Journal Club: First ever sighting of world’s rarest whale on New Zealand coast
SUMMARY: DNA technologies were used by scientists to definitively identify two stranded whales as members of rare species previously known only from a few scattered bones.
Figure 1C: Illustration depicting a generalized external morphology derived from photographs of the adult female spade-toothed beaked whale, Mesoplodon traversii.
A rare whale species that's never before been seen has beached in New Zealand. Two spade-toothed beaked whales, Mesoplodon traversii (pictured above, or view larger), previously known from only a few widely-scattered bone fragments, were positively identified using DNA technologies by a team of scientists from the University of Auckland.
"This is the first time a spade-toothed beaked whale has been seen as a complete specimen, and we were lucky enough to find two of them," said whale expert Rochelle Constantine, a senior lecturer in biology at the University of Auckland and corresponding author on the just-published paper.
Beaked whales are placed into the taxonomic family Ziphiidae, which consists of 21 species of toothed whales, all of which have distinctive elongated beaks. However, due to strong similarities in appearance and habits, beaked whales are cryptic species that are very difficult to distinguish in the wild.
Further, because ziphiids have low population densities and because they are exceptionally deep divers found only in the open ocean, this may explain why they are one of the most enigmatic mammalian families.
"It is remarkable that we know almost nothing about such a large mammal", said Dr Constantine.
Adding to the potential for confusion, New Zealand reports one of the greatest diversities of stranded cetacean species in the world, including 13 of the 21 known species of beaked whales.
"New Zealand is surrounded by massive oceans," explained Dr Constantine, adding; "there is a lot of marine life [in the oceans] that remains unknown to us."
According to Dr Constantine, the adult female whale (5.3 metres/17 feet long; see photograph below) and her male calf (3.5 metres/11 feet) stranded and later died on New Zealand's Opape Beach in the Bay of Plenty on 31 December 2010.
Beached adult female spade-toothed beaked whale, M. traversii.
Photograph: New Zealand Department of Conservation.
After their deaths, the Department of Conservation measured and photographed the whales and tentatively identified them as Gray's beaked whales, M. grayi -- the beaked whale species that most commonly strands on New Zealand's beaches. Unfortunately, the whales were buried without a necropsy, so their cause of death is unknown.
As a routine part of an ongoing 20-year program to amass data on the 13 species of beaked whales found in New Zealand waters, tissue samples were collected and sent to the University of Auckland for DNA analysis to confirm the species identification.
But when Dr Constantine and her team amplified, sequenced and analysed two mitochondrial DNA regions (control region and cytochrome b) from the tissue samples and compared these data to other beaked whale species, they discovered these animals had been misidentified. In fact, the whales were the previously unseen spade-toothed beaked whale.
Never before seen as a complete specimen, no one was even sure if the mysterious spade-toothed beaked whale still lived. This species was known only from a few bone fragments -- a jawbone with teeth from an adult male was found in the Chatham Islands in 1872, and later, two jawless skulls were found; one on White Island in the 1950s and another on Chile's Robinson Crusoe Island in 1986 (figure 1A or view larger):
Figure 1A: Location of partial skulls found on the Chatham and White Islands, New Zealand, on Robinson Crusoe Island, Chile (red squares) and the two recently stranded specimens discovered on Opape Beach, New Zealand (black circle).
"When these specimens came to our lab, we extracted the DNA as we usually do for samples like these, and we were very surprised to find that they were spade-toothed beaked whales," said Dr Constantine. "We ran the samples a few times to make sure before we told everyone."
The team also analysed the DNA data to construct family trees describing the relationships between all known beaked whale species (including the two Opape specimens; green boxes, figure 1B or view larger):
Figure 1B: Neighbour joining tree of control region and cytochrome b sequences from ziphiid species and the two Opape specimens (MtrNZ03 and MtrNZ04) group in a monophyletic clade (green box). Higher-level relationships within the blue shading are not well resolved. Bootstrap values are shown as percentages for a total of 1000 bootstrap replicates. The vertical bars denote several specimens included in the analysis.
The robust data convinced the team that their findings were not an artifact.
"It was pretty exciting news, especially for Anton van Helden and Scott Baker who co-authored the paper describing the three previous [bone fragment] specimens", said Dr Constantine in email.
Based on sightings of only two intact individuals in the last 140 years, the authors consider the spade-toothed whale to be the world's rarest cetacean.
According to the paper's authors, the spade-toothed beaked whale lives in the South Pacific Ocean, a vast and poorly-known area that covers more than 85 million square kilometres -- 14 per cent of the earth's surface -- and includes some of the planet's deepest oceanic trenches. Like its relatives, this whale dives deep in the sea to forage for squid and small fish, and spends little time at the surface. Dr Constantine thinks these habits may be the reason this species has never before been seen alive or intact.
"It may be that they are simply an offshore species that lives and dies in the deep ocean waters and only rarely wash ashore", explained Dr Constantine.
After the whales' true identity had been uncovered, their skeletal remains were exhumed and taken to the Museum of New Zealand Te Papa Tongarewa in Wellington for further morphological analysis.
The identification of these whales underscores the power of using genetic information as a complement to museum and reference collections for uncovering surprises about the natural world.
"This discovery is a real reward. It demonstrates the value of archival collections and the power of DNA as a forensic tool", explained Dr Constantine in email.
So far, New Zealand is unique because it has developed a co-ordinated national response to cetacean strandings. This collaboration between the public, government officials, indigenous peoples and university scientists has provided 20 years of specimens and records on rare whale species.
"I hope [this discovery] stimulates people to report cetaceans when they find them on the beach and for government agencies and scientists to work together to maintain archives like ours because you never know what you might find."
Thompson, K., Baker, C., van Helden, A., Patel, S., Millar, C., & Constantine, R. (2012). The world's rarest whale. Current Biology, 22 (21) R905-R906. doi:10.1016/j.cub.2012.08.055
Rochelle Constantine; emails: 6 November 2012.
University of Auckland press release
NOTE: this piece is slightly edited from the original, which was published on the Guardian.
.. .. .. .. .. .. .. .. .. .. ..