For many years, researchers believed that jellyfish served as an ineffective food option for predatory fish. However, a research team from the Alfred Wegener Institute, partnering with the Thünen Institute, has found that fish in the waters around Greenland do consume jellyfish. In fact, for two of the fish species studied, jellyfish constituted a significant portion of their diet, as highlighted in a study published in the Royal Society Open Science journal. These findings prompt a reassessment of the role jellyfish play in marine food chains, particularly as they may benefit from climate changes and extend their range further north.
The waters around Greenland host numerous types of gelatinous zooplankton. However, the extent to which jellyfish are part of the diet for local fish was previously unknown. “We examined the stomach contents of seven fish species, including commercially important ones like Atlantic cod, haddock, and redfish,” explains Charlotte Havermans. “By using DNA metabarcoding, we were able to accurately identify the diet of these fish.”
The findings were unexpected: “We discovered jellyfish DNA in the stomachs of all the fish species investigated, though in varying amounts,” notes Annkathrin Dischereit, the primary author of the study and a doctoral candidate in ARJEL. For two species, the greater silver smelt and the northern wolffish, jellyfish made up the largest part of their diet. This is despite the common belief that gelatinous zooplankton represents a dietary dead end and serves as merely emergency food for certain fish. “This belief stems from the idea that predator fish quickly digest their tissues, which results in jellyfish being infrequently recorded as prey in research,” explains Annkathrin Dischereit.
Advancements in methods reveal jellyfish’s more significant role than previously recognized
DNA metabarcoding offered a breakthrough: This advanced method allowed researchers to detect short gene fragments within the stomachs, match them against genetic reference databases, and identify the specific prey species corresponding to those fragments. “We found that every species we studied consumed jellyfish or other gelatinous zooplankton,” explains Annkathrin Dischereit. “Our efforts revealed up to 59 types of gelatinous invertebrates in the stomach contents of the fish, indicating that they play an important yet previously underappreciated role in the subarctic food web.” Some of the analyzed stomach contents had never been previously examined in this region.
This study emphasizes the need to reevaluate our perception of jellyfish and similar creatures within marine food webs. Rather than being merely a stopgap food source, gelatinous zooplankton serves as a common prey for higher-level predator fish. “Our findings raise important questions about why fish frequently consume jellyfish,” notes Charlotte Havermans. Despite jellyfish’s low energy density, their contribution to the energy needs of predatory fish might be more substantial than earlier believed: they could be digested efficiently, easier to hunt, becoming more prevalent, and offer energy-rich nutrients due to their feeding habits. “There’s still much to explore in this area,” states Annkathrin Dischereit. “Our research only captures a moment in time, focusing on prey that has recently been digested. We need to gather continuous data year-round to understand how jellyfish populations fluctuate and their relation to fish diets over time. This understanding is essential to clarify the connections between fish and gelatinous zooplankton.”
Such research is vital, as emphasized by Charlotte Havermans: “Only through studying trophic relationships can we reveal the importance of jellyfish for fish and various other marine organisms.” So far, comprehensive trophic studies on several species, like redfish, have yet to be conducted. “The insights from our research challenge our understanding of subpolar ecosystems and the effects of the recent surge in gelatinous zooplankton on these systems.”
