The Mussel Hunters

In the deep, dark sea, where there is no sunlight and very little food, mussels have developed a clever survival strategy: Their energy comes from bacteria sitting in their gills. But how do you find these shells at the bottom of the sea?

By Federal Ministry of Education and Research

Since we are not looking for the shells on the beach, but rather on the seabed in depths of up to 3,000 meters, our shell search is a special challenge. The pilots are currently steering our remotely operated vehicle (ROV) over the seabed to a large hydrothermal vent field. Benedikt Geier and Maximillian Franke, both doctoral candidates at the Max-Planck Institute for Marine Microbiology, are on the lookout for the object of their desire: They are looking for a deep-sea mussel called "Bathymodiolus".

It is known from other marine areas that mussels live in the deep sea on the so-called cold seeps and hot vents, often even dominating these hydrothermal systems, but can they also be found here in Antarctic waters?

It is rare for a research vessel to travel in this area. It is even rarer that the research then focuses on exploring the seabed.

Art of NatureFederal Ministry of Education and Research

But back to the search for the mussel. It is also well-known that in the deep, dark sea, where there is no sunlight and very little food, the mussel has developed a clever survival strategy. In their gills sit bacteria that "eat", for example, methane and hydrogen sulphide. Of course, scientifically speaking, the bacteria do not eat, but they oxidize these chemicals and give the mussel its necessary life energy.

Consequently, the mussel can only live because it has a lifelong partnership with the bacteria.

This is called symbiosis - and Benedikt Geier and Maximillian Franke work in symbiosis research. A wide field. Basic research. Complex. Super exciting, because how and when do the mussels make the bacteria enter their gill cells? How can it decide which bacteria are good for it? After all, there are also bacteria that could cause a horrible death to mussels, as it sometimes does in humans.

As we continue to search the ocean floor, we fleetingly think about how many bacteria are on our skin or play an important role in our intestines, and how little we know about symbiotic processes in our own body.

Suddenly, dense, white smoke. Our ROV approaches a large hydrothermal field. We are in the Kemp Caldera, a volcanic crater structure on the seabed that lies at the southern end of the South Sandwich Arc.

The chimneys emit fine, light-colored mineral particles, which we perceive with our human eye as white smoke. The individual chimneys are thickly covered with yellow sulfur and white bacterial mats. Actually, the right environment for our sought after "mussels". Yes, mussels are animals, perhaps that should also be mentioned.

Clam-Animation by MPI BremenFederal Ministry of Education and Research

While we are exploring the smoking hydrothermal field underwater, a situation is developing at the surface. About 2.5 nautical miles south of the location where we are currently working on the seabed with the diving robot, a table iceberg, approximately 700 meters long, drifts directly toward our ship. There is fierce concentration on the bridge, because if this iceberg came too close to us, in the worst case we would have to stop the dive.

For now, however, we get the green light from the captain and continue to work in the deep sea. The bottom water of Kemp Caldera has a temperature of 0°C. The influence of the volcano heats the sediment to 5°C. Obviously shellfish like this, because suddenly we see many small "snorkels" out of the greenish-brown mud - the siphons of clams.

Little can be seen of their shells, but our biologist from the British Antarctic Survey and our two microbiologists are certain that these are live shells.

The scientists ask the pilot of the ROV to unpack the net. With the manipulator arm of the ROV, it is retrieved from the drawer of our underwater vehicle, and with great care, it becomes possible to sample some of the shellfish. As inexperienced observers, we are surprised that the joy about this "catch" is quite limited.

Deep sea clamsFederal Ministry of Education and Research

Yes, they are clams, and yes, these shellfish live chemosynthetically, but it is not the shellfish that Maximillian and Benedict would like to find. What is the difference? "These shellfish belong to the family of vesicomyids. They also live in symbiosis with bacteria, but the symbionts they carry are passed between clams from generation to generation. Our mussel, however, does not get the bacteria from its parents. It does not bury itself in the sediment, but sits on the bottom of the sea and filters the water”, explains Benedikt Geier. Aha, even in the deep sea, there are different survival strategies.

Deep sea clamFederal Ministry of Education and Research

There remain the questions of how the clams find the right hydrothermal field and at what time, biologists call this the development stage, do the bacteria colonize the clam then? Or is she looking for the right living environment as a tiny larva even before she matures into the clam? How does she manage this, facing the tremendous water pressure and currents in several thousand meters? After all, they are not well-known as good swimmers.

Questions upon questions, making it clear again how great scientific work is, because it obviously consists of asking questions and then finding answers that are sure to raise new questions. "All these questions also show how unexplored many systems are today, and that sometimes we cannot fully explain the "simplest" things," says Maximillian Franke.

Look into a deep sea clamFederal Ministry of Education and Research

The endlessness of the scientific struggle constantly ensures that the researching human spirit will retain its two noblest impulses and will be fanned over again and again: enthusiasm and awe.
Max Planck (1858-1947), founder of quantum physics and Nobel laureate

In order to clarify some questions and to better understand the life cycle, as well as the special interaction between shellfish and bacteria, Benedikt and Maximillian together with their colleagues from the symbiosis working group onshore, use modern molecular techniques to reconstruct the various aspects of the clam, and to make them visible at the molecular level. For this, they use samples that had already been found in other marine areas. And it would just be too nice if they could bring a "Bathymodiolus" from the Antarctic to their Bremen home institution.

Maximilian Franke from MPI Bremen during the "Polarstern"-Expedition PS 119Federal Ministry of Education and Research

A few hours later, it's already dark, our ROV reappears behind the ship. The huge table iceberg in front of us was not that fast, and we still have a safe distance of 1.6 nautical miles. The ROV team and the deck crew secure the robot. The scientists can take their samples from the underwater vehicle's drawer and take them to laboratories.

Katrin Linse from the British Antarctic SurveyFederal Ministry of Education and Research

It will be a long night for Maximillian and Benedict. They dissect some of the clams and fix the samples, so that they can be further examined onshore after the expedition.

FixationFederal Ministry of Education and Research

Although these are not the mussels they would have loved to find, those who watch them at work in the lab, see and feel the excitement and awe they work with. Something that they carry deep within them, and that they share with us during the long weeks at sea. Bringing us new thoughts along with it, also because they are open to questions and engage in discussions that go far beyond their scientific horizons.

For that you can, should, you just have to say THANK YOU. Thanks to two young scientists who stand up for helping us all to gather the “web of life” better and in short moments they even give us the chance to grasp the meaning of our own life.

In the Bio-Lab of "Polarstern"Federal Ministry of Education and Research

Credits: Story

ANIMATION Deep-Sea Mussel:
Max Planck Institute for Marine Microbiology, Department Symbiosis, Maximilian Franke & Benedikt Geier,

UNDERWATER FOOTAGE: MARUM-QUEST 4000, MARUM – Center for Marine Environmental Science, University of Bremen,

PHOTOGRAPHY: Holger von Neuhoff
TEXT: Stephanie von Neuhoff

Credits: All media
The story featured may in some cases have been created by an independent third party and may not always represent the views of the institutions, listed below, who have supplied the content.
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