Freshwater stingray discovered by Alfred Wallace over 160 years ago is described

Despite having been mentioned and illustrated countless times since the nineteenth century, only now has Potamotrygon wallacei been officially named.

By Peter Moon  |  Agência FAPESP – Nature’s riches in Brazil’s Rio Negro basin, one of the world’s top biodiversity hotspots, have just become even greater as far as officially recognized aquatic species are concerned. A team of Brazilian researchers has described a new species of freshwater stingray, named Potamotrygon wallacei, in homage to the English naturalist Alfred Russel Wallace (1823-1913), co-discoverer with Charles Darwin of the mechanism of natural selection. Wallace collected and drew a specimen more than 160 years ago while exploring the Negro, a tributary of the Amazon.

The species, now scientifically described and catalogued, is very well known locally. Its vernacular name is raia cururu, cururu being a common name for the cane toad Rhinella marina, to which the dorsum of P. wallacei bears some resemblance, according to the researchers.

It is also familiar to ornamental fish lovers worldwide who buy these little creatures exported by Brazil. Despite having been mentioned and illustrated countless times since the nineteenth century in both the scientific and aquarium literature, it is always either misidentified or not identified scientifically.

“The first time I saw a specimen was in 1990, when I was an undergraduate. I thought then that it was a new species because of the adult male’s small size and its unique coloration. But I only began studying it more closely in 1997,” said Marcelo Carvalho, an ichthyologist affiliated with the University of São Paulo’s Bioscience Institute (IB-USP) and first author of the paper published by Zootaxa.

The research was supported by FAPESP through a Young Investigator Grant and three Regular Grants for the projects “Molecular evolution of regulatory regions of HOX genes associated with the morphology of fish fins, with special emphasis on Chondrichthyes”, “Comparative analysis of the morphology of the feeding apparatus in Myliobatiform rays (Chondrichthyes: Batoidea)”, and “Taxonomic review of the family Gymnuridae (Chondrichthyes: Myliobatoidei)”.

P. wallacei inhabits the blackwaters of the Middle Negro in the vicinity of Barcelos, Amazonas State, preferring flooded forest areas (igapós) and inlets (igarapés).

“It’s perfectly camouflaged by the leaves that fall from the trees in these areas,” said Maria Lúcia Góes de Araújo, an oceanographer at the Federal Rural University of Pernambuco (UFRPE), co-author of the paper and the researcher responsible for analyzing the biology of the new species.

“It’s very small,” Carvalho said. “As far as we know, it’s the smallest species of the genus Potamotrygon. The largest specimen we’ve found to date has a disc width of 30 cm. Some freshwater stingrays are huge. We once captured one that was bigger than a pickup’s hood.”

Exports of the species were outlawed between 1990 and 1997. The law changed to permit exports in 1998. “Traders need a scientific name for export purposes,” Carvalho said. “That’s why they’ve always used erroneous names until now.” P. wallacei typically ships out of Brazil misnamed as the porcupine river stingray or raia porco-espinho (P. histrix), a species found in the Paraná and Paraguay basins.

Araújo is currently in Recife, Pernambuco, but worked in Manaus for 18 years at the Federal University of Amazonas (UFAM) and Amazonas State University (UEA). She was a member of the annual stingray collection team in Barcelos until 2011. Her focus was researching the animal’s biology, but she also wanted to educate local fishers on the right time of year to catch it and the best type of specimens to collect.

The animals are captured between February and July. Fishing is illegal for them during the mating season, between August and January. Collection is sustainable only because capturing females and adult males is banned. Stingrays, like all rays, are oviviparous: their eggs develop and hatch inside the mother, who gives birth to live young. Gestation periods range from three to six months depending on the species. They do not lay eggs in the water, as most fish do, and some species provide parental care.

Legal and illegal trading

Annual stingray export quotas were introduced in 2003 by IBAMA, the national environmental protection agency. The quotas are based on reproductive potential and demographic studies conducted by specialists like Araújo. The latest revision of the export quota dates from 2009 and corresponds to 5,000 individuals of the species popularly known as raia cururu, valid only for Amazonas and Pará.

According to statistics available from CITES (the Convention on International Trade in Endangered Species of Wild Flora & Fauna), Brazil legally exported 17,000 freshwater stingrays in 2006, while illegal exports are thought to have reached 25,000-30,000. In the same year, Peru exported 15,000 specimens. Colombia’s market share has risen strongly since then, but there are no official statistics on its exports.

“The market for ornamental fish is modish,” Araújo said. “Today, the most exported species are the black diamond, P. leopoldi, and the black stingray, P. henlei, both from the Xingu in Pará. They’re larger and more handsome.”

Nevertheless, P. wallacei remains the freshwater ray with the largest export quota. “It’s not the most brightly colored species or the prettiest, but it’s easier to transport because of its small size and resilience,” Araújo explained.

Fishers try to find younger specimens with a disc width of only 6 cm. They are transported in plastic boxes containing up to six individuals. During the 30-hour boat journey to Manaus, their water has to be renewed and the temperature must be kept constant. From there, they will be flown to Europe or Asia – this leg can last 12-24 hours.

“Not all the Manaus and Belém firms that export ornamental fish have the necessary structure, so mortality during transportation varies considerably,” Araújo said. “It’s very high for some species, with half of the individuals dying during the journey. That’s fatal for business, as the importer pays only for animals delivered alive. P. wallacei is highly resilient, with only 2%-5% mortality. That’s low indeed.”

That P. wallacei is exported while very young and is highly resilient can be a problem. It continues to grow in aquaria worldwide, living about ten years and reaching 25 cm on average, although there are reports of individuals with a disc width of 31 cm. “When a specimen becomes too large for the aquarium, there’s always a risk that the owner will throw it out into a stream or pond,” Carvalho said.

In 2009, peacock-eye stingrays (P. motoro), a species that occurs throughout the Amazon basin, were found to be living and reproducing freely in a reservoir in Singapore. “This kind of thing is a threat to local fauna because stingrays are predators. So much so that importing rays is banned in Australia and several US states.”

Freshwater rays are found only in South America. They evolved from a marine ancestor that penetrated inland with marine invasions that occurred during the Eocene, some 50 million years ago, and possibly the Miocene, approximately 20 million years ago. When the sea arm that had penetrated Amazonia retreated, saltwater species had to adapt to a freshwater environment to survive. This is what the rays did, as did the Amazon pink river dolphin and the manatee.

Wallace 160 years on

In 2015, scientists described another fish species first collected and drawn by Wallace (Crenicichla monicae, a pike cichlid – read more at agencia.fapesp.br/22564). The great naturalist’s 1850-52 expedition to the Upper Rio Negro in the Brazilian Amazon continues to produce surprises more than 160 years later.

Wallace set out to assemble a large collection of local flora and fauna, both for his own research on organic evolution and to sell as a way of paying for more scientific voyages. However, his expedition ended in tragedy. Having become seriously ill, he decided to return to England. On July 12, 1852, he boarded the brig Helen with his collection of living and preserved specimens. After 26 days, a fire in the mid-Atlantic forced passengers and crew to abandon ship. Wallace was only able to save part of his diaries and sketches of fish collected on the Negro. They drifted in an open boat for ten days before being rescued by a passing ship from Cuba.

Wallace eventually made it to London, where he gave his fish drawings to the Natural History Museum. In 2002, zoologist Mônica de Toledo-Piza Ragazzo from the University of São Paulo’s Bioscience Institute (IB-USP) edited a book containing reproductions of all 221 drawings and translated Wallace’s descriptions (Peixes do Rio Negro, EDUSP). The drawing of P. wallacei is plate no. 4 on page 71. His description reads as follows: “Color brown pale edges reddish. Eyes yellowish pupil blackish. Black markings rather pale (…) Tail spine single rather long serrated (...) Younger specimens have the markings much finer and fainter”.

The article “A new species of Neotropical freshwater stingray (Chondrichthyes: Potamotrygonidae) from the Rio Negro, Amazonas, Brazil: the smallest species of Potamotrygon” by Marcelo R. de Carvalho et al., published in Zootaxa, can be read at biotaxa.org/Zootaxa/article/view/zootaxa.4107.4.5.