Port Phillip Bay dolphin identified as unique species

Melburnians appear to have been living with an unrecognised new species of dolphin, unique to Port Phillip Bay.

DNA studies by Monash University PhD student Kate Charlton have shown that the diminutive bottlenose dolphin, until now considered to be a local form of Tursiops aduncus, the common inshore bottlenose dolphin of the Indo-Pacific, differs from that species by 9.1 per cent, as measured by divergence in their mitochondrial DNA sequences.

Its appears to be more closely related to the open-ocean or offshore bottlenose dolphin, T. truncates, but its mitochondrial DNA also differs by 5 per cent from that that species, which itself has only been recognized as distinct from T. aduncus in recent times.

Charlton said the inshore bottlenose, which occasionally enters Port Phillip Bay, is markedly larger than the Bay dolphin, being about a metre longer and much broader in the body. The two species also differ in coloration - the inshore bottlenose dolphin is very dark above and pale below, where the Bay dolphin has relatively uniform dark coloration.

"We originally thought it might have come down from the NSW coast and entered Port Phillip Bay after the last glacial period," Charlton said.

"But the pronounced genetic divergence between the Bay dolphin and the other two bottlenose species equates to between about 5 and 10 million years, indicating there is strong reproductive isolation."

Charlton said the findings suggested that the Bay dolphin had a unique biogeographic and evolutionary history, that may have involved repeated episodes of isolation in Port Phillip Bay at times of low sea level during glacial periods.

During the last glacial period, which ended 10,000 years ago, global sea levels were up to 120 metres lower, blocking the Bay's current entrance from Bass Strait, which in any case was blocked by a land bridge between Victoria and Tasmania, with a huge lake, Lake Bass, at its centre.

Repeated episodes of confinement to Port Phillip Bay would have resulted in strong selection pressure for pre-mating isolating mechanisms in the small Bay dolphin population.

Charlton says the unique population is now at high risk of extinction or inbreeding. It comprises no more than 100 individuals, and the effective size of the female breeding population from year to year may be no more than half a dozen.