Paper in a Nutshell: Nielsen, Christiansen, et.al. 2019. Greenland Shark (Somniosus microcephalus) Stomach Contents and Stable Isotope Values Reveal an Ontogenetic Dietary Shift. Frontiers in Marine Science.

What can sharks tell us about ocean food webs?

The ocean is full of creatures, from benthic invertebrates to pelagic fish to the largest mammals in the world. However, there are many creatures that we rarely see or that we don’t even know exist yet. The Greenland shark (Somniosus microcephalus) is a long-living shark species that lives in the deep sea of the Arctic and the coldest parts of the Atlantic. Due to the potential length of their life the Greenland shark is a key predator to study in order to see how an ecosystem changes over time.

A Greenland shark swimming away from a boat back down to its normal depth. Image from the Washington Post.

One way to study the food web dynamics of an ecosystem is to analyze stable isotopes inside an animal in order to find out about their diets. Stable isotopes are different forms of elements with slightly different molecular weights than the regular element weight. These isotopes can be found naturally in the environment, and they do not decay radioactively overtime.

You are what you eat

By analyzing the levels of certain stable isotopes from a tissue sample, you can tell which other animals or plants that a certain animal is eating over a long period of time instead of just getting a snapshot when you dissect an animal and look inside the stomach. This method lets us know what trophic level (e.g where on the ‘food chain’) animals are foraging at–e.g. eating plants, or things that eat plants (rabbit), or things that eat things that eat plants (wolf); and what environment they are foraging in—e.g. coastal or open ocean. 

This is an example of a stable isotope graph. As you move up the graph on the y-axis the higher δ15N level corresponds to an animal eating at a higher trophic-level. As you move from left to right on the x-axis the lower δ13C level corresponds to an animal living in the open ocean and the higher δ13C level corresponds to an animal living in coastal habitats or on the seafloor. The red numbers here are showing enrichment factors between predator and prey. Image from Marine Strategy Framework Directive.

What’s on the Greenland Shark menu?

There has been some previous research done on the diets of small Greenland sharks (200-400cm), but it is thought that larger sharks may have a different, higher trophic level diets.

In a study by Nielsen et. al, stomach contents and stable isotopes were both measured in Greenland sharks. Through analyzing the stomach contents of each shark, the researchers measured the number of prey items, the reconstructed biomass of the prey items, and the frequency of occurrence of prey items. Stable isotope analysis was used to determine if the prey type was reflected in each shark’s stable nitrogen levels.

The results of the study were somewhat as expected. Smaller (<200cm) Greenland sharks feed at a lower trophic level in terms of prey items found in their stomachs and isotopic δ15N levels, relative to larger (>200cm) Greenland sharks.

Plot of  δ15N Isotope percentage vs. shark size categories (N being the sample size for that particular category). The variance in δ15N shows a difference in trophic level of prey. The different letters show significant differences between size categories. Figure from Nielsen, Christiansen, et.al. 2019, Frontiers in Marine Science.

The study also found an ontogenetic shift—a shift across age. They observed that small (<200cm) sharks ate mostly squid; while larger (>200cm) sharks ate mostly fish and mammals (Fig 1 below). The specific squid diet in younger Greenland sharks may suggest a specialized feeding behavior in smaller animals, and a more generalized feeding behavior in larger animals.

Fig 1 Reconstructed biomass of each prey category vs. shark size categories (N being the sample size for that particular category). Figure from Nielsen, Christiansen, et.al. 2019, Frontiers in Marine Science.

More Questions?

With changing occurring in Arctic oceans due to climate change more information on the Arctic ecosystem as a whole is needed. Information on Greenland sharks in particular can help us understand trophic level relationships in this ever-so-changing habitat in the Arctic.

Although this study did help understand the trophic level dynamics in larger (>200cm) Greenland sharks, there are still many questions to explore.

Image from Giphy.com

Research we’ve done here at the Alaska SeaLife Center has suggested that a close relative of the Greenland shark, Pacific Sleeper Sharks, are likely able to catch freely-swimming juvenile Steller sea lions. The study by Nielsen et al. similarly provided some evidence that Greenland sharks prey upon fast moving prey; but it is unknown how these slow moving animals are capable of catching their prey or the mechanics of their feeding. One interesting observation was that whole seals were found in the Greenland sharks’ stomachs. Maybe the sharks preyed upon them while they were sleeping in the water column and swallowed them whole?

How would you design a study to figure out this question?

Written by: Mary Keenan, ASLC Science Communication Intern 2020

Feature image credit: Encyclopedia Britannica

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