The two subspecies of the South Asian river dolphin (Platanista gangetica) — the Indus river dolphin (Platanista gangetica minor) and the Ganges river dolphin (Platanista gangetica gangetica) — should each be recognized as distinct full species, according to a new study published in the journal Marine Mammal Science.
The dolphins inhabiting the Ganges River were named Platanista by Pliny the Elder in his Historia Naturalis published around 77 CE, meaning that this is one of the earliest cetaceans to be named.
They are the only extant members of the superfamily Platanistoidea, which was one of the earliest cetacean lineages to diverge, between 34 and 24 million years ago (Oligocene epoch), and previously included numerous species widely distributed across the world’s oceans.
Until now, only a single relict species in the family was recognized: the South Asian river dolphin.
It was composed of two subspecies that occur in geographically separate, adjacent, freshwater river systems:
(i) the Indus river dolphin, which is endemic to the Indus River system principally in Pakistan;
(ii) and the Ganges river dolphin, which inhabits only the Ganges-Brahmaputra-Meghna, and nearby Karnaphuli-Sangu river systems of Bangladesh, India, and Nepal.
These dolphins are among the most endangered of the world’s cetaceans; each number only a few thousand individuals, and are both listed as Endangered on the IUCN Red List.
They are threatened by dams and barrages, declining river flows, fisheries bycatch, and pollution.
The new research shows that Indus and Ganges river dolphins have clear genetic differences, as well as different numbers of teeth, coloration, growth patterns and skull shapes.
“The genetic tools we have today help us extract new information from samples collected years ago,” said co-author Dr. Eric Archer, leader of the Marine Mammal Genetics Program at NOAA Fisheries’ Southwest Fisheries Science Center.
In the study, Dr. Archer and colleagues examined differences in external and skull morphology between Indus and Ganges river dolphins to clarify their taxonomic status.
They also analyzed DNA extracted from the tissue samples from the Marine Mammal and Sea Turtle Research Tissue Collection of NOAA Fisheries’ Southwest Fisheries Science Center, the largest of its kind in the world.
“Without collections such as this and those at other museums around the world, it would be impossible to identify difficult to study species such as these dolphins,” Dr. Archer said.
“Recognizing the species-level differences between Indus and Ganges river dolphins is extremely important, as only a few thousand individual dolphins of each species remain,” said lead author Dr. Gill Braulik, a researcher in the Sea Mammal Research Unit at the University of St. Andrews.
“My hope is that our findings will bring much-needed attention to these remarkable animals, helping to prevent them sliding towards extinction.”
“Serious challenges still face this incredible species and all other river dolphin populations, but we can save them,” added Dr. Uzma Kahn, Asia Coordinator of the WWF River Dolphin Initiative.
“By doing so we’ll save so much more, since hundreds of millions of people and countless other species depend on the health of similar rivers around the world.”
“The rapid decline and extinction of the Yangtze River dolphin this century was a very clear warning: we need to act quickly to protect the remaining species of river dolphins, including the Indus and Ganges, all of which are seriously threatened,” said Dr. Randall Reeves, Chair of the IUCN Cetacean Specialist Group, who was not involved in the study.
“The freshwater systems they inhabit must be managed with biodiversity as a top priority.”
Gill T. Braulik et al. Taxonomic revision of the South Asian River dolphins (Platanista): Indus and Ganges River dolphins are separate species. Marine Mammal Science, published online March 23, 2021; doi: 10.1111/mms.12801