Review
A Paradigm Shift in the Trophic Importance of Jellyfish?

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Highlights

Jellyfish are ubiquitous in the world’s oceans, can occur in very high densities and are increasing in abundance in some areas. Yet they have long been considered trophic dead ends that are ignored by most predators because of their low nutritional content.

New approaches are being used to examine the diet of marine predators including stable isotope analysis of predator tissues, animal-borne cameras, and DNA analysis of fecal and gut samples (metabarcoding). These new approaches have revealed that around the world’s oceans jellyfish are frequently consumed by a diverse range of marine predators including fish, birds, turtles, and various invertebrates including octopus, sea cucumbers, crabs, and amphipods.

Taken together, there is now overwhelming evidence that jellyfish are integral components of marine food webs and warrant attention for reasons far exceeding their impacts on human enterprise. Armed with an array of cutting-edge techniques we now have the capacity to drive such investigations forward like never before.

The past 30 years have seen several paradigm shifts in our understanding of how ocean ecosystems function. Now recent technological advances add to an overwhelming body of evidence for another paradigm shift in terms of the role of gelatinous plankton (jellyfish) in marine food webs. Traditionally viewed as trophic dead ends, stable isotope analysis of predator tissues, animal-borne cameras, and DNA analysis of fecal and gut samples (metabarcoding) are all indicating that many taxa routinely consume jellyfish. Despite their low energy density, the contribution of jellyfish to the energy budgets of predators may be much greater than assumed because of rapid digestion, low capture costs, availability, and selective feeding on the more energy-rich components. Feeding on jellyfish may make marine predators susceptible to ingestion of plastics.

Section snippets

Jellyfish as Trophic Dead Ends: A ‘Belly-Full-of-Jelly’?

Gelatinous zooplankton, loosely termed ‘jellyfish’, are a taxonomically diverse group including many cnidarians, such as scyphozoan jellyfish and siphonophores; ctenophores; and also chordates, such as salps, pyrosomes, and appendicularians (Box 1 and Figure 1). Jellyfish are ubiquitous in the world’s oceans and can occur in very high densities in large blooms 1, 2. For example, distinct coastal jellyfish hotspots can occur in coastal waters, such as large aggregations of the barrel jellyfish

Old Question, New Methodologies: Recent Evidence for Jellyfish Consumption

While evidence of predation on jellyfish is certainly not a new phenomenon 10, 11, 12, 13, new technologies are revealing just how widespread consumption of jellyfish is among marine predators, including such charismatic taxa as tuna, albatrosses, and penguins. Methodologies that are transforming our view of jellyfish as prey include stable isotope analysis (SIA) of the tissues of predators 15, 16, animal-borne cameras 17, 18, remotely operated vehicles (ROVs) 19, 20, and molecular analysis of

Blending Old and New Evidence: Can Marine Animals Survive on a Diet of Jellyfish?

While the modern techniques are increasingly revealing the presence of jellyfish in the diets of marine predators, it may be informative to turn to some of the classic older literature to assess the potential relative importance of this prey group to energy budgets. At first glance it would appear that the low energy density of jellyfish would likely mean that they are simply ingested incidentally and other prey, such as fish and crustaceans, will contribute the bulk of the energy requirements

Constraints to Being a Specialist versus Generalist Predator of Jellyfish?

The evidence that a range of marine predators will feed on jellyfish challenges the view that the need to have a ‘belly-full-of-jelly’ limits the number of jelly-feeders. Fast pursuit predators such as penguins and tuna will likely avoid having such pitfalls even when feeding on jellyfish due to rapid digestion. Hence we need to look for alternative explanation for why the leatherback turtle, which is thought to feed almost exclusively on jellyfish, is so large. The answer may be linked to the

Concluding Remarks and Future Directions

There is now overwhelming evidence suggesting that jellyfish cannot simply be ignored as trophic dead ends but rather are opportunistically consumed by a wide range of marine predators. Such trophic pathways reflect the abundance, aggregation, and seasonality of jellyfish and challenge the intuitive view that the reward from feeding on jellyfish would be outstripped by the effort. Indeed, when considering selective foraging on gonadal tissue, for example, the energetic reward and accessibility

Authors’ Contributions

G.C.H. conceived the study and led the writing with input from T.K.D. and J.D.R.H.

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