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For years—once in 2008 , and a second time in 2011—conservationists have spotted mysterious visitors in the Thames.

Now,

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The short-snouted seahorse

they’ve confirmed that two species of seahorses have found a home in the river, the short-snouted seahorse and the spiny seahorse. They’ve been in Greenwich and near the South Bank. Scientists now suspect that the seahorses—which have been seen on six separate occasions over the last two months—are permanent residents of the area. In the past, observers have only found one or two per year.

More than 100 species of fish, gray seals, and sharks also live in the Thames. Back in 2008, Jessica Aldred

Scientists at the ZSL [Zoological Society of London] say the presence of the seahorses in the Thames estuary is a good sign that river quality is improving, but warned that any disturbance to their habitats could be disastrous. […] Seahorses are threatened by overexploitation for traditional medicines, aquariums and curiosities, accidental capture by fishing fleets, and degradation of their habitats.

reported for the The Guardian that signs of seahorses meant that the river water was becoming clearer:

Seahorses are known among aquarists to sticklers for clean water, so the fact that seahorses have taken up residence in the Thames showsjust how drastically the water quality has improved over the decades.

Check out this video from BBC News about the finding: