Exploring the Mysteries of Gelatinous Zooplankton: Nature’s Translucent Marvels
By Chronicle Ono
The Diverse World of Gelatinous Zooplankton
Gelatinous zooplankton, commonly known as jellies, represent a fascinating and highly varied group of aquatic creatures characterized by their soft, transparent bodies predominantly composed of water. While many are familiar with the iconic jellyfish-recognized by their bell-shaped bodies and stinging tentacles-this group also includes a range of other enigmatic species. These range from worm-like organisms and primitive chordates to winged snails, illustrating the incredible diversity within this seemingly simple category.
!A fish bites into a gelatinous creature in the ocean
An example of predator-prey interaction: a California sheephead (Semicossyphus pulcher) preying on a twin-tailed salp (Thetys vagina). Jellies often become easy targets when they venture closer to shore, especially in kelp forests where they are less adapted to such environments.
“If being 95% water is what unites these creatures, that’s where their similarities end,” explains Grace Cawley, a doctoral researcher at the Scripps Institution of Oceanography.
Some gelatinous zooplankton drift passively, grazing on microscopic phytoplankton, while others actively hunt their prey. Their sizes vary dramatically-from tiny, thimble-sized forms to giants that surpass the length of a blue whale. Some species inhabit the surface waters, skimming the interface between air and sea, whereas others dwell hundreds of meters beneath the ocean’s surface.
Cawley humorously recalls how the common reaction to jellies is often, “Oh, it’s gooey,” reflecting their unassuming appearance. Yet, despite their simplicity, these creatures play crucial roles in marine ecosystems. Historically, humans have overlooked many of these ancient species, partly due to the difficulty in studying them. However, technological advancements are now enabling scientists to unravel their mysteries and understand their influence on ocean health.
The Hidden Lives of Jelly-like Creatures
The scientific exploration of gelatinous zooplankton began in the late 19th century, with researchers collecting specimens from docks and ships. Early inquiries often revolved around basic questions-where are these creatures found, and how many are there? However, their fragile bodies, lacking bones or shells, made fossilization nearly impossible, and laboratory studies were limited to brief observations in jars that rarely captured their true nature.
The mid-20th century saw the advent of larger, faster research vessels, allowing scientists to explore remote ocean regions. This era marked a shift from simply counting jelly populations to understanding their ecological roles. As Steven Haddock from MBARI notes, researchers initially focused on “how many” and “how much,” before delving into “who” and “how” these animals fit into the broader marine web.
!A translucent, long-tentacled gelatinous organism
The lemon jelly (Aegina citrea), a seemingly harmless creature, is actually a predator targeting other gelatinous zooplankton like salps and ctenophores.
Some species, such as siphonophores, inhabit depths exceeding 700 meters, only occasionally surfacing due to ocean currents. Others, like transparent pelagic snails, have evolved to blend seamlessly into their environment, blurring the lines between different zooplankton groups. Many of these animals exhibit complex life cycles, alternating between sexual and asexual reproduction, which adds to the challenge of understanding their population dynamics.
The Overlooked Significance of Jelly Ecosystems
When Elizabeth Hetherington of Scripps began studying gelatinous zooplankton, she was struck by how little was known about their biology and ecological roles. “There were so many straightforward questions-about their distribution and abundance-that remained unanswered,” she recalls.
Recent technological breakthroughs have revealed that jellies are more integral to ocean ecosystems than previously believed. A 2022 study suggested that pelagic tunicates-free-floating, gelatinous animals-could be responsible for transporting over 10% of the carbon that eventually sinks to the ocean floor. This process, known as the biological carbon pump, is vital for regulating Earth’s climate by sequestering atmospheric CO₂. The realization that jellies contribute significantly to this cycle has surprised many scientists.
Furthermore, DNA analysis techniques have provided new insights into jelly populations. In 2023, researchers discovered that ctenophores-delicate, comb-bearing gelatinous animals-may be among the oldest surviving animal lineages on Earth, dating back over 600 million years.
Rethinking the Ocean’s Food Web
The understanding of gelatinous zooplankton’s role in marine food webs has undergone a paradigm shift. Previously considered passive drifters or simple prey, jellies are now recognized as active participants-both as predators and prey. DNA metabarcoding has shown that many predators, including fish, seabirds, and sea turtles, consume jelly-like creatures, often without us realizing it. Their DNA remnants are frequently found in stomach contents, indicating their importance in energy transfer within the ocean.
This new perspective underscores the importance of studying these organisms in their natural habitats. Many species, such as the pyrosome-a large, glowing tunicate-can form massive blooms, sometimes comprising up to 80% of the biomass in certain regions. These blooms can dramatically alter local ecosystems, affecting everything from nutrient cycling to the availability of food for other marine life.
Challenges and Opportunities in Jelly Research
Despite these advances, many fundamental questions remain. Scientists are still working to determine the total number of jelly species, their population sizes, and how these are changing over time. Climate change, overfishing, and pollution are rapidly transforming ocean environments, making it even more critical to establish baseline data.
As Cawley emphasizes, “The ocean is a dynamic, complex system, not a static one. We need to shift from thinking about fixed baselines to understanding the range of natural variability.” This approach involves recognizing the fluidity of ocean ecosystems and the role of jelly populations within this variability.
Understanding where these creatures live and how they behave is the first step toward predicting their future impacts. Haddock notes that it’s an exciting era for jelly research, with new species being discovered even near major urban centers like New York City and Tokyo-highlighting that these enigmatic animals are more widespread and influential than ever imagined.
The Future of Jelly Studies
As scientists continue to explore the depths of the ocean and refine their methods, the picture of gelatinous zooplankton’s ecological significance becomes clearer. Their contributions to carbon cycling, their role as both predators and prey, and their responses to environmental changes are vital pieces of the puzzle in understanding global ocean health.
In the words of Haddock, “There’s a lot we still don’t know, but that’s what makes this field so thrilling. Every discovery brings us closer to comprehending the complex web of life beneath the waves.” With ongoing technological innovations, the once-mysterious world of jellies is gradually revealing its secrets, promising new insights into the future of our planet’s oceans.
