Two New Species of Sixgill Sawshark Found in Southwest Indian Ocean
Two new species of the rare six-gill sawshark have been discovered in the waters around Madagascar and Zanzibar. The discovery, published in the open-access research journal PLOS ONE, was spearheaded by the Elasmobranch Research Laboratory in Hamburg, Germany, and involved scientists from the South African Institute for Aquatic Biodiversity, Zanzibar's Institute of Fisheries Research, London's Natural History Museum, and a team from the School of Natural and Environmental Sciences at the UK's Newcastle University.
Pliotrema kajae and Pliotrema annae – also known as Kaja’s and Anna’s six-gill sawsharks – were discovered by researchers investigating fisheries operation off the coasts of Madagascar and Zanzibar.
In a press release on the Newcastle University website, Dr Andrew Temple, Research Associate at Newcastle University and one of the study's co-authors said: 'Last year our team highlighted the massive underreporting of sharks and rays caught in the south-west Indian Ocean and the urgent need to expand efforts globally to assess the impact of these fisheries on vulnerable species. The discovery re-enforces both how important the western Indian Ocean is in terms of shark and ray biodiversity, but also how much we still don’t know.'
Sawsharks are found in deep offshore waters throughout the Indian and Pacific oceans, with one species also encountered in the Bahamas. There were eight known species of sawshark known to science prior to the description of Anna's and Kaja's sawsharks, bringing the total to ten. Only one of the previously known species - Pliotrema warreni, the sixgill sawshark – had six gills.
Sawsharks are similar in appearance to the more commonly encountered shallow-water, coastal dwelling sawfish, however, sawsharks are members of the superorder Selachimorpha - sharks - while sawfish are members of the superorder Batoidea, or rays.
Notable differences between the two include the location of the sawshark's gill slits which are - like all sharks - located on the sides of the animal's body, whereas the sawfish's gills are, like most species of ray, located on the underside. Sawsharks are much smaller in size than sawfish and have a pair of barbels located along the snout, which sawfish do not.
The position of the barbels is one of the most easily observed physical differences between Warren's sixgill shark and the two new species, which have their barbels situated much further along the saw from their mouths.
'The six-gill sawsharks are really quite extraordinary as most sawsharks have five gill-slits per side,' said lead author Dr Simon Weigmann, 'So it was really exciting to find a new six-gill sawshark species and to find two new species - well that was simply astonishing!'
'Knowledge of sawsharks in the western Indian Ocean is generelly still scarce,' said Dr Weigmann, 'but considering their known depth distributions, both new species are likely affected by fishing operations. This assumption, combined with the limited range and apparent rarity of both new species, raises concerns that they are vulnerable to overfishing and might be in continuing decline.'
'This project is also testament to the value of scientists working with local communities,' said study co-author Dr Per Berggrenof Newcastle University. 'Without the fishers help we would not have discovered these animals. Their knowledge of their environment is unparalleled and it is our mission to help them preserve the marine animals and ecosystems they rely on to survive.'
The complete paper 'Revision of the sixgill sawsharks, genus Pliotrema (Chondrichthyes, Pristiophoriformes), with descriptions of two new species and a redescription of P. warreni Regan' by Wiegmann et al. is published under a Creative Commons license at www.journals.plos.org/plosone/article?id=10.1371/journal.pone.0228791