Discovered in the deep: the extraordinary sawshark with a weapon-like snout
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.
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.
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, a Malagasy fisher 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.
“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 westGermanic 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 Pannae, 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.
The three species of sixgill sawsharks live in different parts of the Indian Ocean. The original species, Pwarreni, 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.
It’s important just to give the species a name, to bring attention to it
Simon Weigmann