The extraordinary saw shark with the weapon-shaped snout

With the help of fishermen from Madagascar and Tanzania, scientists have named two new species of six-gilled saw shark.

Floating in the ocean, sharks appear to have a hedge trimmer attached to their heads and a mustache hanging down the middle. They are saw sharks and they use their formidable hull to bring down 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,” Weigmann says, because most sharks have five gill slits. With the help of fishermen from Madagascar and Tanzania, two more species of six-gilled saw shark have been discovered.

female saw sharkSpecimen of a young female Kaja sixgill sawshark. Photography: Simon Weigmann

Long before western scientists named them, the six-gilled, meter-long saw sharks were known to people in fishing communities in southwestern Madagascar and were called vae vae .

In 2017, Malagasy fishermen donated 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, he sent them to Weigmann. He tracked down more preserved specimens that were on museum shelves and realized that they belonged to a different species of six-gilled saw shark.

“Previously, we thought we only had one species in South Africa, Mozambique and Madagascar. We now know that Madagascar has a different species,” Weigmann says.

The name he chose for scientific literature is pliotrema kajae , Kaja’s six-gilled saw shark, named after her young daughter, who watched with great interest as she 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.

saw shark x-rayX-rays of the heads of three species of Pliotrema. Photography: Simon Weigmann

A specimen of a third species of six-gilled saw shark has found its way to Weigmann after his colleagues visited a fish market on the Tanzanian island of Zanzibar. He called it Pannae, after Kaya’s cousin Anna.

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

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 sixgill 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 straddling the Seychelles and Mauritius. So far, Annas has only been found off Zanzibar, in much shallower water between 20 and 35 meters.

“It’s important to give the species a name, to attract attention,” says Weigmann. The next steps will be to determine how threatened the species is and if it needs protection.

By Helen Scales. Articles in English

Leave a Comment