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In the Twilight Zone – Q&A with Dr Xabier Irigoien

The ocean can be an enigmatic place, and like much of nature, it comes in layers with great hidden depths.

Exploring these layers is one of the great scientific projects of our time. With so much marine life not fully accounted for yet in our scientific records, it’s an exciting field to behold, but also one carrying a great deal of responsibilities. Those who study layers of the ocean such as the twilight zone can find themselves observing important natural processes taking place behind the scenes which help keep our planet in check.

The twilight zone is where sunlight grows scarce beneath the surface, and photosynthesis stops working, limiting the kinds of organisms which can live here. Life takes new and unusual shapes to survive, showing just what lengths evolution must take to ensure survival down here. Dr Xabier Irigoien is Science Director at AZTI, a science and technology organisation which develops projects with others in alignment with the 2030 Sustainable Development Goals (SDGs) of the UN.

Our knowledge [mesopelagic environment] of that layer is so limited that we still do not know the fish biomass down there… If we do not know something as basic as biomass, imagine about the rest.

– Dr Xabier Irigoien, Science Director, AZTI

AZTI specialises in changing the way we consider the marine environment and our relationship to it in its work through food, as just one example. Fish and other organisms make up the bread and butter of many peoples’ diets, but much of this fauna passes through or even lives in the twilight zone, of which we know so little. We recently spoke with Xabier to learn more about his work, the twilight zone itself and why it matters.

Tell us about your life. What drew you to becoming a person with a fascination with all things marine to begin with?

Xabier: Well, I would say that the sea is fascinating by definition. Huge, changing, unknown…on the other hand I was born and I have always lived on the coast and there have always been sailors in my family (my great grandfather, grandfather, uncle). Let’s say I come from a maritime culture; the Basque country has always been closely related to the sea through fishing and exploration and the sea has always been around me.

The subject of this discussion is the oceanic twilight zone, but could you explain what this actually is?

Xabier: The technical name is mesopelagic zone, and the technical definition is the depth range of the sea where there is not enough light for photosynthesis (algae cannot grow) but there is still enough light to see. In practical terms the depth ranges between 200-1,000 metres.

I understand you are leading an EU project to explore the twilight zone. What’s it like to be part of such a project, discovering parts of our environment that no human has explored before?

Xabier: Yes, we have an EU-funded project called SUMMER: Sustainable Management of Mesopelagic Resources. The overall objective of SUMMER is to evaluate whether and how mesopelagic resources can be exploited without compromising the essential ecosystem services they provide. Well, we cannot say that the twilight zone has not been explored before. Work in that region started after the Second World War, when the Sonars looking for German submarines detected a layer at those depths that moved up and down during the day. 

The overall objective of SUMMER is to evaluate whether and how mesopelagic resources can be exploited without compromising the essential ecosystem services they provide.

– Dr Xabier Irigoien, Science Director, AZTI

There is a fascinating book about the research on the deep ocean by Tony Koslow, ‘The Silent Deep’, that I would recommend to anybody. However, it is true that the mesopelagic environment has never been intensively studied and is still full of surprises. Our knowledge of that layer is so limited that we still do not know the fish biomass down there by one order of magnitude. The fish biomass on the planet is larger than the human biomass, but it’s as if I were to tell you that we are not sure whether there are eight or 80 billion people on the planet.

If we do not know something as basic as biomass, imagine about the rest. Biodiversity, adaptations to live at depth and more importantly, function in the ecosystem and regulation on the planet´s biogeochemical cycles. There are amazing adaptations like bioluminescence, fantastic organisms that we have not been able to really observe alive like giant squids and the largest migration of the planet, with billions of single cm-sized organisms moving to the surface at night and coming back to deep waters at day. So yes, for a curious mind it’s like a dream, an empty canvas with a few dots that suggest an amazing landscape but one where we do not even know the size of the canvas.

What does a dark part of the ocean have to do with the levels of carbon in our atmosphere?

Xabier: Through photosynthesis, phytoplankton fixes CO2 from the atmosphere, but if the organisms die and are decomposed by bacteria in the surface, the CO2 is back in the surface water and an equilibrium is established between the atmosphere and ocean surface. However, any mechanism that can take organic matter fixed by microalgae from the surface and move it to deep waters or the bottom of the ocean alters that equilibrium and the ocean can absorb more CO2. 

One mechanism is particle sinking, another is transport of dissolved CO2 by sinking water masses and another one is the transport by organisms that migrate daily from the deep waters to the surface, the so-called biological pump. To avoid visual predators, mesopelagic organisms spend the day in deep waters (often more than 500m deep) and come to the surface to eat during the night. Through this migration, they are taking organic matter in at the surface and moving it very fast, in a few hours, down to deep waters, therefore increasing and accelerating the transport of CO2 to the deep waters. 

However, as I said earlier, we do not know what is the biomass of mesopelagic organisms and therefore we do not know their contribution to the transport of CO2 to deep waters.

When many of us think of the darkest parts of the ocean like the midnight zone, we think of all the sharp-toothed fish with large eyes and glow-in-the-dark body parts. How biodiverse is the ocean twilight zone by comparison?

Xabier: I would say that we do not really know. For fish, we know there are about 30 families and about 160 genera. Species is more difficult as there are many that are probably not described or cryptic, but it could be the hundreds. However, as an example, there are species such as the colossal squid Mesonychoteuthis hamiltoni, probably the largest squid, that could reach 15 metres, that we have only captured a handful of times and we know they are a major prey for sperm whales. So, there are organisms down there that elude us, and these are the giants. If we enter the field of the small organisms such a zooplankton, we know very little.

How does human activity directly impact what happens in the twilight zone?

Xabier: In different, poorly known ways. At the moment, there are no fisheries on the mesopelagic. There have been some attempts in the Indian Ocean and around Iceland but they did not appear to be economically viable or other species became more interesting. However, the marine ecosystem is not separated into shallow and deep, as we have indicated earlier. Deep species migrate to the surface to eat and therefore, they link the surface to the deep ecosystem. Warming, acidification and pollution certainly affect mesopelagic species the same way they affect other species.

We know that the Paris Agreement intends to keep global temperatures from rising no higher than 1.5 degrees celsius above pre-industrial levels. Any higher and organisms such as coral could experience large-scale bleaching events. How will rising temperatures impact organisms living in the twilight zone?

Xabier: Higher temperatures imply higher metabolic rates, as well as an extension of the anoxic [depleted of oxygen] layers, but the fact is that we know so little about the physiology of the mesopelagic organisms that is difficult to know how flexible and adaptable they are. A mesopelagic fish migrating in a tropical zone is moving daily from four degrees celsius to 25 degrees, as if from the pole to the tropics. Does that mean that they are highly adaptable? Or they have stretched their adaptability to such a limit that any increase may overwhelm them? We really do not know.

Does the ocean twilight zone tell us anything about how life could exist on other planets?

Xabier: Directly probably not, unless it’s a very Earth-like planet. Not like the thermal vents and similar features that present very different form of life. But the twilight zone shows us how evolution can play to adapt life to almost any environment.



Hydrophones can help pick up sounds in the marine environment, but by its very nature, the twilight zone is a gloomy place to try and explore. What kind of technology is best-suited to helping explore it while causing minimal disruption to the organisms that live down there?

Xabier: Echosounders have been the main tool to explore the mesopelagic because sound travels farther than light in water. However, from the surface, we are limited to a few frequencies to reach deep waters. Nets have shown their limitations because many organisms elude them, and we need to deploy many different nets which is really time consuming. In my view, lowered echosounders that can take different frequencies to deep waters, combined with environmental DNA, which only needs water to detect the presence of organisms in the environment, is the way to the future.

Would you say you are future-positive about the environment, including our oceans, and the way the world is going at the moment?

Xabier: Mixed feelings. I think there is a significant improvement in factors that can be managed locally; fisheries are much better managed than before and although problems persist, there is a clear improvement. Coastal pollution is also improving around the planet with better water management of discharges in many countries. Even in terms of eutrophication and nutrient discharge, there are improvements. Let’s say coastal and fisheries management is improving. On the other hand, at the planet-wide scale, global warming is not improving, I doubt we will reach the objectives of the Paris agreements, and therefore we may see major changes in all ecosystems, including the ocean.

The twilight zone doesn’t exist in isolation; think of it more as a bridge between the surface world and the deep, dark depths and the sea floor itself. Life crosses the twilight zone on a regular basis but we don’t even have names or words to describe much of this life

– Dr Xabier Irigoien, Science Director, AZTI

Our conversation with Xabier gave us a fraction of an idea about what could lurk down in the ocean twilight zone. It is a somewhat gloomier place quite literally, and life has been sustained down there in complex ways we still don’t fully understand. What we can tell is that the twilight zone is but one layer of a diverse, rich ocean home to multitudes of marine flora and fauna. 

The twilight zone doesn’t exist in isolation; think of it more as a bridge between the surface world and the deep, dark depths and the sea floor itself. Life crosses the twilight zone on a regular basis but we don’t even have names or words to describe much of this life. Human activity could do much to disrupt the delicate balance existing between each layer of the ocean, as they are all interconnected.

The work of Xabier and SUMMER will do much to unravel the mysteries of this twilight zone over the coming years. Let’s just hope we can explore it before climate change potentially erases whole species from the ocean before we even know about them.

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