Five, six and seven-gill shark research

The vast majority of the 500+ species of shark have five pairs of gills, but there are a small number of sharks have six or seven gill pairs. This is the case for all six species in the order Hexanchiformes. The group’s scientific name means “six arches,” referring to the gills, however, four species have six gill pairs and two have seven gill pairs. The Hexanchiformes are the most ancient lineage of modern sharks, with fossils dating back to the Lower Jurassic (~190 mya).

Work to date on these species has been very limited. Biopixel Oceans Foundation’s lead scientist, Dr Adam Barnett, has conducted research on the ecology and role of the seven-gill shark Notorynchus cepedianus in coastal habitats across three continents and a couple of research groups have concluded some research on the bluntnose sixgill shark.

Barnett et al. 2012 review “An overview on the role of the Hexanchiformes in marine ecosystems: biology, ecology and conservation status of a primitive order of modern sharks,” collated the early work on Hexanchiformes and their role in ecosystem dynamics, highlighting the areas where critical information is required to stimulate research directions.

The only other family of sharks to have a species with six pairs of gills is the sawshark. So, the recent revision of the sixgill saw sharks’, genus Pilotrema (Chondrichthyes, Pristiophoriformes), with descriptions of two new species and a redescription of P. warreni Regan, is a very significant announcement and of interest to Barnett’s Hexanchiform research. The following release by New Castle University outlines the findings.

Importance of the western Indian Ocean 

Dr Andrew Temple, Research Associate at Newcastle University, a co-author on the paper, 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 reinforces both how important the western Indian Ocean is in terms of shark and ray biodiversity, but also how much we still don’t know.” 

Two rostra of P. kajae were collected by Dr Ruth Leeney based at the Natural History Museum in London and many further specimens of this new species were found in different museum collections. The two specimens of P.annae were collected by PhD student Ellen Barrowcroft, Dr Andrew Temple and Dr Per Berggren from Newcastle University, UK, and Dr Narriman Jiddawi from the Institute of Fisheries Research, Zanzibar. 

Lead author Dr Simon Weigmann, based at the Elasmobranch Research Laboratory in Hamburg, explained: 

“The six-gill sawsharks are really quite extraordinary as most sawsharks have five gill slits per side. 

“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 generally still scarce. 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 could be particularly alarming for Anna’s six-gill sawshark due to its very small known range, rarity and occurrence in shallow waters as the species is only known from depths of 20 to 35m.” 

Dr Berggren, a co-author on the paper and leader of Newcastle’s Marine MEGAfauna lab, adds: 

“This project is also a testament to the value of scientists working with local communities. Without the fishers’ help we would not have discovered these animals. Their knowledge of their environment is unparalelled and it is our mission to help them preserve the marine animals and ecosystems they rely on to survive.” 

Story source: 

Materials provided by Newcastle University. 

Read the original press release here. 

Journal reference: 

Simon Weigmann, Ofer Gon, Ruth H. Leeney, Ellen Barrowclift, Per Berggren, Narriman Jiddawi, Andrew J. Temple. Revision of the sixgill sawsharks, genus Pliotrema (Chondrichthyes, Prsitiophoriformes), with descriptions of two new species and a redescription of P. warreny Regan. PLOS ONE, 2020: 15 (3): e0228791 DOI: 10.1371/journal.pone.0228791 

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