Jellyfish are finally giving up their secrets

National Geographic

09-06-2025

Gelatinous zooplankton, colloquially known as jellies, are an evolutionary hodge-podge of squishy, translucent creatures composed mostly of water. While this group does include 'true jellyfish' with their iconic rounded bell and stinging tentacles, it is also host to a smattering of creatures such as worms, primitive chordates, and snails with wings. A California sheephead (Semicossyphus pulcher) takes a bite out of a twin-tailed salp (Thetys vagina.) Jellies are poorly suited to life near shore, so when they find themselves in a kelp forest, they can make easy prey.
'If being 95 percent water is what unites the group, that is where it ends,' says Grace Cawley, a PhD candidate at Scripps Institution of Oceanography.
Some are passive grazers, others track down their prey. Some are the size of a thimble, others can grow longer than a blue whale. Some cruise along the air-sea interface, others live thousands of meters beneath the surface.
Cawley joked that the common reaction with jellies was ''oh, it's gooey?' Throw it with the gelatinous zooplankton.'
In failing to recognize their diversity, humankind has overlooked some of the most ancient creatures on our planet. But thanks to advances in technology, scientists are now racing to decipher how jellies will shape the future of Earth's oceans. The hard part about squishy bodies
The study of gelatinous zooplankton began in the late 1800s by scooping specimens out of the water from docks and ships. 'A lot of [early inquiry] was really just, 'what is this thing'?' says Steven Haddock, a leader in zooplankton biology at Monterey Bay Aquarium Research Institute (MBARI).
Typical methods for investigating evolutionary history simply didn't work.
Jellies lack the bones and shells that make for good fossils—scientists struggled to keep them alive in the lab long enough to observe their life cycles—and attempts to preserve them resulted in jars of cloudy film that bore no resemblance to the original creature.
The proliferation of larger, faster research vessels around the mid-1900s meant that it became possible to sample new and remote regions of the ocean. Scientists rushed to ask ''how many?' and 'how much?' before having answered 'who?' and 'how?'' wrote Haddock in an early paper . This 1 inch lemon jelly (Aegina citrea) may not seem intimidating, but it is a predator. Its prey are other gelatinous zooplankton like salps and ctenophores. Hula-skirt siphonophores (Physophora hydrostatica) normally live deeper than 700 meters, but strong currents will occasionally carry them to the surface. Many different groups have made the transition to life in the water column. This pelagic snail has evolved to be transparent, but it still retains its shell. These animals blur the line of what is considered a gelatinous zooplankton. In their case, it largely comes down to the context in which they are being studied. Many species of salp (colonial tunicates) have a complex life cycle that alternates between sexual and asexual reproduction. Once the individuals in the colony mature, they will break off to begin reproducing sexually.
When Scripps Institution ecologist Elizabeth Hetherington began studying gelatinous zooplankton, she was shocked by how little was known about their lives. 'There were so many questions that seemed pretty simple, like basic questions about distribution and abundance … that I couldn't find answers to.'
Since the early 2000s, advances in technology have revealed that they play a more vital role in the ocean's food web than scientists thought.
One paper from 2022 suggested that pelagic tunicates—gelatinous sea creatures that float in the open ocean—could be responsible for transporting more than 10 percent of carbon that is eventually stored in the ocean floor. The discovery that this single group of jellies could play such an influential role in the carbon cycle surprised scientists.
The significance of all the ocean's jellies combined is unclear; however, the role they play in helping store carbon is probably underestimated. New technology that allowed scientists to study tiny bits of DNA also yielded new insights into jellies themselves.
One study published in 2023 found that ctenophores, the most fragile of the gelatinous zooplankton, may be the oldest animal species living on Earth.
Not only did these new methods revolutionize the study of individual species, but they also transformed our understanding of the open ocean. Jellies were more prevalent than previously thought and enthusiastic participants in the food web , hunting and being hunted.
Using DNA metabarcoding, a technique used to identify multiple species within a mixed sample, 'we [could] detect gelatinous zooplankton in the guts of predators' explains Hetherington. Though the remnants of jellies were rarely visible, their DNA has been found in stomach contents of a wide variety of birds, fish, and sea turtles, disproving the idea that they were just dead-ends in the food chain. As larvae, many fish species mimic the traits of gelatinous zooplankton to decrease their chances of being eaten. This larval cusk eel is nearly transparent which helps it hide in the open ocean.
Scientists are still trying to answer major questions about how many species exist, in what numbers, and how those populations might be changing.
'In an oceanographic context, we're still a long way from having the big picture biogeochemistry stuff figured out' remarks Haddock. 'Questions like 'Are jellyfish increasing?', 'How much jellyfish biomass is there relative to fish biomass?', 'What is the true diversity of jellies?' … we're still struggling to answer those.'
To answer these questions about jelly species, scientists must also learn more about how they fit in their ocean habitats.
'The ocean is not a stagnant place where nothing happens, the ocean is this dynamic, complicated system,' says Cawley. Jellies are no exception. Instead of maintaining a consistent, predictable population, many gelatinous zooplankton follow extreme boom and bust cycles that scientists are still trying to understand.
One species of pelagic tunicate called a pyrosome can bloom with such intensity that it will make up 80 percent of the biomass in a given area. When blooms like this occur, they affect every aspect of an ecosystem from the food web to the chemistry of the water.
With warming temperatures, overfishing, and pollution rapidly changing our oceans, answering these questions is becoming even more difficult.
'All of these ecosystems are impacted by warming and by pollution so it's important to get the baseline of where we are now,' but in a system as fluid as the ocean, a baseline is more complicated than a set of measurements, clarifies Hetherington. 'We should shift our thinking from baseline to baselines, that it's not this one thing, it's this dynamic range.' A baseline needs to capture the underlying patterns of our oceans.
It's an intimidating challenge that begins with demystifying where these jellies are living and what they are doing.
Still, in Haddock's eyes, it's an exciting time to study gelatinous zooplankton. 'There are new species within a stone's throw of New York City or Tokyo … if you just look in the right ways'