Tiny crustaceans found feeding on live jellyfish during harsh Arctic night

In the darkness and cold of the polar night which lasts for months, food resources are limited. Some groups of marine organisms in polar regions overcome this challenge by entering a state of metabolic quiescence in winter, surviving on reserves accumulated during the short growing season.

But others, such as several species of marine zooplankton, have evolved a different strategy: they switch from a specialized to an omnivorous diet during the polar night, taking advantage of a wide range of potentially less remunerative foods available throughout. of the year.

Now, scientists have shown that a key food source for these seasonal Arctic omnivores has been overlooked until now: dead and living jellyfish. The results are published in Frontiers of Marine Science.

“Here we show for the first time that jellyfish, considered generally poor in nutrients, nevertheless constitute an important food source for amphipods during the Arctic polar night,” said Annkathrin Dischereit, doctoral student at the Alfred Wegener Institute in Germany, and the first author of the article.

“For example, we found evidence that some amphipods feast on “jelly-falls,” naturally sunken jellyfish carcasses. Other species may also prey on live jellyfish.”

In January and February 2022, Dischereit and other members of the Helmholtz ARJEL young researcher group from the Alfred Wegener Institute took part in an expedition to the French-German research station AWIPEV in Svalbard.

Venturing in a small boat from Kongsfjorden, Svalbard, researchers discovered that its waters were teeming with jellyfish: not only “real jellyfish” like the lion’s mane jellyfish, but also hydrozoans like the pink helmet jellyfish in thimble-shaped, colonial siphonophores and unrelated comb jellies or ctenophores.

The researchers sampled the local amphipods – crustaceans measuring between five and 20 millimeters long – with nets and baited traps. They chose to focus on amphipods because they are locally abundant and important components of fjord systems. The captures consisted mainly of four species: Orchomenella minuta and Anonyx sarsi, scavenging amphipods of the superfamily Lysianassoidea, and two distantly related species of Gammarus.

DNA metabarcoding of ingested prey

The researchers dissected the intestines of each amphipod, then used DNA metabarcoding to identify the prey remains inside.

Jellyfish DNA from several species predominated in the intestines of both Gammarus species, along with traces of algae and crustaceans. Jellyfish DNA was also found, but less abundant, in A. sarsi and O. minuta, proving that all four species studied regularly consume jellyfish tissue. The authors conclude that A. sarsi and O. minuta appear to feed opportunistically on jellyfish, while both Gammarus may, in addition, feed on live jellyfish.

Fish, live or dead, were also an important food for A. sarsi and O. minuta, as well as polychaete worms, crustaceans and molluscs.

Discherit et al. also found that between 27% and 60% of sampled amphipods had empty intestines. This confirms that food shortage and starvation pose a challenge for marine invertebrates during the polar night, even for species that may adopt a more omnivorous diet.

Paradigm shift on jellyfish

“There has recently been a paradigm shift in the marine biology literature that recognizes that far from being a ‘trophic impasse’, jellyfish are in fact eaten by a wide range of organisms. Our observations support this major shift in how scientists perceive the role of jellyfish in the food chain,” said Dr. Charlotte Havermans, leader of the 2022 expedition and final author of the study.

“As jellyfish tissues are rapidly digested, they may have been overlooked as prey in previous studies that, unlike our study, relied on visual identification of foods to determine the diet of jellyfish invertebrates. Arctic.”

“Atlantification” and the new Arctic

Today, the Arctic is warming at a record rate compared to the rest of the world, and jellyfish species from the Atlantic Ocean have been observed to spread northward. This “Atlantification” could make jellyfish even more important as a resource within Arctic food webs.

“To get a better idea of ​​the role of jellyfish in the Arctic marine food web and how this might change as their populations increase in the ‘new Arctic,’ we plan to study the diet of jellyfish in more detail. “other invertebrates and fish that potentially feed on jellyfish. Questions remain, for example, whether jellyfish are an integral part of the amphipod diet or simply a survival food during the polar night,” concluded Dishereit.