Consider the Copepod: Researching the Base of the Food Web (with Dr. Nancy Mercado-Salas)

Image Credit: Andrei Savitsky (left and right), CC BY-SA 4.0 ; Uwe Kils (centre), CC BY-SA 3.0

The deep sea is a wondrous world of biodiversity, darkness, and mysteries we still know very little about. Despite the fact that we rely on the deep sea as a sink for carbon dioxide – and increasingly as a source of natural gases and minerals – we have very little understanding of how our actions will affect its intricate food web.

Near the base of the food web sits an incredibly diverse group of animals called copepods. They are so abundant and have such sweeping variety that we are still struggling to come up with a way to classify them. Dr. Nancy Mercado-Salas has worked with these tiny creatures since her bachelor’s thesis, both in freshwater and in marine ecosystems, and her message is clear: We need to increase our knowledge on this group of animals before it is too late.

Nancy is the curator for crustaceans at the zoological collection in Hamburg (CeNak) in Germany. I met Nancy during my master’s thesis in Hamburg and we both joined the IceAGE 3 expedition to the deep sea around Iceland this summer, and I was fascinated by her love for copepods. Read on to learn about why they are so important and how difficult it can be to learn about them, even as a researcher in this field.

Eva Paulus (EP): Nancy, you grew up in Mexico and already started working with copepods during your bachelor’s degree. How did you end up working with such tiny animals and what was your project about?

Dr. Nancy Mercado-Salas (NM-S): I always wanted to be a marine biologist, but the University I went to was in my hometown, which was in the middle of Mexico. It wasn’t possible to go to the coast, so I decided to work with smaller freshwater animals. You can also find similar versions of them in the ocean, and I thought they could be the link that would help me transition to marine biology eventually.

Then I fell in love with copepods! My project was on the diversity of freshwater cyclopoids, which are a type of copepods that you can find in most freshwater bodies around the world. I was studying the diversity of these animals in the state of Aguascalientes in Mexico. During my master’s and doctoral thesis, I continued studying freshwater ecosystems, and later on when I moved to Germany for my postdoc, I got the opportunity to work with marine copepods at the Senckenberg am Meer Research Institute in Wilhelmshaven.  

EP: In the past, often even marine biologists completely disregarded copepods as they are so numerous, and they weren’t even sorted in plankton samples. Why do you study such a tiny animal, and why are they so important?

NM-S: Copepods are very important in the ecosystems they inhabit. When you look at them from an evolutionary perspective, how they have invaded so many different habitats, how versatile they are, they’re just incredible. They come in so many different shapes, have different lifestyles, some look like tiny balls, others look like aliens. Copepods went through so many shifts during their evolution and this is reflected in how diverse they are.

They are also the foundation of a multitude of ecological processes. They are the main source of protein on Earth, their role in the trophic food net is so important, both in freshwater and the marine environment. Even their fecal pellets are important as they sink down into the deep sea and store carbon there, helping us mitigate climate change. They are also great indicators of how healthy an environment is.

EP: What are some of the differences, for instance in methodology, with freshwater and marine copepods?

NM-S: With freshwater copepods, sampling is easier in many ways. If you have a permit, you can drive to any water body, bring a small plankton net, and you can have your sample within a few minutes. In the ocean, and especially the deep sea, everything is a lot more complicated. You need a lot of funding, a research ship, and expensive equipment, such as plankton nets, box cores or epibenthic sledges. Just to reach the sampling areas can take days or weeks of steaming time. Resource limits – both in money and time – are one of the main reasons we know so little about the deep sea.

Venturing Into The Earth’s Great Unknown: The Mesopelagic Zone

Also, the biodiversity is totally different in freshwater and marine ecosystems. In freshwater, most species are exclusively found in the freshwater environment, and the diversity is lower in comparison to marine systems. We can say we know the diversity of some freshwater bodies after a few sampling trips, but this is impossible in the ocean. In the deep sea, there are so many species and different shapes of copepods that nobody has ever seen before. For example, for my master’s thesis, I was able to collect about 50 different cyclopoid species in the entire Chihuahuan Desert. In the Pacific, we found more than 500 species within just one 300m x 300m sampling plot.

EP: Have the methods changed a lot since you started working with copepods?

NM-S: Before I moved to Germany, my work focused very much on the taxonomy of copepods, identifying and describing new species. I was working with a more traditional approach to assessing biodiversity; it involved working with a lot of zoological collections, for instance by comparing my specimens to those deposited in the collections. I was also trying to define the distributional patterns of different species. Once in Germany, I started to use more genetic methods, which is often not possible with specimens found in zoological collections. Now, I use a lot of different genetic methods, such as barcoding and next generation sequencing. We can see how different species have separated, how gene flow works in the open-ocean ecosystem, and how well-connected the populations are, even in the deep sea.

Dr. Mercado-Salas assembling the equipment for sampling deep-sea sediment and the precious copepods within it during the 2020 IceAGE 3 cruise (Image Credit: Viola Siegler, CC BY 2.0)

EP: Tell me about a typical sampling trip on a research cruise.

NM-S: Research cruises are a lot of fun, but also very intense. You work a lot, and you learn a lot from many different areas of expertise and other scientists who are super passionate about their work. Usually you are involved in many different teams on board a research ship. In my case, I usually work with the equipment that samples copepods living in the water column, but my main research  focuses on the meiobenthos, animals that are smaller than 1mm and live in the sediment of the ocean floor. Once you retrieve the samples from the ocean, you have to make sure it is all fixed and stored properly. If possible, you try to get a first look at your samples and get an overview of what you can expect once you get back home. Then, you bring your treasures home to the lab and try to take advantage of every single animal you have collected.

EP: Once you have them in your lab, what happens?

NM-S: As copepods are so small and diverse, we try to look at every single specimen. Most of them are only about 1mm in size, so you have to treat them with the utmost care and prevent any damage to be able to identify them correctly. With benthic copepods, the ones that are directly associated with the sediment, you have to separate the animals from the sediment first, and then carefully sort through your samples. I personally try to extract DNA from every single animal I collect, and then recover the animal to work with them morphologically.

It’s very important to me to use both genetic and morphological methods. I think they are both equally important, even though the morphological work has been pushed to the background with the newer and modern genetic tools we have now. All these new techniques still rely on and are rooted in the morphological work.

In the deep sea, about 95% of the copepods you collect are new species. So you really have to mount every single specimen on a microscope slide and try to identify which family it belongs to, and ideally even the genus. Then you try to describe as many as you can, and focus on the groups you have expertise in already. The other ones you will keep or try to find experts who would be interested in describing them.

EP: Your recent work has been in areas of the Pacific where there are plans for deep-sea mining. On the ocean floor, there are manganese nodules that developed over millions of years that contain valuable metals, and different countries have already laid claims to mine them. What do you think the future for your copepods and the wider ecosystem is here?

NM-S: We are really racing to investigate the diversity in this area of the world and try to predict more precisely what will happen and how much diversity we may lose due to deep-sea mining. In the last decade, several exploration cruises have taken place to study the diversity and ecological processes between Mexico and Hawaii. We now know that if the manganese nodules are mined, we will lose many species that live on the nodules or in the sediment that will also be removed during the mining process. Most benthic animals live in the first 5 cm of the sediment and removing the manganese nodules will disrupt that layer.

Our main goal is to generate basic knowledge that can be used either to stop the mining or – in case mining is inevitable – design a better legislation groundwork that can avoid the loss of diversity and through that preserve the different ecological processes in which these species are involved. We will be able to designate areas to be mined, but also marine protected areas that will serve to preserve the diversity of species and potentially restock the mined habitats later on. It is obviously important to have a lot of baseline data in order to judge what effects deep-sea mining could and will have. We should really make sure that we do not lose such an amazing environment before we even understand what lives in it. Deep-sea mining will not only affect animals that are associated with the sediment, it will likely also release a lot of sediment into the water column. This will damage the animals living in the water column, which we understand even less than the benthic animals, with unknown consequences.

Eva Paulus is a recent MSc graduate in marine biology. She is interested in the ecology and evolution of deep-sea mesopelagic fishes, and has recently worked aboard the German vessel FS Sonne on a research cruise exploring the deep sea around Iceland. Follow her on Twitter @Deep_Sea_Dirndl.

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