How to Discover a New Species of Fish, by the Academy of Natural Sciences of Drexel University

This is Mark Sabaj Perez, interim curator of fishes at the Academy of Natural Sciences of Drexel University in Philadelphia.

The second way taxonomists discover new species is through fieldwork at places unexplored or poorly sampled. The following images are from expeditions to the Xingu River, Brazil, an Amazonian tributary that was poorly sampled until recently. Funded by the National Science Foundation, the so-called iXingu Project was jointly led by Mark Sabaj Perez, the author; Lúcia Rapp Py-Daniel of the Instituto Nacional de Pesquisas da Amazônia, Brazil, and Leandro Sousa of Universidade Federal do Pará, Campus de Altamira, Brazil. In addition to yielding valuable new museum specimens to study, the fieldwork documented the natural aquatic diversity of the Xingu River prior to its disruption by Belo Monte, the world’s third largest dam complex.

Aspredo eggs incubate while attached to the underside of the mother.

Taxonomists sample the calm margins and backwaters of large rivers with gill nets, which are checked throughout the day and night. Gill nets are effective for snaring deep-bodied fish like pacus and piranhas in the family Serrasalmidae.

Large gill nets are effective at catching channel dwellers like this ripsaw catfish. Its common name comes from the row of bony plates along its sides, each with a central thorn pointed toward its tail.

Sandy beaches are best sampled with large bag seines pulled by hand. The best time to sample beaches is at night when many fishes move into the shallows to feed in the cover of darkness. Examples are freshwater stingrays and drum. Others, like the knifefish, rise up from their daytime retreats in the sand.

Among the most difficult habitats to sample are large river rapids. One effective technique is cast-netting. Lead weights surround the perimeter of the circular cast net. They quickly sink to the bottom when thrown, trapping swift-water fishes like Leporellus vittatus and Leporinus fasciatus, two colorful members of the family Anostomidae.

Hand lines are another good way to sample large rapids, particularly deep swirling pools where large predatory fishes lurk during the day. Examples are peacock bass and sabertooth characin.

A specialized technique for sampling rapids is with bow and arrow or a homemade spear-gun. Deep bodied fishes like this new pacu turn on their sides to swim past shallow rapids. That makes them easy targets for a skillful shot.

Streams are often sampled using small seines tied to brails pulled between two people. The seine can be set downstream of a particular habitat, like a submerged woody snag. The habitat is kicked and overturned, startling resident fishes into the net which is then lifted out of the water. Examples of fishes caught in this small blackwater stream are the catfish Helogenes marmoratus and tetra Hyphessobrycon heterorhabdus.

Other habitats are only available in the high water season when the river floods its banks into the surrounding forest. Many fishes, including the arowana and tiger shovelnose catfishes, reproduce during this time, and the flooded habitats provide refuge for their young to mature.

The clear waters of the Xingu River provide ample clarity for diving and hand-picking fishes, especially plecos, an archetype of the tropical fish hobby. Ornamental fishermen carve slender pointed sticks to pin and grab or coax plecos from their hiding places in rocky crevices.

For centuries taxonomists have relied on the skills and expertise of local fishermen. They know the river, specifically where, when and how to fish for particular species. Dani, shown here with his son, is an ornamental fisherman from Altamira. He collects fishes, especially plecos, to sell to the tropical fish trade. Prior to sale, the plecos are kept in wooden coffins submerged in the river near his home.

Our expeditions to the Xingu River have yielded thousands of new specimens to study at the Academy of Natural Sciences and at other natural history museums in the U.S. and Brazil. After the specimens are sorted into jars, identified and cataloged, another expedition beckons.