the extraordinary saw shark with the weapon-shaped snout

Sharks swimming in the ocean look like they have a hedge trimmer strapped to their head and a mustache hanging halfway down. They are saw sharks and they use their formidable headgear to slice through schools of fish. The whisker is a sensory device that helps sharks detect their prey.

“Saw sharks are something extraordinary,” says Simon Weigmann of the Elasmobranch Research Laboratory in Hamburg, Germany.

Until recently, scientists knew of eight species of saw shark, including one that had six gill slits on the side of its body. “It’s unusual in sharks,” says Weigmann – because most sharks have five gill slits. With the help of fishermen from Madagascar and Tanzania, two more species of six-gilled saw sharks have been discovered.

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Long before Western scientists named them, people in fishing communities in southwestern Madagascar knew about the meter-long, six-gilled saw sharks and called them vae vae. In 2017, a Malagasy fisherman gave two of the saw-like snouts (called rostra) to Ruth Leeney, a visiting biologist at the Natural History Museum in London. Realizing they were something different, she sent them to Weigmann. He tracked down more preserved specimens that were on shelves in museums and realized that they belonged to a separate species of six-gilled saw shark.

“Previously we thought there was only one species off South Africa, Mozambique and Madagascar. Now we know that Madagascar has a different species,” says Weigmann.

It is important to give a name to the species, to draw attention to it

Simon Weigman

The name he chose for scientific literature is Pliotrema kajae – Kaja’s six-gilled saw shark – according to his young daughter who watched him with great interest as he examined the shark specimens kept at home. Kaja also means warrior in Frisian, a West Germanic language, which Weigmann thought appropriate given the weapon-like snouts of sharks.

A specimen of a third species of six-gilled saw shark came to Weigmann after colleagues visited a fish market on the Tanzanian island of Zanzibar. He named this one P annaaccording to Anna, Kaya’s cousin.

Something that sets these species apart from previously known saw sharks is that their whiskers (technically known as barbels) sit closer to the tip of their snout, but Weigmann doesn’t yet know the relevance of that.

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Likewise, there is no obvious explanation as to why these saw sharks have six gill slits. Of more than 1,000 species of sharks and rays, only a handful have six or seven gill slits.

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

“It’s important to give the species a name, to draw attention to it,” Weigmann explains. The next steps will be to determine how threatened the species is and whether it should be protected.

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