We've got an actual photograph of it too, but it's not pretty, so you might want to stop eating before you scroll down...

Considered the least known and rarest species of whale, and one of the world’s rarest living mammals, the spade-toothed whale (Mesoplodon traversii) has been seen for the first time after a mother and her male calf beached and died on a New Zealand beach.

Not only are beaked whales of the family Ziphiidae rare, their ability to dive down to exceptionally deep areas of the ocean in search of squid and other deep-sea fish means they are also very elusive. They can reportedly dive to more than 800m below the surface and dives can last up to 87 minutes. Superficially, beaked whale species look pretty similar, and for many of the 21 known species, we have frustratingly few records. So when a new beaked whale specimen is discovered, figuring out which species to assign it to can be quite a task.

Until now, we've only known about the spade-toothed beaked whale from a few bone samples, as no intact specimens have been discovered. This has made identifying the species extremely complicated. In 1872, a partially damaged mandible and set of teeth were picked up on Pitt Island, of the Chatham Island archipelago in New Zealand, and described by the director of the Colonial Museum of New Zealand at the time, James Hector, the following year. As no one had ever seen a spade-toothed whale before, Hector assigned the bones to the Scamperdown whale (Dolichodon layardii), which had been discovered eight years prior by British zoologist John Edward Gray. This species was the most commonly beached beaked whale around the New Zealand coast at the time.

Gray caught wind of this and examined the bones himself, and in 1874 assigned them to an entirely new species, which he called Dolichodon traversii. The correct genus was a point of contention for these whales, so Dolichodon traversii was later corrected to Mesoplodon traversii. In response to Hector's analysis, and explaining the need to name a new species, Gray noted that, "‘Mesoplodon layardi (or as I should call it, Dolichodon layardi) has a much longer and attenuated lower jaw, and much more slender teeth than the Chatham Island specimen", in Transactions of the New Zealand Institute (PDF).

During the 1950s, a second beaked whale specimen was collected on New Zealand's White Island and stored in the MacGregor Collection of the University of Auckland. After a flurry of analysis, both genetic and morphological, from 1999 to 2002, the skull bone specimen was assigned to a number of different species including Bahamonde's beaked whale (Mesoplodon bahamondi). Bahamonde's beaked whale was known from a damaged skull bone found in 1986 on Robinson Crusoe Island in Chile. Finally, in 2002, new research by a team led by Anton van Helden from the Museum of New Zealand Te Papa Tongarewa unified the Pitt Island, White Island and Robinson Crusoe Island specimens as belonging to the one species, and suggested that Bahamonde's beaked whale was actually the same species as Mesoplodon traversii.

"Our results combine morphological and molecular evidence to unify three fragmentary and disparate museum-held beaked whale specimens resulting in the synonymy of M. bahamondi with M. traversii," the team wrote in Marine Mammal Science (PDF). They gave M. traversii the common name, the spade-toothed whale, which refers to its extraordinary teeth that are shaped like "the oblong blade of a fensing knife (known as a 'spade') used by North American whalers in the 19th Century". Thanks to the discovery, New Zealand gained the reputation of being the richest region in the world for beaked whale species diversity.

So after many years, the enigmatic spade-toothed beaked whale had three sets of specimens - two adult skull fragments, adult teeth and a mandible - assigned to it, but besides the basic skull morphology, no one knew much of anything about it.

In December 2010, an incredible discovery was made when two whales stranded themselves and died on Opape Beach in New Zealand. The adult female was 5.3m long and the young male was 3.5m long. Initially identified as the common Gray's beaked whale (Mesoplodon grayi), a team of researchers led by Kirsten Thompson from the School of Biological Sciences at the University of Auckland sequenced two mitochondrial DNA regions and compared them to the existing three bone specimens to discover that these were never-before-seen spade-toothed whales.

The researchers also figured out what distinguished the spade-toothed whales physically from other species, having seen the species' external flesh for the first time. They noted that the colouration of the rostrum, or 'beak, is dark gray or black, rather than the white of the adult Gray’s beaked whale. It also has a dark eye-patch, white belly and dark flippers.

The team credited the discovery to New Zealand's long-term and coordinated response to whale strandings, where they are reported regularly by the public to the Department of Conservation, whose researchers collect information and tissue samples from each one. The government has now accumulated some 20 years' of tissue samples and records on a number of species.

We still have no idea how many of these whales exist in our oceans, but, the researchers conclude in Current Biology today, "We can now confirm that the spade-toothed whale is extant and for the first time we have a description of the world’s rarest and perhaps most enigmatic marine mammal."

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