The jellyfish arrived in clouds. Thousands of translucent bells pulsing through shallow Pacific Northwest bays, so thick in places that researchers couldn't sample through them.

Inside these swarms, something was disappearing.

Copepods—the tiny crustaceans that fuel marine food webs from anchovies to salmon—were vanishing at a pace no one had properly measured. Not until now.

A Two-Hour Feast

Scientists placed moon jellyfish into tanks filled with natural seawater teeming with zooplankton. The setup mimicked conditions in Puget Sound's protected inlets, where jellyfish congregate each summer in numbers that have seemingly grown over recent decades.

Within two hours, the jellyfish consumed up to 75% of available copepods.

That's not a slow drain. It's a biological sprint that outpaces the ability of copepod populations to recover, since these animals reproduce over weeks or years rather than hours.

The experiments used living zooplankton communities pulled straight from the sea—not lab-reared prey or single species in isolation. This approach reveals what actually happens when jellyfish encounter the full complexity of their natural menu.

And they ate voraciously.

What Lives in a Jellyfish Swarm

Fieldwork across four Washington inlets between 2019 and 2021 painted a consistent picture. Inside jellyfish aggregations, copepod densities dropped by up to 73% compared to areas just outside the swarms.

The pattern held across seasons, across bays, across varying water conditions.

Two copepod groups bore the brunt: medium-sized calanoids of the genus Paracalanus, and a smaller cyclopoid called Ditrichocorycaeus anglicus. Both are energy-dense prey favored by juvenile fish and forage species that underpin the region's food web.

Statistical models confirmed what the experiments suggested. Jellyfish biomass was the primary predictor of copepod abundance in inlets where aggregations occurred. The more jellyfish, the fewer copepods—a relationship both logarithmic and stark.

At the highest jellyfish densities recorded, researchers calculated that half the copepod population could vanish in as little as five hours.

Five hours.

The Appetite Decoded

Clearance rates—the volume of water a jellyfish can sweep clean of prey per day—reached 322 liters for medium calanoid copepods and 284 liters for D. anglicus. Individual jellyfish, in other words, can filter hundreds of liters daily.

These rates exceed those reported in some earlier studies, possibly because past experiments used narrower size ranges of jellyfish or different prey communities. The discrepancy underscores a critical point: jellyfish predation isn't universal. It's context-dependent, shaped by local prey, local temperatures, local jellyfish sizes.

But when conditions align—as they do in Puget Sound's shallow, warming embayments—the impact scales fast.

Selectivity analyses revealed preferences. Medium calanoids, cyclopoid copepods, copepod larvae, larval shrimp, and larval crabs were positively selected. Smaller harpacticoid copepods, polychaete larvae, and bryozoans were avoided.

Why? Possibly swimming behavior. Calanoid copepods can "jump" to escape predators, but jellyfish tentacles are passive traps, and evasion may fail more often than it succeeds. Alternatively, some copepods migrate vertically through the water column to avoid visual hunters—a strategy that may incidentally reduce daytime encounters with jellyfish, which tend to concentrate near the surface except during high tide.

A Cascade Taking Shape

Something else changed inside the swarms.

Phytoplankton levels—measured as chlorophyll and microplankton biovolume—were significantly higher inside jellyfish aggregations than outside.

This hints at a trophic cascade. Fewer copepods means less grazing on phytoplankton. Phytoplankton blooms. The microscopic forest thickens.

Whether jellyfish directly cause this by depleting copepods, or whether they congregate in areas already experiencing algal blooms, remains ambiguous. But the correlation is strong and consistent across sites.

There's another possibility. Jellyfish excrete ammonia, which bacteria convert to nitrate and nitrite—nutrients that fuel phytoplankton growth. Inlets hosting jellyfish aggregations showed elevated nitrogen levels compared to jellyfish-free bays.

The jellyfish, in effect, may fertilize the waters they inhabit while simultaneously removing the grazers that would otherwise keep algae in check.

Competing for Survival

Moon jellyfish and forage fish—herring, sand lance, young salmon—eat similar prey. In Prince William Sound, Alaska, researchers found roughly 70% dietary overlap between moon jellyfish and several fish species.

Puget Sound likely mirrors this competitive dynamic, though direct comparisons weren't part of the current study.

What's clear is this: if jellyfish populations continue increasing, they could limit prey availability for fish in affected areas, particularly juvenile fish whose survival depends on abundant copepods during critical early life stages.

Pacific salmon survival in Puget Sound has declined in recent decades. The causes are many—habitat loss, pollution, warming waters, shifting ocean conditions. Jellyfish predation on shared prey may compound these pressures, especially in shallow nursery habitats where young fish feed.

Why Now, Why Here

Jellyfish aren't new to Puget Sound. But their numbers appear to have grown since the late 1970s, particularly in southern and central basins.

Conditions associated with climate change and human activity—higher temperatures, increased salinity, eutrophication, hypoxia—favor some jellyfish species. Moon jellyfish polyps, the sessile life stage that clings to hard surfaces, strobilate earlier and more frequently in warmer water. This allows multiple reproductive cycles per year instead of one.

Coastal development also plays a role. Docks, pilings, and artificial structures provide substrate for polyps, expanding available habitat for the immature stage that produces free-swimming medusae.

During the 2015–2016 Northeast Pacific marine heatwave, moon jellyfish aggregations in Puget Sound became larger, more abundant, and more persistent than in pre-heatwave years, according to aerial photograph analysis.

The trend may not be monotonic—jellyfish populations fluctuate naturally over long timescales—but the trajectory is worrying.

What Happens Next

The question isn't whether jellyfish eat copepods. They do, and they do it efficiently.

The question is whether rising jellyfish abundance will fundamentally reshape food webs in estuaries like Puget Sound, redirecting energy away from fish and toward gelatinous dead ends.

Or perhaps not dead ends. New research using stable isotopes and DNA metabarcoding suggests jellyfish have more predators than once thought. Their role in ecosystems remains poorly understood, even as their visibility—and their appetite—becomes harder to ignore.

For now, the moon jellyfish pulse through Puget Sound each summer, filtering hundreds of liters daily, depleting copepods faster than tides can replenish them, and leaving behind water greener with phytoplankton and emptier of the small crustaceans that young fish need to survive.

The swarms are winning.

Credit & Disclaimer: This article is a popular science summary written to make peer-reviewed research accessible to a broad audience. All scientific facts, findings, and conclusions presented here are drawn directly and accurately from the original research paper. Readers are strongly encouraged to consult the full research article for complete data, methodologies, and scientific detail. The article can be accessed through https://doi.org/10.3354/meps14824