Scientist splits Amazonian giants into separate species

Jeremy Hance
October 23, 2013

It's hard to mistake an arapaima for anything else: these massive, heavily-armored, air-breathing fish (they have to surface every few minutes) are the megafauna of the Amazon's rivers. But despite their unmistakability, and the fact that they have been hunted by indigenous people for millennia, scientists still know relatively little about arapaima, including just how many species there are. Since the mid-Nineteenth Century, scientists have lumped all arapaima into one species: Arapaima gigas. However, two recent studies in Copeia split the arapaimas into at least five total species—and more may be coming.

In the most recent study, researcher Donald Stewart with SUNY College of Environmental Science and Forestry (ESF), describes an entirely new species of arapaima based on a specimen held in the Instituto Nacional de Pesquisas da Amazônia in Brazil. Dubbed Arapaima leptosoma, the new species is more slender than Arapaima gigas and possesses other important physical differences.

"This species was collected near the confluence of the Solimões and Purus rivers in Amazonas State, Brazil," Stewart writes. "It is the first new species of Arapaima to be described since 1847."

But, Stewart proposes resurrecting an additional three arapaima species that were described at one time in the Nineteenth Century, but never fully accepted by the scientific community. For example, earlier in the year Stewart also published a paper that argues for resurrecting Arapaima agassizii that was described in 1847. The species was based on a specimen collected by a French biologist in the early 19th Century, however the specimen in question was destroyed by a bomb in World War II. Looking back at original sketches made of the now lost specimen, Stewart contends that the species is in fact distinct. However, no one knows exactly where to find Arapaima agassizii.

A new species of arapaima: Arapaima leptosoma. This species is housed at Sevastopol Sea Aquarium in the Ukraine, but long conflated with Arampaima gigas. Photo by: George Chernilevsky.
A new species of arapaima: Arapaima leptosoma. This species is housed at Sevastopol Sea Aquarium in the Ukraine, but long conflated with Arampaima gigas. Photo by: George Chernilevsky.

"Arapaima agassizii still is known only from the holotype [i.e. specimen], which was collected in 1817–20 somewhere in lowlands of the Brazilian Amazon," Stewart writes in his paper. "It thus is important to locate this taxon to determine its distribution and conservation status."

If arapaima are decidedly a number of distinct species, instead of one single type it would change the way the fish are managed and their conservation classifications. In many parts of the Amazon, arapaima were long overfished, but some populations are beginning to make a comeback due to better management and conservation work.

Arapaima are massive freshwater fish, weighing around 200-400 pounds (90-180 kilogram) and measuring 2-2.5 meters (6.5-8 feet). They are air-breathers, which helps them survive in the sometimes oxygen-depleted rivers of the Amazon; while they don't have lungs, they have a swim bladder that has evolved to take-in oxygen. As if that weren't enough, arapaima are armored by incredibly tough, yet flexible scales—so strong that not even piranhas can make a dent. Scientists say arapaima are virtually unchanged in the fossil record for the last 23 million years.

Close-up of arapaima at Smithsonian National Zoo. Photo by: Tiffany Roufs.
Close-up of arapaima at Smithsonian National Zoo. Photo by: Tiffany Roufs.

  • Donald J. Stewart (2013) Re-description of Arapaima agassizii (Valenciennes), a Rare Fish from Brazil (Osteoglossomorpha: Osteoglossidae). Copeia: March 2013, Vol. 2013, No. 1, pp. 38-51.
  • Donald J. Stewart (2013) A New Species of Arapaima (Osteoglossomorpha: Osteoglossidae) from the Solimões River, Amazonas State, Brazil. Copeia: September 2013, Vol. 2013, No. 3, pp. 470-476.

AUTHOR: Jeremy Hance joined Mongabay full-time in 2009. He currently serves as senior writer and editor. He has also authored a book.

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Jeremy Hance
mongabay.com (October 23, 2013).

Scientist splits Amazonian giants into separate species.