highlight all of the incredible diversity of the Xingu River. Beyond the rapids, the Brazilian Shield begins to flatten out, and this change in habitat has produced an astounding number of endemic species in the upper tributaries, where spectacular rivers with crystal clear water and dense forests of aquatic plants are home to a totally different fish community.