One of the paradigms in marine ecology is that the animals’ diet can be linked to their size. In other words, it is understood that “big eats small”, but “big” is hunted and eaten by something bigger, and so food and energy migrate up the food chain. Studies that use new methods and technology indicate that this paradigm does not necessarily provide a good understanding of reality, and that we can occasionally replace “big eats small” with “small eats big”.
How can that be?
Let us start with the marine food chain in the Barents Sea. Most life so far north depends on nutrients and energy produced by single-celled algae. Every spring, when the sea ice melts and the sun returns, the algae start the machinery that ensures that energy-rich substances are formed from CO2, water and sunlight.
Over the course of weeks, even days, the water column can go from being nutrient-poor to becoming a feast full of calories. The menu features freshly prepared dinner portions with algae that are rich in sugar and fat. These substances migrate further in the food chain when the algae are eaten by single-celled animals and small zooplankton. These are then eaten by slightly larger zooplankton, which in turn are eaten by fish and birds, then seals, and finally we reach the hierarchy’s top predator, the polar bear.
This linear model has stood firm for centuries and can be used to advantage to show how the food chain is roughly connected. But we know today that nature is incredibly much more complex than the model suggests. Therefore, we are moving further and further away from the linear model, and closer to a food network where prey-predator interactions can also occur across trophic levels and size ratios. Central to this development is the use of new technology in research that allows us to uncover hitherto unknown species interactions.
Fact box
Trophic levels are used to describe the steps in the transfer of nutrients and energy in a food chain. As a rule, a food chain consists of four to five steps, with the producers (in this case algae) at the bottom, and predators at the top.
Traditional studies are limited by our level of knowledge
To understand the advantages of today’s technology, we must first look at the older, traditional methods. Previous studies of diet have been based on visually recognizing species in the stomach, in faeces or by directly seeing that one species eats another. For many animals, such methods can give good indications. For others, especially when moving down to the millimetre-sized copepod, it quickly becomes a difficult task. These creatures can be so small that their behaviour is difficult to observe. And the smaller they are, the smaller the stomach. Imagine dissecting the animal and identifying all kinds of species in a stomach the size of a pinhead.
After staring into the microscope for two hours, you have square eyes, and what you think is a species may just as well be remnants from the antenna of a krill.
Especially small animals can be difficult to work with in the traditional way. In beautiful association with single-celled protists, we find one of the smallest, but most numerous copepods in the southern Barents Sea – the Microsetella norvegica (to the left). Protists are a common term for single-celled “eukaryotic” organisms, which – in contrast to bacteria – have one cell nucleus and organelles (e.g. mitochondria or chloroplasts). Photo: Snorre Flo/UNIS
Another problem with the older methods is that they require an enormous amount of knowledge. It often takes several years of practice to be able to determine the species in a water sample with certainty. It is even more difficult to recognize species from material that is more or less digested. Particularly soft animals such as jellyfish and eggs have presented problems because they end up looking like an unrecognizable, transparent lump only after a short time in the stomach. The fact that they break down quickly can also lead to us underestimating the amounts of them relative to animals with hard outer skeletons (shellfish) or bones (fish).
Molecular methods contribute to problem solving
New methods are based on studying molecules that survive longer in the stomach than a visually characteristic shape or structure. A good example can be found in a study conducted on the diet of albatrosses in South America. We have long believed that these birds feed on fish and shellfish, but by studying the DNA in the albatross’ faeces, researchers found that a significant part of the diet consisted of jellyfish. This is new because jellyfish had not previously been identified in either faeces or stomach. After all, they are transparent, and consist mainly of water.
From DNA to diet
The researchers who studied the diet of the albatross used a method that could identify species without having to resort to a microscope or eyes. This method, called “metabarcoding”, involves reading the order of the letters in short stretches of DNA (sequencing) from the contents of the stomach, or, as in the albatross study – in the faeces.
Fact box
Sequencing means reading a DNA molecule. There are now many different sequencing machines with different behaviours, but they have in common that they describe the DNA base by base, or letter by letter if you will. Unlike our alphabet, DNA has only four letters; A, T, G and C.
By reading the DNA from the stomach, one can be left with millions of sequences with slightly different sequences of the letters, such as “AATGGCTACGGA” and “AATGACTACGGA”. Understanding which species the DNA molecules come from is nevertheless demanding, and here we need help from the world of computer science. Using advanced computer programs and supercomputers, we can compare the millions of sequences we find in the sample against sequences of known species stored in databases. Since the sequences are more or less unique to each species, we can thus determine where they come from, and form a picture of which species are included in the diet.
Examples from the Arctic
Some of the most important key species in the marine Arctic are found in a genus of copepods called Calanus. These are primarily considered grazing animals – that is, they feed on algae. Nevertheless, recent studies suggest that they may also eat larger animals. DNA from bristle worms (Polychaetes) and marine worms (Chaetognatha) has been found in the stomach of the Arctic copepod Calanus glacialis. In a similar study, it has been found that the more southern copepod species Calanus finmarchicus can eat jellyfish and nettles.
These animals, due to their size, have been considered unlikely prey for the much smaller Calanus. It should also be mentioned that Calanus probably preys on the young life stages of the larger animals, such as eggs or larvae. In that sense, the classic paradigm that nutrients move from small to large will continue to be valid, but that is not the point either. Using the DNA method, we have seen that the migration of nutrients and energy not only directs upwards in the food chain, but that it can go a little cross-sectionally. Thus, we see that the linear model (from small to large) does not always fit with the nutrients exchange that takes place.
From studies such as those on C. finmarchicus and C. glacialis, we now see that research no longer supports a simple model (on the left). Instead of a linear migration from small to large, nutrition and energy also move from large to small (on the right.). For example, in that the early life stages of larger animals (here represented by the bristle worm Tomopteris) are eaten by C. glacialis.
We still know very little
A common denominator with these studies is that they contribute to new knowledge, which in turn reminds us that we know quite little.
Therefore, in the large research project The Nansen Legacy, we are interested in utilizing the new technology when we go out into the Barents Sea. By studying small but numerous zooplankton such as Microsetella, we hope to be able to contribute to an ever better understanding of how nutrients and energy move in these bodies of water. Could it be that their appetite is also in contrast to the paradigm that big eats small, in that they eat eggs and larvae of larger animals?
References:
- C. McInnes, R. Alderman, M. A. Lea, B. Raymond, B. E. Deagle, R. A. Phillips, A. Stanworth, D. R. Thompson, P. Catry, H. Weimerskirch, C. G. Suazo, M. Gras, S. N, Jarman: “High occurrence of jellyfish predation by black-browed and Campbell albatross identified by DNA metabarcoding”, Molecular Ecology, 16. August 2017
- C. Cleary, J. E. Søreide, D. Freese, B. Niehoff, T. M. Gabrielsen: “Feeding by Calanus glacialis in a high arctic fjord: Potential seasonal importance of alternative prey”, ICES Journal of Marine Science, 25. Juli 2017
- D. Yeh, J. M. Questel, K. R. Maas, A. Bucklin: “Metabarcoding analysis of regional variation in gut contents of the copepod Calanus finmarchicus in the North Atlantic Ocean”, Deep Sea Research II, 21. Januar 2020