An international team of marine scientists has discovered two new species of a rare shark in the western Indian Ocean: the six-gill saw shark. The discovery of those already affectionately known as Kaja and Anna (the scientific names are Pliotrema kajae and Pliotrema annae ) came during an investigation of small-scale fisheries off the coast of Madagascar and Zanzibar.
As its name suggests, the saw shark is characterized by having a snout in the shape of this tool . Their teeth are sharp and the smallest ones are among the largest. This shark can reach one and a half meters in length and lives in the temperate waters of the three main oceans, especially the western Pacific Ocean.
Another of the saw shark’s hallmarks is the pair of barbels it has in the center of its snout . Barbels are whisker-shaped sensory organs that are located around the shark’s mouth and that serve to detect prey. This type of shark feeds on fish, crustaceans, and squid.
The newly discovered new species of saw shark, Pliotrema kajae and Pliotrema annae , differ from the only previously known six-gill saw shark species, Pliotrema warreni , in the position of their barbels. In Pliotrema kajae and Pliotrema annae , these are located roughly halfway from the tip of the sierra – the rostral tip – to the mouth, compared to Pliotrema warreni , where they are about two-thirds of the way down, much closer from the mouth.
A special feature of P. annae is the generally shorter rostra compared to P. kajae and P. warreni .
The discovery of the two new species of saw shark has been published in PLOS ONE and, according to the research team, this finding highlights how little we still know about life in the ocean and the impact we are having on it . “Last year our team highlighted the massive underreporting of sharks and rays caught in the southwestern Indian Ocean and the urgent need to scale up efforts globally to assess the impact of these fisheries on vulnerable species,” says Andrew Temple, Research Associate at the University of Newcastle, and co-author of the paper “The discovery reinforces both the importance of the western Indian Ocean in terms of shark and ray biodiversity, as well as how much we still do not know.”
Overfishing, the main threat to the saw shark
Small-scale fishing employs around 95% of the world’s fishermen , is a great source of money as well as food, especially for developing tropical countries. These fisheries tend to work near the coast of some of the most diverse spots on the planet such as coral reefs, mangroves and seagrass beds.
According to Newscastle University, there is very little data on small-scale fisheries: how many fishers are employed, where, when and how they fish, what exactly they catch, etc. This makes it difficult for governments to act to ensure sustainable fishing, the protection of ecosystems and the livelihoods of fishermen and the communities that depend on them.
“While the small-scale fisheries in East Africa and nearby islands are not well documented, we know that there are at least half a million small-scale fishermen using more than 150,000 boats. That’s a lot of fishing . While every fisherman and boat may not catch as many fish each day, with so many operating, it really starts to add up. Many use nets, either driftnets that float on the surface or gillnets, which are anchored close to the sea floor. Both are cheap but not very selective about what they catch. Some use longlines, which are effective in catching large fish, including sharks and rays ”, can be read in an article published on the website of the University of Newscastle.
Referencia: Weigmann S, Gon O, Leeney RH, Barrowclift E, Berggren P, Jiddawi N, et al. (2020) Revision of the sixgill sawsharks, genus Pliotrema (Chondrichthyes, Pristiophoriformes), with descriptions of two new species and a redescription of P. warreni Regan. PLoS ONE 15(3): e0228791. https://doi.org/10.1371/journal.pone.0228791
