Imagine: You are a zoologist on a research expedition, exploring the remote riverine ecosystems in some of the remotest parts of the Brazilian Amazon river basin…You have heard rumors from the local tribes people of a large, strange, long-nosed creature that inhabits the waters here (it’s bad luck to kill a such a creature)…You know of only two such other river dolphins, but each is found in distant and distinct basins of the Brazilian Amazon River…not here in the Araguaia River basin of Brazil (isolated by several powerful rapids from the main Amazon River basin)…Such a dolphin would almost necessarily be a distinct species, being so isolated from the others…and extremely rare… And then, suddenly, one of the very same creatures pokes its head through the surface right next to your boat…
That is pretty much what happened to a six member team of zoologists while on expedition in the remote river basins of Brazil’s Amazon Rain Forest late last year. The team, led by Tomas Hrbek and Vera Maria Ferreira da Silva, reported the results of their discovery and genetic tests this past week in the open source journal PLoS ONE, stating:
“We report the discovery of a new species of a river dolphin from the Araguaia River basin of Brazil, the first such discovery in nearly 100 years.”
Subsequent DNA analysis of the dolphin’s nuclear DNA and DNA “bar code” (a block of code in its mitochondrial DNA) and comparison with genetic sequences from other specimens showed that the newly found river dolphin — officially named Inia araguaiaensis — is indeed a new species that has been separated from the other two known species (I. geoffrensis and I. boliviensis) by at least two million years of evolution.
And, as a result the dolphin has distinct variations on its physiology — such as a wider skull and differently shaped/numbered teeth. These distinct features were used in the physiological comparison with the other two river dolphin species — giving support for the new species claim, even though the comparison samples were, necessarily, quite small due to the difficultly in finding dead animal specimens. “These are occasional finds, so you cannot do a proper sampling design,” says lead researcher Tomas Hrbek, an evolutionary biologist at the Federal University of Amazonas in Manaus, Brazil.
One of the other species of river dolphin (I. geoffrensis), whose range is closest to that of the newly found I. araguaiaensis, probably diverged on the evolutionary family tree when the mouth of the Araguaia River shifted to the east, consequently emptying into the Atlantic Ocean instead of the Amazon River. The teams’ estimation of the date of this divergence is 2.08 million years, which corresponds quite precisely with the separation of the Araguaia-Tocantins basin from the Amazon basin.
Despite the apparent isolation and geographical obstacles to interbreeding, the paper does note that “The habitat ranges of the two species ( I. araguaiaensis, I. geoffrensis) potentially overlap in a small area downstream of the rapids where a narrow canal connects the mouth of the Araguaia River with the Amazon River delta.” Thus, there is a small possibility of more recent interbreeding, which would argue against the new species designation.
To answer this question definitively, the research team plans several surveys of the potential overlap areas to see if in fact both species inhabit the overlap area (located between two tributary rivers located just within the outer-most borders of the Amazon River Basin), and, do so without any interbreeding [see the insert map, below]. Looking at the broader picture, the researchers hope to encourage other researchers so as to “stimulate historical bio-geographical analyses of the two basins.”
Distribution map of all known species and subspecies of Inia. Black outline denotes the limit of the Amazon basin. Question marks denote uncertainty as to which species occurs in the Tocantins River downstream of the Tucuruí dam which potentially delimits the distributions of I. geoffrensis and I. araguaiaensis sp. nov. Bars on the Madeira River represent a series of rapids that delimit the distribution of I. geoffrensis and I. boliviensis. The single bar on the northern limit of the Amazon basin represents the Casiquiare canal which connects the Amazon and Orinoco basins, and is thought to delimit the I. g. humboldtiana subspecies from I. g. geoffrensis [source: doi:10.1371/journal.pone.0083623.g001]
Although river dolphins are widely distributed through out the Amazon River basin region, their numbers are unknown but believed to be vastly smaller than their marine counterparts. River dolphins face existential threats from fishermen (who often kill the mammals and use them for bait), over-harvesting of their food source, and habitat fragmentation (e.g., from dams).
Such finds of large, water-dwelling mammals are rare — and river dolphins are amongst the rarest of the rare — and, they reveal our sheer ignorance of the full range of fauna that inhabit the remaining and increasingly-less-remote tropical forest regions of our globe.
In the words of the research team:
“River dolphins…comprise relict evolutionary lineages of high taxonomic distinctness and conservation value, but are afforded little protection…”
“This discovery highlights the immensity of the deficit in our knowledge of Neotropical biodiversity, as well as vulnerability of biodiversity to anthropogenic actions in an increasingly threatened landscape.” [source: paper abstract, see link below]
Let us hope that this new discovery will call yet more attention to the wondrous and rare biodiversity hotspots that constitute the larger Amazon River basin, and so perhaps stall, or even reverse, their encroachment, contamination, and destruction.
The expedition team members included: Tomas Hrbek, Vera Maria Ferreira da Silva, Nicole Dutra, Waleska Gravena, Anthony R. Martin, Izeni Pires Farias. Their results were published under the title: ‘A New Species of River Dolphin from Brazil or: How Little Do We Know Our Biodiversity‘ [Published: January 22, 2014; DOI: 10.1371/journal.pone.0083623]
Some additional source material for this post came from the Science Magazine article: ‘New Dolphin Discovered in Brazil’
Top Image: (I. araguaiaensis) credit: Nicole Dutra via Science Magazine
Bottom Chart (Amazon River basin listing river dolphin habitats); from the paper by Hrbek et al [see link, above]