Jellyfish are more than just a snack food for predatory fish

For the longest time, scientists have believed that jellyfish are a non-viable food source for predatory fish. This long-held assumption has recently been dismantled by a team of researchers from the renowned Alfred Wegener Institute and the Thünen Institute.

The study reveals that fish in the frosty Greenland waters do indeed feed on jellyfish. For two of the analyzed species, jellyfish even made up the majority of their diet.

The findings carry significant implications, suggesting that the role of jellyfish as prey within marine food webs deserves reconsideration.

This is particularly important given the potential for climate change to benefit these gelatinous creatures, enabling them to expand their territories.

Jellyfish thriving in a changing climate

Dr. Charlotte Havermans is head of the ARJEL junior research group at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI).

“It is important that we rethink and understand how jellyfish and gelatinous zooplankton generally fit into marine food webs,” said Dr. Havermans.

Jellyfish are found in all oceans, from polar to tropical regions. Unlike most species, jellyfish thrive as the world’s oceans heat up and acidify, making them one of the few winners of climate change.

This unique resilience suggests that jellyfish or other gelatinous zooplankton could spread even further as global warming progresses, potentially reshaping marine ecosystems.

Breakthrough findings in Greenlandic waters

The Greenlandic waters, home to large quantities of various types of gelatinous zooplankton, have long been thought to contribute little to the diet of local fish.

Dr. Havermans and her team set out to challenge this notion by meticulously analyzing the stomach contents of seven fish species, including commercially significant species like Atlantic cod, haddock, and redfish.

“With the help of DNA metabarcoding, we were able to determine very precisely what the animals fed on,” noted Dr. Havermans.

The results were surprising: “We found DNA of jellyfish in the stomachs of all examined fish species, albeit in varying quantities,” said Annkathrin Dischereit, first author of the study and a doctoral student in ARJEL.

For two species, the greater silver smelt and the northern wolffish, jellyfish even constituted the largest proportion of their diet.

Jellyfish are not just emergency food

The findings are surprising when you consider the fact that gelatinous zooplankton are widely considered a trophic dead end and emergency food for some fish species at best.

“This assumption is based on the fact that their tissue is quickly digested by predator fish, and jellyfish are therefore rarely recorded as prey in studies,” explained Dischereit.

Modern DNA metabarcoding has provided new insights into this overlooked aspect of the marine food chain.

“We detected up to 59 species of gelatinous invertebrates in the stomachs of the fish,” noted Dischereit, highlighting that jellyfish play a significant but previously underestimated role in the subarctic food web.

The DNA metabarcoding method allows researchers to identify consumed prey by comparing short gene fragments found in fish stomachs to genetic reference databases.

“Our results raise the question of why fish seem to eat jellyfish surprisingly frequently,” said Dr. Havermans.

Jellyfish as a preferred food source

Despite their low energy density, jellyfish might offer more to the energy budget of predator fish than previously thought.

Factors such as their ease of hunting, increased prevalence, and diet-induced energy-rich components might make them preferable prey. However, Dr. Havermans emphasized that there is still a need for further research.

“Our study provides a snapshot in time that only takes into account recently digested prey. We need to collect continuous samples throughout the year and link these to how the gelatinous zooplankton communities change over this period of time,” said Dr. Havermans.

“Only then can we understand the trophic links between fish and gelatinous zooplankton. Only trophic insights can provide information on why jellyfish are important for fish and other organisms.”

The findings of this study challenge long-held views on marine food webs and highlight the need to reassess our understanding of subpolar ecosystems, particularly in light of the recent increase in gelatinous zooplankton populations.

The study is published in the journal Royal Society Open Science.

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