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Protecting the strange sea pangolin and other animals: Q&A with deep sea biologist Chong Chen

Mining licences have been issued for two of the three known sites of the scaly foot snail. Image by Chong CHEN.

  • Creatures living in deep-sea hydrothermal vents lead a unique life that researchers are only now beginning to understand. Yet these animals are at risk of disappearing because of deep-sea mining before we even learn about them.
  • A deep-sea hydrothermal vent mollusk, the scaly-foot snail (Chrysomallon squamiferum), for example, debuted as endangered on the IUCN Red List this year because of threats from mining.
  • Mongabay spoke with deep-sea biologist Chong Chen, who has been assessing deep-sea hydrothermal vent species for the IUCN Red List, about his work and why listing these species on the IUCN Red List matters.

When Chong Chen was in high school, he came across a paper published in 2003 that described a peculiar species of snail from the Indian Ocean, thousands of feet below the water’s surface. The snail lived at the base of hydrothermal vents — fissures on the ocean floor that spew out a hot mix of minerals like iron, copper and nickel — and had a foot covered in armor-like overlapping scales of hardened iron sulfides, the same minerals that spurted out of the vents. Most other snails by contrast have a smooth muscular foot that they stick out of their shells to help them crawl across surfaces. The deep-sea snail’s shell was also made of iron sulfides, the only living animal known to use this material in its skeleton.

Chen had always been interested in mollusks, a group of invertebrates (animals lacking a backbone) that includes snails, slugs, clams and octopuses. But the scaly-foot snail was “one of the coolest ones” he’d ever read about, Chen, now a deep-sea biologist at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), told Mongabay.

“I always wanted to study molluscs and I had this interest in the deep sea, but of course, it was not developed,” he said.

But then he came across an opportunity to do a Ph.D. in deep-sea biology at the University of Oxford, and he jumped on it. Chen went on to study the scaly-foot snail in detail, visiting seamounts and hydrothermal vents at the bottom of the Indian Ocean, and collecting specimens of the animal. He even formally described the snail as Chrysomallon squamiferum in a paper published in 2015, with Chrysomallon translating to golden-haired, in reference to the golden metal coating of the snail’s shell. The golden color comes from pyrite, one of the iron sulfide compounds contained in the shell, commonly called fool’s gold.

The scaly-foot snail has a foot covered in armor-like scales of hardened iron sulfides. Image by Chong Chen.

Sixteen years after the scaly-foot snail was first described, it finally debuted on the IUCN Red List in July this year as a species that’s endangered. The species, also referred to as the sea pangolin for its scaled appearance, is known from just three hydrothermal vent sites at depths of 2,400 to 2,900 meters (7,800 to 9,500 feet) in the Indian Ocean, covering a tiny area of around 2 hectares (5 acres). Two of these three sites are now under deep-sea mining exploration licenses, Chen said.

In addition to the scaly-foot snail, deep-sea hydrothermal vents harbor numerous other animals with distinctive lives and adaptations that scientists are only now beginning to understand. But with a rise in interest in deep-sea mining of the minerals that occur in the vents, researchers worry that we could lose these unique animals and habitats before we even have the chance to know about their existence.

“Now, it’s a race between how fast we can do the scientific work and how fast people can send big machines to mine these areas,” Chen said.

In a letter published in Nature Ecology & Evolution, Chen and his colleagues say that assessing deep-sea hydrothermal vent species for the IUCN Red List could bring more visibility to the species and help garner much-needed attention and support for their conservation.

Mongabay spoke with Chen about his work on creatures living in the deep-sea hydrothermal vents and why listing them on the IUCN Red List matters.

Deep-sea biologist Chong Chen. Image by JAMSTEC.

Mongabay: What do you study in the deep sea?

Chong Chen: I focus on hydrothermal vent molluscs and I work on many aspects of their life. I work on their adaptation in terms of morphology and genetics. I also look at how different populations are connected with each other in terms of dispersal. That then tells us how closely related they are and how much larval dispersal occurs between two areas. I also have quite a bit of interest in their symbiosis. Often in these deep-sea environments, animals live with bacteria; sometimes bacteria live in their cells, like in the case of the scaly foot snail. So I’m interested in how the symbiont and animal live together as one system.

What kinds of tools do you use to study these animals?

I do dissection, but not just with standard methods that we think of like with a scalpel and forceps. I also do digital dissection using CT scans, which help us get a stack of images through an entire specimen, which we use to digitally reconstruct the specimen’s anatomy. This gives us quantitative data on the organization of the organs inside the animal, how big the organs are, how they grow and how that relates to symbiosis. When I use that method together with an electron microscope to see how the bacteria is changing inside the specimen, then we have an understanding of how the animal develops through its life and also how it modifies its anatomy to suit what it has to do in this special environment which is the hydrothermal vent.

I also a use genetic sequencing to infer evolutionary relationships between species, for example, between the scaly foot snail and other snails. This basically tells us things like how special they are in their group, when they evolved and how long it has been since they speciated. I also use sequencing to sequence the same gene from many individuals, which is a proxy of how closely related they are. If you have that data for many populations, then you know how closely related each population is, which is very important with conservation because you want to know how unique each population is.

What about expeditions? How many have you been on?

I’ve been on over 20 expeditions now.

Do you often dive or use remotely operated vehicles?

Both actually. Nowadays, I rely more on remotely operated vehicles but I also dive in the Japanese submersible, the Shinkai 6500. I actually have a dive coming up next month, so I’m very excited.

How deep do you dive usually?

The deepest I’ve been is 3,700 meters [12,000 feet]. The submersibles go till around 6,500 meters [21,300 feet]. But the hydrothermal vents are usually around 1,000 to 3,000 meters [3,300 feet to 9,850 feet].  The deepest one is 5,000 meters [16,400 feet], but that’s very rare.

How understudied are deep-sea species generally? And what are some reasons for this?

The deep sea is very understudied, mostly because the opportunity to access the deep sea is very rare and very expensive. This is especially true for species in special habitats, such as hydrothermal vents and cold seeps where animals rely on chemical energy from the earth instead of light from the sun. These habitats have systems where bacteria use hydrogen sulfide or methane, chemicals coming out from the earth to do primary production just like plants do on land with light but in a different way. But these systems are often very small, only about the size of a quarter of a football field, or the size of the auditorium in the vast ocean floor. So, finding these sites is very difficult. And we still have very little idea about some of these sites, and what lives there. We don’t have a species list in many of these areas. And many of the species don’t have names.

We also have very little idea about how resilient these species are to potential environmental impacts. Unfortunately, these systems, especially hydrogen vents and cold seeps, have been targeted for deep sea mining. These are very small sites, so if mining happens, it’s very likely that the entire site will be gone before we even know what lives there. This is a large part of why the scaly foot snail was listed as endangered because we only know it from three sites. Two of the sites are already licensed for mining exploration. If the mining actually happens, maybe two out of three of the snail’s habitats will be gone.

Deep-sea creatures are extremely understudied. Image by Chong Chen.

Could you tell us more about these deep-sea hydrothermal vents, the kind of life they support, and what’s unique about the animals that are found there?

Deep sea hydrothermal vents are found in places where there’s tectonic activity and ocean water seeps down from Earth’s crust and hits the active magma. Heated things rise, so the heated water rises up. But at that time, because it contacts magma, it contains a lot of chemicals and metals that were originally in the magma, such as hydrogen sulfide, iron, and copper. When the water comes out and hits the normal ocean floor, it precipitates there because the surrounding sea water is very cold whereas the water heated by magma is usually over 300 degrees [Celcius, about 570 degrees Fahrenheit]. Because the fluid contains hydrogen sulfide, it becomes an energy source for bacteria that are able to do chemosynthetic, primary production. So, we have a lot of bacteria living around these areas, and where there’s bacteria, there are animals who eat these bacteria. Then we have huge biodiversity and a huge biomass supported by these bacteria.

Hydrogen sulfide, however, is toxic to humans because it binds to the same sites on our red blood cells that otherwise bind to oxygen, which we definitely need. So hydrogen sulfide basically blocks you from breathing. But in habitats like deep-sea hydrothermal vents, there are special animals that have adapted to living there. Tubeworms, for example, have red blood cells that have special binding sites for hydrogen sulfide so that oxygen isn’t blocked. A single red blood cell can carry both oxygen and hydrogen sulfide, which is transported by blood to cells containing bacteria. The bacteria then use the oxygen and hydrogen sulfide to make energy. The scaly foot does exactly the same.

The snail has a pouch in its esophagus that is hugely expanded, about over 10 percent of its body volume, with bacteria living inside. The snail extracts hydrogen sulfide from the hydrothermal vent fluid using a giant gill, and then uses a really big heart to pump it to the bacteria in its cells. The bacteria use the hydrogen sulfide to make energy, which gets passed on to the snail in the bloodstream. So, the snail basically sustains itself on this energy and does not have to eat. This has a huge energetic advantage because the energy production occurs directly in the cells, which means that instead of having to eat and lose some energy through that process, the snail just makes energy within its own body. We think it’s because of this partly that the scaly foot snail is a lot larger than other snails of its close relatives — over 10 times the body size of other snails. While other snails are usually few millimeters, the scaly foot is a few centimeters. In general, species that have this kind of special relationship with bacteria are able to grow very large, such as giant tubeworms that grow two over two meters [6 feet] in length.

Animals like these snails and tubeworms can only live in the hydrothermal vents because in other places there is no supply of hydrogen sulfide. The bacteria they need, live only in the hydrothermal vents.

Another interesting thing about hydrothermal vents is that they’re now considered one of the most likely places where life originated on Earth. There are these chimney-like structures in hydrothermal vents where metals precipitate. These structures contain high amounts of copper, iron, nickel, silver, gold, which people are trying to mine. So maybe there’s a chance that we actually originated from this environment, which we’re trying to destroy.

How many species are known from these hydrothermal vents?

There are over 650 species of invertebrate animals that are known to be specially adapted to the hydrothermal vents. But this is only a fraction because we know a lot of them are not described.

How many hydrothermal vents have been mapped around the world?

We have good knowledge of about 300 hydrothermal vent sites, although we know that there are definitely 600, maybe up to 1,000 hydrothermal vents, around the world. But if you consider how small these vents are, like a fraction of a football field, it is really unlikely that mining the hydrothermal vent will be a long term sustainable thing to do. At first, it’ll probably be like, oh, there are a lot of minerals there, so it’s good. But then how long will it last? And then the animals will be all dead by the time we finish. So is mining there a sustainable thing to do for human beings and for the earth?

In deep-sea hydrothermal vents, bacteria are primary producers. Image by Chong Chen/University of Southampton.

The scaly-foot snail was first described in 2003. Why do you think it’s taken so long for it to make it to the IUCN red list?

I was actually quite surprised. The reason why we did this assessment was because when we were in an expedition in 2016, we couldn’t find a colony of the scaly foot snail in a place where we knew it occurred before. It had actually moved about two meters and its density was much lower. We found that there was some kind of natural fluctuation in their density and numbers, but we don’t actually understand why. But at first not finding the colony gave us cold chills because we thought, what if they were just gone? And if they’re so easily damaged by small environmental changes, they definitely cannot survive mining.

So we thought, what is something that we can do that will raise awareness to people, and the IUCN Red List came to mind. We expected someone to have done it already, because it’s so obvious, right? But no, nobody has done an assessment for any of the hydrothermal vent animals in the light of mining for the IUCN Red List. So we got in touch with IUCN.

One reason the scaly foot snail may not have been assessed before, say, 2010, is because hydrothermal vent mining was not a big thing then, and people were not worried about these animals because they were so remote, and people cannot access them. Their habitat was not under danger. It is only in the recent years that hydrothermal vent mining has become such a hot topic, with exploration licenses that have been granted for mining, and mining risks having become apparent in these past few years. This could be why it took so long for people to start protecting these animals.

We happen to be apparently the first ones to think of the Red List as a way of communicating this idea with the general public and stakeholders.

You also discovered four new species of deep-seas snails from Japan’s hydrothermal vents recently. How common are such discoveries?

A part of my job is doing taxonomy and describing new things from habitats we don’t know. So far, I’ve described over 20 species. But these environments are so poorly known that every time you go, you’re bound to find new species. Finding new species is not rare at all; you look everywhere you find a new species because people have not been to these places. Now, it’s a race between how fast we can do the scientific work and how fast people can send big machines to mine these areas.

How hard is it to assess the deep-sea hydrothermal vent species for the IUCN Red List, especially since they’re so difficult to find?

The IUCN Red List is developed mostly for terrestrial life, so while applying its criteria to the deep sea, we had a lot of discussion. But in the end, we find it’s not actually that difficult.

Hydrothermal vents and their species actually function quite similar to special habitats like limestone karsts and animals that only live there. These limestone caves or karsts are sparsely distributed in an area, and hydrothermal vents are also sparsely distributed. As long as we know how many sites each species lives in, how big these areas are, how common these species are, and maybe the potential individual number in each site, then we can assess them just like these special ecosystems on land. The assessment of the deep-sea hydrothermal vent species is partly made easy because they only live in these special spots, and not all over the sea floor. If you try to assess deep sea animals that live in a general seafloor, it’s going to be a lot more challenging.

How many deep-sea species from the vents have been assessed for the IUCN Red List to date?

There are only three hydrothermal vent endemic species on the IUCN Red List, and the scaly foot snail was the first one that was assessed with consideration of mining risk. The two other species that have been assessed is a lobster and an octopus, but these were done a few years back, so they did not consider the risk of hydrothermal vent mining at all.

We currently have 14 species that we have submitted to IUCN, which will come out in the next round of release. All of them are either vulnerable, endangered, or critically endangered even because of threats from mining. Of course, we are continuing to do more. We hope that in the next release, we will have more than 14 that we can release to the world.

You mentioned that licenses for mining exploration have been issued for two of the three known scaly-foot snail habitats. Do you have details of whom the licenses have been issued to?

One is issued to Germany and one to China. The third site lies within the exclusive economic zone of Mauritius, so it’s under Mauritian waters. We guess that one is probably safe for now, but we don’t know when Mauritius will be interested in mining.

Can you tell us more about mining licenses for deep-sea hydrothermal vents?

The sites in international waters are governed by the International Seabed Authority, which has released many mining exploration licenses to many countries and companies around the world. Hydrothermal vents that are within a country’s national jurisdiction are not safe either. For example, Japan has become the first country to try and send machines down to do hydrothermal vent mining. When the hydrothermal vents are in the waters of countries that clearly have a deep-sea mining interest, then the risk is potentially even larger than those ones in international waters. The ones we are assessing now from the Pacific, in many cases, they are not in international waters, but they fall under the waters of countries with mining interests.

How does listing deep-sea hydrothermal vent species help?

I think it’s helping us a lot. It’s very difficult to communicate the idea of what a hydrothermal vent system is and why we should protect them. But if you say a species is on the IUCN Red List as endangered then people just understand it because it’s a very well understood, well respected system. People then ask, so they’re endangered like tigers, for example? So we’re very glad that this listing is working out quite well in terms of communicating with policymakers.

What do you find most exciting about the work that you do? And what do you find most challenging?

As I said, hydrothermal vent animals have unique adaptations. So the part I find most exciting is to find how animals do things that we never thought life could do. Studying these animals really expand our views on what animals are capable of through evolution. And I think that is the most exciting thing for me.

The most challenging thing is that we can’t keep the deep sea hydrothermal vent species alive for very long. So it’s very difficult to do experiments, or try and look at their behavior. We can’t keep the scaly foot snail alive for more than weeks, for example. It’s because these deep sea animals require such a unique environment, and they just can’t survive in laboratory situations. That is definitely the biggest challenge.

Banner image of sea snails by Chong Chen.


Warén, A., Bengtson, S., Goffredi, S. K., & Van Dover, C. L. (2003). A hot-vent gastropod with iron sulfide dermal sclerites. Science, 302(5647), 1007-1007. doi:10.1126/science.1087696

Chen, C., Linse, K., Copley, J. T., & Rogers, A. D. (2015). The ‘scaly-foot gastropod’: A new genus and species of hydrothermal vent-endemic gastropod (Neomphalina: Peltospiridae) from the Indian Ocean. Journal of Molluscan Studies, 81(3), 322-334. doi:10.1093/mollus/eyv013

Sigwart, J. D., Chen, C., Thomas, E. A., Allcock, A. L., Böhm, M., & Seddon, M. (2019). Red Listing can protect deep-sea biodiversity. Nature Ecology & Evolution, 3(1134). doi:10.1038/s41559-019-0930-2

Correction: A previous version of the article quoted Dr. Cheng saying that the International Seabed Authority has released many mining licenses. It has been changed to “mining exploration licenses.”

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