Evidence of a colony of rare seahorses has been discovered in the Thames, the Environment Agency has revealed.
The short-snouted seahorse grows up to 15cm and is more commonly found in the warm waters of the Mediterranean and Canary Islands.
The Hippocampus hippocampus has often been a visitor to coastal waters around Britain but the agency said it had never been found so far upriver.
The seahorse, which was released back into the river, was spotted during a routine fisheries survey at Greenwich, south east London.
Their findings suggest there may be permanent populations in the UK and that the seahorses, which mate for life after elaborate courtship rituals, may be breeding in the Thames.
Emma Barton, Environment Agency fisheries officer, said: 'The seahorse we found was only 5cm (2 inches) long, a juvenile, suggesting that they may be breeding nearby.
'This is a really good sign that seahorse populations are not only increasing but spreading to locations where they haven't been seen before.
'We routinely survey the Thames at this time of year and this is a really exciting discovery.'
The last sighting of seahorses in the Thames was much further down the river in Dagenham in 2008. They have not seen since.
The Thames has been reformed from a 'biologically dead' river in the 1950s, as a result of sewage and industrial pollution, to a much cleaner waterway today.
Ms Barton added: 'We hope that further improvements to water quality and habitat in the Thames will encourage more of these rare species to take up residence in the river.'
