Marine biologists working off Hawaii have confirmed the sighting of a hybrid of two dolphin species.

In August last year, a group of marine biologists was working off the island of Kauai when they spotted something a little odd.

"We came across a mixed group with two species: rough-toothed dolphins, and what appeared, at first, to be two melon-headed whales," said Robin Baird, a research biologist with the Cascadia Research Collective.

Melon-headed whales are generally found in very large groups of their own kind.

Genetic testing has confirmed the hybrid's father is a rough-toothed dolphin like these pictured ( Cascadia Research Collective )

"Seeing just two individuals mixed in with the rough-toothed dolphins was a bit surprising," he said.

"Then, on closer observation, the field team noted one of the melon-headed whales had a very unusual head."

Melon-headed whales have a rounded head, and rough-toothed dolphins have a long, gently sloping rostrum, or "beak".

But one of the two creatures had a head that was neither one nor the other.

"It had a gently sloping beak, but much shorter than a normal rough-toothed dolphin," Dr Baird explained.

The hybrid's mother was a melon-headed whale (which is really a dolphin, not a whale at all) ( Cascadia Research Collective )

The team thought it could be a hybrid between the two species.

A skin sample was collected and DNA testing, as a newly published paper outlines, confirmed their suspicions.

The genetic test showed the hybrid had the mitochondrial haplotype of a melon-headed whale (actually a species of dolphin), which means the mother was a melon-headed whale, and the father was a rough-toothed dolphin.

The test also revealed the hybrid was male.

"It was the first time that a hybrid has been recorded between these two species" Dr Baird said.

"And hybrids in general in oceanic dolphins have only been confirmed genetically on a few prior occasions, it's quite unusual."

It's the first time a hybrid (foreground) has been seen between a melon-headed whale and a rough-toothed dolphin. ( Cascadia Research Collective )

A number of species will hybridise in captivity, where mating opportunities are limited, but these two species are not usually held in captivity, said Dr Baird.

He said it was a much more surprising occurrence in the wild.

Dolphin hybrid 'just as unique'

The term "wolphin" was first applied to the false killer whale and the bottlenose dolphin hybrid born in captivity in the 1980s.

The mammal, named Kekaimalu, is still a major attraction at Hawaii's Sea Life Park.

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Dr Baird said he thought using the term "wholphin" to describe the product of two dolphins just muddied the waters.

"It's a funny thing because back when many of these species were given common names, things like killer whales, pilot whales, melon-headed whales were all called whales," he said.

"But as the knowledge of their relationships with other species became clear, they are all actually oceanic dolphins, and they are all relatively closely related.

"So it's really that there's hybridisation between two different species of dolphins."

And that in itself was "just as unique", he said, and its rarity should be appreciated.