How and why do so many organisms glow in the ocean?

The sea walnut cannot see, but it still emits light. How does it do it and what does it use the light for? A new research project will try to solve the mystery.

En amerikansk ribbegoble (Mnemiopsis leidyi) i mørk vand. Goblen lyser op i strenge ned langs kroppen

Our eyes and minds are drawn to light when it is dark. It may be stars in the sky or light made animals, algae and bacteria in the oceans.

In fact, scientists estimate that at ocean depths where sunlight cannot penetrate, at least 30 and perhaps as many as 70 percent of all species are able to produce light. This is called bioluminescence, and on land we experience it, for example, in the common glow-worm (Lampyris noctiluca).

In the ocean, far more species can produce light. We just don't know exactly how they do it.

»We have an extremely widespread phenomenon, but it is not very well described on many levels. All the way from behavioural biology to hard chemistry. From a biochemical point of view, it is actually quite unusual to have a substance that is extremely widespread and has been known for decades, but where its biochemical origin is unknown,« says Jakob R. Winther, Professor at the Department of Biology at the University of Copenhagen.

He leads a project that, with support from the Independent Research Fund Denmark, will try to uncover many aspects, ranging from how light is produced to what role light plays in regards to the animal’s behaviour. 

The species that has been selected to help unravel the mystery is a warty comb jelly or sea walnut (Mnemiopsis leidyi).

In Denmark, the sea walnut has been designated as an invasive species. Partly because it did not live here originally, and partly because it eats considerable amounts of small fish, fish eggs and fry.

However, the fascination is also literally visible: If you give the jellyfish a little push, it emits blue light.

Furthermore, it is easy to breed and therefore suitable for research.

Photo of sea walnut that lights up
The sea walnut is capable of emitting light. In the small images B to E, all ambient light is turned off, and the sea walnut's flash of blue light, over a few seconds, is clearly visible.

Lights up only when it bumps into something other than its conspecifics

When the sea walnut wants to produce light, it uses an enzyme called luciferase. This is well described. But in order for the enzyme to produce the light, the jellyfish must also produce a substance that can help produce light when it reacts with oxygen in the water.

In the sea walnut and many other marine animals, it is the substance coelenterazine that plays this role. The substance has been known since the 1970s.

»But it has only been possible to either extract it from animals or produce it synthetically. We have no idea how the animals themselves produce it,« adds Jakob R. Winther.

Therefore, one half of the project will try to find out what kind of proteins in the sea walnut are responsible for producing coelenterazine.

The other half is about what happens to their behaviour when the jellyfish is not able to light up. The researchers are able to turn off certain genes in the animal and thus literally able to turn off the light.

»One of the strange things is that the jellyfish itself is blind. And it is also not clear whether its prey can see anything. In addition, there are some strange effects. For example, our preliminary results show that it does not light up if it bumps into another sea walnut. If, on the other hand, it bumps into something that is not a conspecific, it lights up,« explains Jakob R. Winther, who adds that the sea walnut’s blue light shines quite brightly.

May solve the mystery of many other sea creatures

The research project is being carried out as a collaboration between researchers from the University of Copenhagen, the Technical University of Denmark (DTU) and the University of Southern Denmark and is planned to run for three years.

Jakob R. Winther hopes that the researchers will be able to prove a very special theory.

»In this case, it looks as if the jellyfish turns a large protein into a small molecule, which is very strange and very different from what you see otherwise. It could be incredibly exciting if we could see that it is happening in this way,« says Jakob R. Winther and continues:

»But the second best result is also really interesting. If we can understand how this molecule, coelenterazine, is produced in the jellyfish, it can probably provide insight into how it is produced in other animals as well. Considering how widespread it is in the ocean, it will be a major breakthrough.«

The research may also benefit our society. Today, luminous substances are important in research, for example if you want to investigate the spread of a tumour in a laboratory animal. But the drugs are very expensive to produce chemically.

»If you can produce it biochemically, it will be a great benefit. This means that you can get bacteria or yeast to produce it. Then you can make it cheaper and make bioluminescence applicable in different areas,« concludes Jakob R. Winther.

The project group and its partners

Jakob R. Winther leads the research. In addition, the project group consists of:

  • Anders Garm, Associate Professor at the Department of Biology at the University of Copenhagen.
  • Cornelia Jaspers, Senior Researcher at DTU Aqua.

The project also involves the following partners:

  • Ole Nørgaard Jensen, Professor at the Department of Biochemistry and Molecular Biology at the University of Southern Denmark.
  • Martin Marek, Head of The Structural Biology Group within the Loschmidt Laboratories at the RECETOX Research Institute of Masaryk University in Brno, Czech Republic.