Lurking beneath the surface: 7 surprising creatures you’d never guess live in the River Thames

The tidal Thames and its tributaries are finally thriving again. The famous river was declared to be “biologically dead” in the 1950s, but thanks to a concerted effort by conservation organisations and the government, sewerage systems have been rebuilt and life has finally returned to the Thames.

ZSL’s The State of the Thames report was released in January 2026, and showed how decades of dedicated action have improved the water quality and wildlife presence in the Thames.

So what are those species that lurk beneath the surface in England’s longest river? Some of the animals that live in the Thames might surprise you…

Surprising animals that live in the Thames

Sharks

Around five shark species are thought to live in the Thames, with top and starry smooth-hound varieties using the Greater Thames Estuary as a nursery ground. This is because adult sharks tend to prefer pupping in shallow bays and estuaries, which makes the Thames an ideal habitat. The young sharks are able to remain in the nursery ground for up to two years.  

• What’s the most common shark in Britain?
• The rise of the shark in British waters

Short-snouted seahorses

Seahorses tend to have small home ranges, so if they’re based in a particular location they tend to live there all year round. Several juvenile short-snouted seahorses have been reported in the outer tidal Thames and Thames estuary.

Seahorses are a sign of clean water and a healthy ecosystem, as they are particularly sensitive to environmental pollution, and rely on thriving ecosystems such as seagrass beds – which only survive in clean conditions. You’re most likely to see seahorses in shallow waters, as they aren’t particularly good swimmers and use their tails to grip onto seaweed.

• Monumental rise in one of Britain's weirdest and most endearing tiny sea creatures: the seahorse

Seals

Two species of seals call the Thames home: harbour seals and grey seals, with a combined Greater Thames Estuary population of about 4,000 individuals. Most seals live on the sandbanks and coastal creeks in the Thames’s outer estuary, but some have been spotted following fish even as far as Richmond. Both species are among the UK’s top predators, feasting on a variety of fish and crustaceans in the Thames’s waters.

The harbour seals have used the Thames as an important breeding ground, using the sandbanks exposed by the tides as pupping sites.

• Is it normal to see a seal on the beach? Yes – but here are the warning signs to look out for
• Britain's best seal watching spots and how to avoid disturbing the colony
• 10 weird and spectacular sea creatures found in British waters

Wels catfish

This non-native, invasive species is predominantly found in the tidal Thames – although they are fairly rare. These massive fish can weigh up to 50kg, and the Environment Agency monitors their impact as they have been known to disrupt native fish populations. They were introduced to the Thames in the early 20^th century for recreational angling, with anglers considering them a “trophy prize” due to their large size.

European eels

These snake-like fish are born in the Sargasso Sea, just off the coast of the United States of America, and are a migratory species, travelling over 5,000km to European estuaries – including the Thames. They can stay in their new homes for up to 20 years, travelling all the way back to the Sargasso Sea to reproduce and then die.

Once a common species, the European eel is now considered critically endangered, with its population declining by 90-95% since the 1980s. This is due to habitat destruction, physical barriers to their migration and overfishing.

125 species of fish – including the smelt (which smells of cucumber)

You’d expect to find fish in the Thames, but would you expect to find as many as 125 species? Some of these are familiar species, such as the sea bass, Dover sole and flounder, while others, such as the smelt (which is renowned for smelling of cucumber), are perhaps more surprising.

The Thames acts as a nursery ground for many of these species of fish to grow and develop. In fact, the Thames is one of only five known nursery grounds for Dover sole in the UK.

• Best fish identification books

Oysters

Native oyster populations in the UK have been decimated over the last century, but there is still a population living in the outer Thames estuary. Oysters provide a vital source of food and habitat for young fish, and also play a role in filtering the water around them. They can also protect the city from flood damage, as they can act as natural breakwaters and act as a sustainable, “living” barrier.  

In September 2024, it was announced that London Mayor Sadiq Khan planned to reintroduce oysters to the Thames as part of a 10-year plan to clean up the city’s rivers. A similar scheme took place in the Hudson River in New York, with a billion oysters reintroduced from 2014.