2013 Expedition to the Lower Xingu River

Overview

No other river in South America is as famous for its myriad rapids and rapids-specialized fishes as the Xingu, a right-bank (southern) tributary of the lower Amazon River that drains the northern slope of the Brazilian Shield in northern Mato Grosso and southern Para states, Brazil. Within the Xingu, the largest stretch of nearly contiguous rapids is the Volta Grande ('Big Bend'), a 130 km long horseshoe-shaped bend in the main channel of the lower Xingu. Habitats within the Volta Grande host more than 300 fish species, many of which are specialized rheophilic (i.e., current loving) species that are evolutionarily adapted for life in close association to rocks and fast flow. Among these, many are surface-grazing detritivores, algivores and invertivores from the families Anostomidae, Cichlidae, Loricariidae and Serrasalmidae. 

        Species of the suckermouth armored catfish family Loricariidae are more diverse in the Volta Grande than anywhere else throughout this family's range. Many, like the iconic zebra pleco (Hypancistrus zebra), are boldly patterned and/or brightly colored. Because of their beauty and generally small size, loricariids from the Volta Grande are very popular among global aquarists. Almost two dozen loricariid species are commercially harvested from the Volta Grande for export and sale to the international ornamental aquarium fish industry. Some of these (e.g., Pseudacanthicus sp. L25) regularly sell for over US$1000 in American, European and Asian retail shops.

        The many bright colors and intriguingly specialized morphologies of fishes in the Volta Grande appear to be evolutionary responses to the unique ecological conditions of the rapids, in particular the combination in these habitats of high water transparency over a wide array of different shallow, crystalline, metamorphic and conglomerate rock types. This combination may promote 1) exceptional rates of attached primary production plus 2) the evolutionary specialization of jaw morphologies to graze attached algae from a wide range of surfaces and 3) sexual selection for a broad range of bold color patterns.

       To date, very little ecological or evolutionary research has focused on the Volta Grande and, unfortunately, the ecology of the entire system is being threatened by the now near complete Belo Monte hydroelectric dam complex. Once complete and operational, Belo Monte will flood much of the upper Volta Grande, and heavily reduce flows to the remaining portions. To address the urgent need for basic ecological and evolutionary research in the Volta Grande, the United States National Science Foundation awarded funds to the iXingu Project, comprising an interdisciplinary group of researchers at the Academy of Natural Sciences and Texas A&M University.

        I have been a post-doc on the iXingu Project studying gene flow among a half-dozen fish and invertebrate species that are widely distributed throughout the Volta Grande and span a wide range of life history characteristics (e.g., body size, fecundity, vagility). My goal is to identify suites of biotic and abiotic factors that contribute to accelerated speciation within the rapids. I am conducting this research in the lab of collaborator Brice Noonan at the University of Mississippi. Other scientists collaborating on this project include John Lundberg and Mark Sabaj Pérez (co-PIs at the Academy of Natural Sciences; ANSP), Kirk Winemiller (co-PI at Texas A&M University; TAMU), Dan Fitzgerald (PhD student at TAMU), Leandro Sousa (Professor at the Federal University of Para in Altamira), and Lucia Rapp Py-Daniel (Curator of Fishes at the National Institute for Amazonian Research; INPA). Voucher specimens and tissues collected during this trip are now cataloged at ANSP and INPA (field number prefix iXIN), where they are accesible to international researchers. Published papers that have already benefited from collections and observations made during this trip include the following:

iXingu Expedition II – Rising Water

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1. The first outing team and all its gear just before departing Leandro's house in Altamira. 2. At the Altamira landing waiting to load the boats. 3. An anhinga (Anhinga anhinga) sunning itself. 4. Podostemaceae blooming in the rapids. 5. One of our fishermen coming up with (6.) a Spectracanthicus zuanoni7. Magic hour in the rapids of the lower Xingu. 8. Leandro showing Madoka some of his underwater photos. 9. Dan and Rafael take substrate measurements. 10. Dani, the most interesting fisherman in the Xingu. 11. Pseudacanthicus n.sp. L025. 12. Sponge growth and snails (Doryssa sp.) on the underside of a conglomerate rock plate. 13. A new genus and species of dreissenid mussel found on the underside of a rock, with a mechanical pencil tip for size comparison. 14. The eggs of a Spectracanthicus on the underside of a conglomerate rock plate. 15. The iconic zebra pleco (Hypancistrus zebra). 16. Our field kitchen. 17. Preparing to tissue a group of snails (Doryssa sp.). 18. With a Cichla pinima. 19. Ecotoxicology students Dani and Taís dissect tissue samples from Cichla. 20. Belo Monte Dam. 21. A tray full of loricariids, including Ancistrus ranunculus, Baryancistrus xanthellus, Leporacanthicus heterodon, Parancistrus nudiventris, Spectracanthicus punctatissimus, and S. zuanoni. 22. The Xingu endemic serrasalmid Ossubtus xinguense23. Tissue team on the move upriver. 24. Potamotrygonid stringrays (Potamotrygon spp.). 25. A cymothoid parasite attached to the tongue of a Cichla. 26. A rubber tree on the banks of the lower Xingu that bears the scars of having been tapped. 27. The Xingu River at Porto de Moz (near its mouth). 28. A group photo before our third outing. 29. Leandro takes fish photos. 30. Enjoying the magic hour. 31. Our fishermen prepare for another dive, and then return with (32.) an Acanthicus hystrix attached by string to their air hose. 33. Positioning skewered Leporinus to roast by the fire. 34. An old channel of the Xingu that was dewatered to construct Belo Monte Dam. 35. A fish 'rescue' crew pulling in their seine. 36. Mark and Vanessa catalog specimens in preparation for export. 37. At the fish lab at INPA, finishing the sorting and splitting of specimens.

Fishes

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1. Sympatric color and body shape variation within what may be a single species of Hypancistrus2. Pseudacanthicus n.sp. L025, 3. Anostomoides passionis4. Hypomasticus julii5. Ossubtus xinguense, 6. Tometes n.sp., 7. Myleus/Myloplus sp., 8. Ventral view of the mouth of Ossubtus xinguense9. Cichla melaniae10. Crenicichla sp.

Sponsors

This field work was sponsored by NSF grant DEB-1257813 (the iXingu Project). 

The Cachoeira Grande do Iriri