Polar cod is an important part of the Arctic food web. They are being eaten by many species, including birds such as this kittiwake (photo Susanne Kühn).

Compound microscopy and DNA analysis reveal new insights into the diet of arctic cod

Arctic cod is an important part of the Arctic food web. Sarah Maes (University of Leuven) and Fukji Schafsma (Wageningen Marine Research) have investigated the diet of polar cod from the Barents Sea, with the help of colleagues from KU Leuven and the Alfred Wegener Institute. They did this by combining conventional microscopy and DNA analysis. The study yielded new insights regarding the diet of arctic cod, which has now been published in the scientific journal Marine Ecology Progress Series.

Diet investigations are usually done by opening the stomach and examining its interior with a microscope when the animal being examined is young. This way, researchers can see which prey has been consumed and how much. However, some types of prey are more easily digested than others, which leads to a potentially biased view of the importance of some types of prey. Some types of prey can even be overlooked entirely. Sarah Mays of KU Leuven has started an investigation into the diet of arctic cod using DNA analysis to get around this problem. By combining this method with conventional microscopic analysis, the results obtained from the two different methods can be compared.

Diet verification using DNA analysis

The stomach contents examined under a microscope were analyzed using what is called a DNA metabolism process. DNA from the stomach is “read” and compared to a database containing the DNA sequences of many animals from around the world. In this way, predators that once resided in the stomachs of the polar cod become known.

Arctic cod is an important part of the Arctic food web. They are eaten by many species, including birds such as this kittiwake (photo by Susan Cohen).

The study showed that apart from the usual crustacean prey, many arctic cod feed on other fish species, and eggs or larvae are likely to be easy to digest. This indicates that fish are more common prey than previously assumed. Another amazing prey was the barnacle. Although they are commonly known for clinging to rocks or ships, they have larvae that float in the water.

Added value of collection methods

DNA analysis revealed a wide range of prey species including easily digestible ones. It also helped identify prey that could only be partially recognized by microscopy due to digestion, at the species level. In addition, DNA was present for prey species that would normally be well visible with microscopy, but are now not seen. Thus this prey has already been completely digested. This shows that DNA analysis provides information on diet over a longer period of time, while microscopic analysis provides a snapshot. Thus, the combination of both methods may give insight into recent dietary changes. Some information cannot be obtained by DNA analysis, such as the numbers of a particular prey in the stomach or the developmental stage of prey species. However, microscopic analysis of gastric content provides this knowledge. Thus, the combination of both methods is useful for gaining better knowledge about the importance of a species in the diet and the functioning of the food web.

research importance

The Arctic is changing rapidly due to global warming. Several studies have already shown that the distribution range of many species in the oceans is expanding northward. Nutritional studies provide information on the state of the Arctic food web and allow for changes in prey species to be monitored. In this way, the ability of arctic cod to adapt to climatic warming can be studied. This information can be further used for policy formulation related to nature conservation or fisheries. Furthermore, the study shows what information can be obtained from the different methods and how they can be used together.

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