29 January 2010, by Sara Coelho

Scientists have found that the common skate can be split in two genetically distinct species. This is worrying because this bottom-dwelling fish was already critically endangered and the two new species are probably in even greater danger of extinction.

Common skate

The common skate.

'The two groups we found are definitely different species,' says lead author Dr Andrew Griffiths, from the Marine Biological Association of the UK. 'Our findings have major implications for the conservation of the species,' he adds.

The results underline the need to revise the conservation status of the common skate, as well as highlighting the plight of many of the ocean's large predators.

The common skate (Dipturus batis) is the world's largest species of skate and can grow to almost 3 metres in length. Its size makes it especially vulnerable to being caught accidentally as many of the nets used by fisherman can snatch it up. These fish take a long time to grow and produce relatively few young, which makes them vulnerable to overfishing.

'Now the range for each species has been reduced, which means these species are even more vulnerable to extinction.'

Dr Andrew Griffiths,

Marine Biological Association

'It used to be very common - hence the name,' says Griffiths. 'Now it took us over two years to collect enough samples for this study.'

The species is already listed as critically endangered, and can only be found in a fraction of its former range, which once stretched from Scandinavia to the Mediterranean.

Griffiths and his colleagues sampled 123 common skates caught by fishing nets or during scientific cruises around Britain. The team analysed their DNA and looked for genetic differences between the remaining skate populations.

'We identified two different groups that probably split millions of years ago,' says Griffiths. The Northern species lives in the North Sea and Atlantic Ocean between Scotland and Northern Ireland. The Southern species can be found off the Bristol Channel, around England, the south of Ireland and Rockall. 'Their current ranges are only slightly overlapping,' he says.

The two species are physically very similar and it's difficult to tell them apart. They also live in the same habitats and at similar depths, but the differences found in their DNA leave no doubt that they are two separate species.

The fall of the common skate

They are both likely to be at a high risk of extinction. Geographical range is one of the key features considered when scientists assess conservation threats. 'The common skate distribution was already restricted,' says Griffiths. 'Now the range for each species has been reduced, which means these species are even more vulnerable to extinction.'

The findings, reported this week in the Proceedings of the Royal Society B, add up to the tragic conservation history of the common skate. The species has the unfortunate merit of being the first ever fish to be considered locally extinct, after it disappeared from the Irish Sea in the 1980s.

The European Union has now banned commercial fishermen from even bringing common skate ashore, but their recovery is likely to be slow. 'They don't seem to recolonise the regions that they've disappeared from, with most individual fish remaining within very specific areas,' says Griffiths, who compares the fish's life-style to mammals.

The discovery of these species could be a case of history repeating, because what we now know as common skate was once considered, by some, as several species. A hundred years ago numerous large skate had been described across Europe and their naming had become very confused, but eventually these various species were grouped under the name Dipturus batis.