To celebrate World Habitat Day we’re going underwater to one of the largest, but least understood living places on our planet – the deep sea.

Astonishingly in the deep sea there is more life than anywhere else on earth! An alien world where strange creatures have found a way to survive extreme conditions.

Our deep-sea voyage descends 3,000 meters into the waters surrounding the UK overseas territory, and world’s most remote inhabited island – Tristan da Cunha.

Situated in the middle of the South Atlantic the Tristan archipelago is unique in having both cold sub-Antarctic waters and warmer subtropical waters which makes the seas around the islands highly productive. The almost pristine waters are an important foraging site for many seabirds, sharks, and marine mammals.

In 2020, after a decade of hard work by the Tristan community, the RSPB and partners the waters around Tristan were designated as a Marine Protection Zone, creating one of the world’s largest sanctuaries for marine life.

Salps
Salp by British Antarctic Survey

The sub-tidal zone

The islands of Tristan da Cunha are volcanic and have steep seabed slopes dropping into very deep water within a few miles of the coast. Over 90% of Tristan da Cunha’s marine territory is over 1,000 metres deep, and most of this is still deeper, below 2,500 metres!

We're starting our journey 50-200 metres below the surface. The slopes have black lava rocky outcrops and sandy ledges that are home to sea fans, cup corals sponges and sea anemones, offering good hiding and feeding places for starfish, sea slugs, urchins, lobsters and fishes.

Cup coral by Camille Moreau, British Antarctic Survey
Lobster larva (c) Katie Brigden
Shipwreck MS Oliva by Sue Scott

 

Cup corals are marine animals that build themselves a hard skeleton. The animals inside the skeleton are called polyps, and their mouths are fringed with tentacles. Cup corals are found over rocks and boulders and provide homes for other marine life. They are commonly found in deeper waters, on the underwater volcanoes around Tristan too!

Lobster is a vital resource for the island community

A lobster egg is the size of a pin head! The tiny larva spend almost a year at sea, drifting with the ocean currents, changing larval stages several times  before it sinks to deeper water to hide in the rocks. Lobsters grow by moulting their shells throughout their life.

The Tristan community rely on the sustainable fishing and export of lobster as their main income.

There’s also a shipwreck!

A cargo ship called Oliva sank in 2011 on the slopes of Nightingale island leaving behind fuel oil and 65,000 tonnes of soya beans. This incident led to Tristan da Cunha Government declaring a 25 mile ‘Area to be Avoided’ around all its islands, to deter oceanic cargo shipping away from its sensitive shoreline.

Twilight zone

Time to travel a little deeper to 200 –1,000 metres where sunlight becomes increasingly dim, the temperature drops and the pressure increases. Survival here means making the most of every glimmer of light. You might expect the waters to be barren but in fact they are teaming with, jellies, squids, and fishes.

The twilight zone is thought to be home to more fish that than the rest of the ocean combined but most are tiny, measuring just a few inches long or less.

Sea angel (Pteropod) by British Antarctic Survey
Salp by British Antarctic Survey
Sea slug (c) Chester Sands

Sea angels

Pteropod (ptero stands for ‘wings’ and pod for ‘foot’), also known as sea angels or sea butterflies flutter gracefully through the water on translucent wings. These sea snails use their feet as wings for swimming.

Salps have a superpower!

Salps are barrel shaped marine animals that can jet propel themselves through the water. Salps have a superpower! They can form a salp bloom! When food is plentiful they can reproduce alone by forming a long chain of identical salps, and each salp can increase rapidly in size – making them one of the fastest growing multicellular animals on Earth.

Most of the animals in the deep-sea are unknown to science

We might have found something new to science! This nudibranch, commonly known as a sea slug is currently under expert investigation! Their name means ‘naked gill’ because they have breathing apparatus on the outside of their bodies. To protect themselves from predators they use camouflage and toxins which they can release from tissue on their backs.

Hatchet fish (c) Katie Brigden
Giant sea urchins by British Antarctic Survey
Deep sea coral reef by British Antarctic Survey

Master of camouflage

Cutting through the water without a trace the deep sea Hatchetfish (Argyropelecus sp) is the master of camouflage! It has light producing organs on its body, that point downwards to hide its silhouette from below. These small, silvery fish live in the deep water during the day but at night they move up to the surface water to feed.

Giant sea urchins

Sea urchins have a special mouth called Artistole’s Latern! Named after the Greek philosopher, scientist and teacher who first described the mouth structure in his book Historia Animalium. In this book, he referred to the "mouth-apparatus" of the urchin as looking like a five-sided "horn lantern."

Deep-sea coral reef

Even at depths of 500m cold-water coral reefs provide shelter for marine life.

Look closely and you might see crinoids, often known as sea lilies, these animals are related to starfish and palaeontologists have dated them back to 490 million years.

Midnight zone

We travel deeper to 1km (1,000 metres), into the world of eternal darkness – the midnight zone. With no light there are no plants or phytoplankton, just animal predators and scavengers.

Lantern shark can glow in the dark!

Lantern sharks are usually found in depths between 200 to 1,000 metres, but they can be found as deep as 4,000 metres (that’s 2.4 miles deep!). Unfortunately lantern sharks are sometimes accidently captured in fishing vessels.

Lantern shark
Lantern shark (c) James Bell

This shark can glow in the dark! Their underbellies are covered with light producing organs called photophores, little round organs with a dark pigment (melanin) that produces light after a chemical reaction in low temperatures.

Very little is known about the species. They are small sharks growing just to over a meter in length with two sharp spines on their dorsal fins. They are usually found in small groups.

Ocean floor

At around 3,000 meters we reach the ocean floor. It’s a vast plain that covers half the surface of our planet. The seabed may at first appear lifeless but its home to a unique cast of ocean floor dwellers!

But this is where our journey ends in the waters of Tristan da Cunha. We’ve never reached the ocean floor!

Discoveries new to science

The deep sea is a challenging place to explore. With each new expedition to explore the deep sea, we make discoveries new to science.

Three scientific research cruises around Tristan da Cunha have given us a tantalising glimpse of what lies beneath the waves but there is so much left to explore.

During the research cruises a team of on-board specialists from different institutions including the RSPB, British Antarctic Survey and the UK Government Blue Belt Programme collected samples of marine life and habitats in the deep sea. By growing our understanding of these unexplored depths, the Tristan da Cunha Government are better able to inform protection and management measures. You can read more about the research cruise findings online.

More information

Research cruises

Deep sea research papers

All images are (c) Tristan da Cunha Government unless stated otherwise.