The genome of the Yangtze River porpoise is so distinct from its ocean-going relatives that it is indeed its own species, which makes it the only species of freshwater porpoise in the world.

This is the conclusion of a new genetic analysis study on the snub-nosed mammal, published this week in Nature Communications. The research shows that rather than being a subspecies of finless porpoise, the Yangtze River variety has been genetically isolated for thousands of years, adapting to its freshwater environment over this period. This is the first time that researchers have been able to conduct a whole genome analysis of the animal, conclusively proving that the porpoises are indeed separate species.

The finless porpoise (Neophocaena phocaenoides) is thought to be the most primitive species of porpoise plying the waves today. Found in shallow coastal waters from Saudi Arabia in the east to Japan in the west, the rotund cetacean has a large range, although their numbers seem to be concentrated around the sandy seabeds of the Korean peninsula.

Since the finless porpoise roams over such wide distances, the population has long been divided into subspecies, but there have been ongoing discussions about whether some should be a distinct species in their own right. This debate has focused on the Yangtze River porpoise, with morphological and mitochondrial data suggesting that it should be. Now, whole genome sequencing has resolved the issue.

Genetics shows that the Yangtze River porpoises likely split from their seagoing cousins between 40,000 and 5,000 years ago, becoming isolated ever since. It seems that the two groups have not been interbreeding for thousands of years, and are now so distinct that the researchers suspect that even if the marine and freshwater cetaceans were to come into contact, they wouldn't be able to produce fertile offspring.

The biggest changes occurred in the genes that now enable the once marine porpoises to survive in a freshwater world. These include alterations to genes that regulate kidney function, as well as those that regulate the water-salt balance in the cetacean’s blood.

The results have big implications for the small cetaceans. It means there should be a step up in conservation efforts to save the endangered porpoises and prevent them from going the same way as the Yangtze River dolphin, which used to share the river with them but was declared functionally extinct in 2006, as horrific amounts of pollution and river traffic killed them off.