Discovered in the deep: the extraordinary sawshark with a weapon-like snout

With the help of fishers in Madagascar and Tanzania, scientists named two new species of rare sixgill sawsharks

Swimming through the ocean are sharks that look as if they have a hedge trimmer fixed to their heads and a dangling moustache part way along it. These are sawsharks and they use their formidable headgear to slash through shoals of fish. The moustache is a sensory device that helps the sharks detect prey.

“Sawsharks are something extraordinary,” says Simon Weigmann from the Elasmobranch Research Laboratory in Hamburg, Germany.

Details of the unique tooth structure of a Kaja’s sixgill sawshark.
Details of the unique tooth structure of a Kaja’s sixgill sawshark. Photograph: Simon Weigmann

Until recently, scientists knew of eight species of sawshark, including one that has six gill slits in the side of its body. “This is unusual among sharks,” says Weigmann – as most sharks have five gill slits. With the help of fishers in Madagascar and Tanzania, two more species of sixgill sawsharks have come to light.

The ocean is one of the world’s last truly wild spaces. It teems with fascinating species that sometimes seems to border on the absurd, from fish that look up through transparent heads to golden snails with iron armour. We know more about deep space than deep oceans, and science is only beginning to scratch the surface of the rich variety of life in the depths.

As mining companies push to industrialise the sea floor and global leaders continue to squabble over how to protect the high seas, the Guardian's Seascape series, Discovered in the deep, will profile some of the most recently discovered weird, wonderful, majestic, ridiculous, hardcore and mind-blowing creatures. They reveal how much there is still to learn about the least known environment on Earth – and how much there is to protect. 

Long before western scientists named them, people in fishing communities in south-west Madagascar already knew about metre-long, sixgill sawsharks and called them vae vae. In 2017, Malagasy fishers gave two of the saw-like snouts (called rostra) to Ruth Leeney, a biologist visiting from London’s Natural History Museum. Realising they were something different, she sent them to Weigmann. He tracked down more preserved specimens that had been sitting on shelves in museums, and realised they belonged to a distinct species of sixgill sawshark.

Specimen of a juvenile female Kaja’s sixgill sawshark.
Specimen of a young female Kaja’s sixgill sawshark. Photograph: Simon Weigmann

“Formerly, we thought that we just had one species occurring off South Africa, Mozambique and Madagascar. Now we know Madagascar has a different species,” says Weigmann.

The name he picked for the scientific literature is Pliotrema kajae – Kaja’s sixgill sawshark – after his young daughter who watched on with great interest while he examined the preserved shark specimens at home. Kaja also means warrior in Frisian, a west-Germanic language, which Weigmann thought was appropriate given sharks’ weapon-like snouts.

A specimen of a third species of sixgill sawshark came to Weigmann after colleagues visited a fish market on the Tanzanian island of Zanzibar. He named this one P annae, after Kaya’s cousin Anna.

Something that sets these species apart from previously known sawsharks is that their moustaches (technically known as barbels) lie closer to the tip of their snouts, but Weigmann doesn’t yet know the relevance of this.

Similarly, there’s no obvious explanation for why these sawsharks have six gill slits. Out of the more than 1,000 species of sharks and rays, only a handful have six or seven gill slits.

Radiographs of the heads of three Pliotrema species
Radiographs of the heads of three Pliotrema species. Photograph: Simon Weigmann

The three species of sixgill sawsharks live in different parts of the Indian Ocean. The original species, P warreni, lives off South Africa and southern Mozambique, down to about 900 metres. Kajas have been found between 200 and 300 metres underwater off Madagascar and on the submarine Mascarene plateau that stretches between Seychelles and Mauritius. Annas have so far only been found off Zanzibar, in much shallower waters of between 20 and 35 metres.

“It’s important to give the species a name, to bring attention to it,” says Weigmann. The next steps will be to work out just how threatened the species is and whether it needs protecting.


Helen Scales

The GuardianTramp

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