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

Academy of Natural Sciences of Drexel University

Meet the Author
This is Mark Sabaj Perez, interim curator of fishes at the Academy of Natural Sciences of Drexel University in Philadelphia.
Fishes in a Pan
Scientists, specifically taxonomists, discover new species in two different ways. One way is by studying plant and animal specimens at the Academy of Natural Sciences and at other museums. The Academy’s collection of more than 18 million specimens constitutes an archives of life. Each specimen is like a book. The more the taxonomist reads and compares one specimen to others, the more he or she is able to distinguish a known species from one not yet discovered.
Expedition Team
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.
Trawling to Sample Habitats
Taxonomists employ a variety of gear and techniques to sample the different habitats where fish live. Here we are using a net (submerged) that opens between two wooden doors (exposed) when dragged along the river bottom. Trawling yields species exclusive to the depths of large river channels, like the nearly blind knifefish and banjo catfish on the next two pages.
Banjo catfish, Aspredo aspredo
Aspredo eggs incubate while attached to the underside of the mother.
Using Baited Long Lines
Taxonomists also use baited long lines to sample large river channels. The line has many hooks and is set overnight in the channel. Long lines are effective for collecting large channel dwellers like the following migratory catfishes.
Sampling with Gill Net
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.
Ripsaw catfish Oxydoras niger
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.
Red-bellied piranha, Pygocentrus nattereri
Smaller gill nets can be set around specific habitat types such as a stand of water hyacinth. Fishes that school and feed under the vegetation, like this red-bellied piranha, become trapped in the net when they exit such habitats.
Stingray, Potamotrygon motoro
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.
Using Hand Lines
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.
New pacu, Tometes kranponhah
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.
Fishing Gaff
Another specialized technique employs the fishing gaff, essentially a large hook lashed to the end of a sturdy pole. The gaff is used to catch electric eels in shallow, quiet backwaters. The eels must surface to breathe air. The trick is to gaff the eel when it rises and pull it to shore without being shocked!
Catfish, Helogenes marmoratus
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.
Flooded Forest Refuge
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.
Hand-picking Fishes
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.
Dani the Fisherman
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.
Fish Specimens Storage and Study
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.
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