The deepest-dwelling fish in the sea is small, pink and delicate

Deploying the fish trap in the Mariana Trench from the R/V Falkor. © Schmidt Ocean Institute.
Paul Yancey, Whitman College., CC BY-ND

 

Mackenzie Gerringer, University of Washington

Thanks to movies and nature videos, many people know that bizarre creatures live in the ocean’s deepest, darkest regions. They include viperfish with huge mouths and big teeth, and anglerfish, which have bioluminescent lures that make their own light in a dark world.

However, the world’s deepest-dwelling fish – known as a hadal snailfish – is small, pink and completely scaleless. Its skin is so transparent that you can see right through to its liver. Nonetheless, hadal snailfish are some of the most successful animals found in the ocean’s deepest places.

Our research team, which includes scientists from the United States, United Kingdom and New Zealand, found a new species of hadal snailfish in 2014 in the Mariana Trench. It has been seen living at depths of almost 27,000 feet (8,200 meters). We recently published its scientific description and officially christened it Pseudoliparis swirei. Studying its adaptations for living at such great depths has provided new insights about what kinds of life can survive in the deep ocean.

The Mariana snailfish, Pseudoliparis swirei, the deepest-living fish. Video by Alan Jamieson and Thomas Linley, University of Aberdeen. Schmidt Ocean Institute.

Exploring the hadal zone

We discovered this fish during a survey of the Mariana Trench in the western Pacific Ocean. Deep-sea trenches form at subduction zones, where one of the tectonic plates that form the Earth’s crust slides beneath another plate. They extend 20,000 to 36,000 feet deep below the ocean’s surface. The Mariana Trench is deeper than Mount Everest is tall.

Ocean waters in these trenches are known as the hadal zone. Our team set out to explore the Mariana Trench from top to bottom in an effort to understand what lives in the hadal zone; how organisms there interact; how they survive under enormous pressure created by six to seven miles of water above them; and what role hadal trenches play in the global ocean ecosystem.

Mariana Trench location.
Dcfleck, CC BY

Getting to the bottom

Sending instruments to the ocean floor is pretty straightforward. Bringing them back up is not. Researchers studying the deep sea often use nets, cameras or robots connected to ships by cables. But a 7-mile-long cable, even if it is very strong, can break under its own weight.

We used free-falling landers – mechanical platforms that carry instruments and steel weights and are not connected to the ship. When we deploy landers, it takes about four hours for them to sink to the bottom. To call them back, we use an acoustic signal that causes them to release their ballast and float to the surface. Then we search for them in the water (each carries an orange flag), retrieve them and collect their data.

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Image from video of Mariana snailfish.
SOI/HADES/University of Aberdeen (Dr. Alan Jamieson) , CC BY-ND

Life in the trenches

Hadal trenches are named after Hades, the Greek god of the underworld. To humans, they are harsh, extreme environments. Pressure is as high as 15,000 pounds per square inch – equivalent to a large elephant standing on your thumb, and 1,100 times greater than atmospheric pressure at sea level. Water temperatures are as low as 33 degrees Fahrenheit (1 degree Celsius). Yet, a host of animals thrive under these conditions.

Our team put down cameras baited with mackerel to attract mobile animals in the trench. At shallower depths, from approximately 16,000 to 21,000 feet (5,000-6,500 meters) on the abyssal plain, we saw large fish such as rattails, cusk eels and eel pouts. At the upper edges of the trench, below 21,000 feet, we found decapod shrimp, supergiant amphipods (swimming crustaceans), and small pink snailfish. This newly discovered species of snailfish that lives to near 27,000 feet (8,200 meters), is now the world’s deepest living fish.

Video footage captured from the University of Aberdeen’s Hadal-Lander in the Mariana Trench from 16,000 to 35,000 feet deep. Video by Alan Jamieson and Thomas Linley.

At the trench’s greatest depths, near 36,000 feet (11,000 meters), we saw only large swarms of small scavenging amphipods, which are somewhat similar to garden pill bugs. Amphipods live all over the ocean but are highly abundant in trenches. The Mariana snailfish that we filmed were eating these amphipods, which make up most of their diet.

The Mariana Trench houses the ocean’s deepest point, at Challenger Deep, named for the HMS Challenger expedition, which discovered the trench in 1875. Their deepest sounding, at nearly 27,000 feet (8,184 meters), was the greatest known ocean depth at that time. The site was named Swire Deep, after Herbert Swire, an officer on the voyage. We named the Mariana snailfish Pseudoliparis swirei in his honor, to acknowledge and thank crew members who have supported oceanographic research throughout history.

Life under pressure

Hadal snailfish have several adaptations to help them live under high pressure. Their bodies do not contain any air spaces, such as the swim bladders that bony fish use to ascend and descend in the water. Instead, hadal snailfish have a layer of gelatinous goo under their skins that aids buoyancy and also makes them more streamlined.

Hadal animals have also adapted to pressure on a molecular level. We’ve even found that some enzymes in the muscles of hadal fish are adapted to function better under high pressure.

Scientific drawing of Pseudoliparis swirei, the Mariana snailfish.
Thomas Linley/Zootaxa, CC BY-ND

Whitman College biologist Paul Yancey, a member of our team, has found that deep-sea fish use a molecule called trimethyl-amine oxide (TMAO) to help stabilize their proteins under pressure.

However, to survive at the highest water pressures in the ocean, fish would need so much TMAO in their systems that their cells would reach higher concentrations than seawater. At that high concentration, water would tend to flow into the cells due to a process called osmosis, in which water flows from areas of high concentration to low concentration to equalize. To keep these highly concentrated cells from rupturing, fish would have to continually pump water out of their cells to survive.

The evidence suggests that fish don’t actually live all the way to the deepest ocean depths because they are not able to keep enough TMAO in their cells to combat the high pressure at that depth. This means that around 27,000 feet (8,200 meters) may be a physiological depth limit for fish.

The ConversationThere may be fish that live at levels as deep, or even slightly deeper, than the Mariana snailfish. Different species of hadal snailfish are found in trenches worldwide, including the Kermadec Trench off New Zealand, the Japan and Kurile-Kamchatka trenches in the northwestern Pacific, and the Peru-Chile Trench. As a group, hadal snailfish seem to have found an unlikely haven in a place named for the proverbial hell.

Mackenzie Gerringer, Postdoctoral Researcher, University of Washington

This article was originally published on The Conversation. Read the original article.

Scientist at work: I’ve dived in hundreds of underwater caves hunting for new forms of life

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Author Tom Iliffe leads scientists on a cave dive.
Jill Heinerth , CC BY-ND

Tom Iliffe, Texas A&M University

Maybe when you picture a university professor doing research it involves test tubes and beakers, or perhaps poring over musty manuscripts in a dimly lit library, or maybe going out into the field to examine new crop-growing techniques or animal-breeding methods. All of it’s good, solid research and I commend them all.

Then there is what I do – cave diving. To study the biology and ecology of coastal, saltwater caves and the marine fauna that inhabit them, my cave diving partners and I head underground and underwater to explore these unique and challenging ecosystems. Often we go to places no other human has been. While the peaks of the tallest mountains can be viewed from an airplane or the depths of the sea mapped with sonar, caves can only be explored firsthand.

Around the globe, from Australia to the Mediterranean, from Hawaii to the Bahamas and throughout the Caribbean, I have explored more than 1,500 such underwater caves over the last 40 years. The experience can be breathtaking. When you are down 60 to 100 feet in a cave that has zero light and is 20 miles long, you never know what you are about to see as you turn the next corner.

The remipede Cryptocorynetes elmorei from Eleuthera, Bahamas. Remipedes are only found in deeper saltwater layers from caves on opposite sides of the Atlantic and from the Indian Ocean coast of Western Australia.
Tom Iliffe, CC BY-ND

My primary focus is searching for new forms of life – mostly white, eyeless crustaceans – that are specifically adapted to this totally dark, food-poor environment. Cave diving is an essential tool in our investigations since the caves I’m interested are filled with water: typically a layer of fresh or brackish water on the surface and then saltwater at depths of 10 to 20 meters or more.

There’s no other way to access these unexplored areas than to strap on your scuba tanks and jump in.

Scientific research as extreme sport

The list of what can go wrong in a cave dive could fill your event planner.

Equipment or light failure, leaking scuba tanks, broken guide lines, getting lost, cave collapse, stirred up silt resulting in zero visibility, poisonous gas mixtures – you get the idea.

It’s fieldwork that can be a matter of life or death. I have had some close calls over the years, and sadly, have lost several good friends and researchers in cave accidents.

Tom Iliffe preparing for a side mount dive at Cliff Pool, Bermuda. Rather than carrying tanks on his back as in conventional scuba, a tank is clipped off under each arm, allowing him to pass through low sections in a cave where it would otherwise be impossible to go.
Gil Nolan, CC BY-ND

To put it mildly, underwater caves can be very hostile and unforgiving. One such cave – the Devil’s system in north-central Florida – has claimed at least 14 lives in the last 30 years, and there are other examples elsewhere in Florida and in Mexico.

Most of the time, human error is to blame, when divers don’t follow the rules they should or lack essential training and experience in cave diving.

My family has gotten used to the idea that what I do is not always a walk in the park. They know that since I’m 69, I stress safety, being physically and mentally prepared, and that I religiously abide by the cardinal rule of cave diving – that you never ever dive alone. My colleagues and I usually go into a cave with teams of two to three divers and constantly look after each other to see if there is anything going wrong during our dives, which usually last about 90 minutes, but can be as long as three hours or more.

Tom Iliffe diving with his Megalodon closed-circuit rebreather in a lava tube cave in the Canary Islands.
Jill Heinerth, CC BY-ND

Death-defying dives pay off in discoveries

It’s not just new species we are discovering, but also higher groups of animals including a new class, orders, families and genera, previously unknown from any other habitat on the planet. Some of our newfound animals have close relatives living in similar caves on opposite margins of the Atlantic Ocean or even the far side of the Earth (such as the Bahamas versus Western Australia).

While most of these caves are formed in limestone, they can also include seawater-flooded lava tubes created by volcanic eruptions. Amazingly, similar types of animals inhabit both.

In the deserts of West Texas, our team discovered and explored the deepest underwater cave in the U.S., reaching a depth of 462 feet.

The graduate students in my lab work on a diverse group of questions. They’re uncovering the nature of chemosynthetic processes in caves – how microorganisms use energy from chemical bonds, rather than light energy as in photosynthesis, to produce organic matter – and their significance to the cave food web.

Other students are examining records of Ice Age sea level history held in cave sediments, as well as the presence of tree roots penetrating into underwater caves and their importance to the overlying tropical forest. We’re finding evidence that sister species of cave animals on opposite shores of the Atlantic separated from one another about 110 million years ago as tectonic plate movements initiated the opening of the Atlantic, as well as determining how environmental and ecological factors affect the abundance and diversity of animals in saltwater caves.

Our research has significant implications, especially concerning endangered species and environmental protection. Since many cave animals occur only in a single cave and nowhere else on Earth, pollution or destruction of caves can result in species extinctions. Unfortunately, the creation of many protected areas and nature reserves failed to take cave species into account.

The remipede Godzillius robustus from Abaco, Bahamas. Note the darker shaded venom-injecting fangs on the first pair of appendages.
Tom Iliffe, CC BY-ND

Some discoveries can be completely unanticipated. For example, when we sequenced DNA from a variety of arthropods, including crustaceans and insects, the data strongly support a sister group relationship between hexapods (the insects) and remipedes, a small and enigmatic group of marine crustaceans exclusively found in underwater caves. This places the remipedes in a pivotal position to understanding the evolution of crustaceans and insects.

The author on a cave dive.
Jill Heinerth, CC BY-ND

Even at this stage of my life, to me the risks attendant to my cave diving research are worth it. It’s like the Star Trek mantra come true – to boldly go where no man has gone before. The chance to discover new forms of marine life, to view never-before-seen underwater formations, vast chambers, endless tunnels and deep chasms, to swim in some of the bluest and purest water on Earth – I will take that sort of research and its challenges any day.

The ConversationYes, it can give new meaning to the old line about “publish or perish” in academia. But I love it, and I will tell you with all honesty, I can’t wait until my next trip.

Tom Iliffe, Professor of Marine Biology, Texas A&M University

This article was originally published on The Conversation. Read the original article.