Jellyfish : grace in the current

Thom Robbins | Story & pictures

Gliding through the green summer currents of the Salish Sea, jellyfish seem to exist somewhere between matter and myth—elegant, translucent, and suspended in a world shaped by light and movement. Though often dismissed as pests or feared for their sting, these gelatinous invertebrates are a vital and beautiful part of the Salish Sea’s marine ecology, especially during the warmer months.

At least 10 known species of jellyfish (Class Scyphozoa and Hydrozoa) are commonly found in the Salish Sea. However, the exact number varies depending on seasonal conditions and depth. The region also hosts a variety of hydromedusae, small jellyfish-like organisms from the class Hydrozoa and ctenophores (comb jellies), which are not true jellyfish but are often seen alongside them.

In the summer, diving in the Salish Sea isn’t about crystal-clear views but seeing what moves through the murk. As the surface waters warm, sunlight and nutrients combine to spark massive phytoplankton and algae blooms. These microscopic organisms flourish in the long daylight hours, turning the water cloudy green and dramatically reducing visibility.

At the same time, nutrient-rich upwelling from deeper waters feeds this explosion of life, creating a cascading effect through the food web. These blooms nourish jellyfish and other filter feeders, yet they can also trigger oxygen depletion in deeper layers—a process known as eutrophication. As algae and plankton die off and decompose, they consume oxygen, sometimes creating low-oxygen zones that drive fish and invertebrates like wolf eels into shallower waters, or out of the area entirely.

Despite the haze, the midwater teems with motion. Jellyfish pulse silently past in the filtered light, their delicate forms catching the sun like drifting ornaments. I’ve hovered motionless on some dives as dozens of moon jellies passed—some brushed softly against my mask. In those moments, the murk becomes a cathedral, lit not by clarity but movement. Among the most commonly seen is the Moon Jelly (Aurelia labiate). Recognizable by its delicate, bell-shaped body and the four horseshoe-shaped gonads visible through its dome, the moon jelly is both harmless and hypnotic. Although capable of stinging, its venom is so weak that divers typically feel nothing more than a light brush, if anything at all.

Another summer highlight is the Fried Egg jelly (Phacellophora camtschatica), which drifts through the current like a golden sun. Its yolk-colored center and frilled white tentacles make it one of the most photogenic creatures in the water column. Despite its size, it poses little threat to swimmers or divers. Less frequent but unforgettable is the Lion’s Mane jellyfish (Cyanea capillata)—the world's largest known jellyfish species. Its tentacles can grow over 30 meters long in Arctic populations, though those found in the Salish Sea are often smaller. Its sting is more noticeable, though rarely dangerous, and it tends to favor colder, deeper waters where few divers tread.

Jellyfish have been drifting through Earth’s oceans for over 500 million years, long before dinosaurs roamed the land or fish evolved bones. Fossil evidence shows that their basic body plan has remained remarkably unchanged, a testament to just how well-adapted they are to life in the water column. These ancient invertebrates are one of the oldest animal groups on Earth. Unlike many marine organisms, jellyfish never developed hard skeletons or complex organs. Instead, they relied on simplicity and flexibility: a hydrostatic body, a decentralized nerve net, and the ability to reproduce both sexually and asexually.

“Their survival lies in elegant simplicity. Jellyfish drift with currents, respond to light and nutrients, and reproduce quickly when conditions align—strategies that have carried them through five mass extinctions. Most jellyfish species live only a few months in their adult medusa form.”

In the Salish Sea, each summer bloom connects us to this deep evolutionary past. To hover among these creatures is to glimpse a lineage nearly as old as the ocean itself. Their movements may be fleeting, but their presence speaks to a legacy stretching back over 500 million years.

Built to Drift

Though they seem simple, jellyfish are highly specialized. With no bones, brains, or hearts, they are built to drift. The jellyfish found in the Salish Sea all share the same basic anatomy, but with fascinating variations.

At the center of a jellyfish’s structure is its bell, the umbrella-like dome that pulsates to move it through the water. This bell is made of a gelatinous material composed chiefly of water, up to 95% in most species. Radial canals branch from the stomach, distributing nutrients throughout the bell.

Larger predators also take an interest. Occasionally, ocean sunfish (Mola mola)—the world’s largest bony fish—wander into the Salish Sea, and jellyfish are a favorite food. Their thick skin protects them from stings as they chomp through even the largest medusae. Historically, leatherback sea turtles, specializing in eating jellyfish, may have traveled this far north, though sightings today are extremely rare.

Even other jellyfish can be predators. With its long, trailing tentacles, the fried egg jelly often feeds on smaller jellyfish that become ensnared in its stinging threads. It’s a quiet cannibalism, part of the natural food web.

Jellyfish can be an unexpected snack for juvenile salmon, especially pink and chum. These young fish sometimes feed on tiny jellies and jelly-like plankton as they migrate through the sea, supplementing their diets during their early life stages. When a jellyfish’s life ends and it sinks to the seafloor, crabs, sea stars, and other scavengers move in. They scavenge the soft tissue, recycling the jelly’s nutrients into the ecosystem.

For all their fragile, ghost-like appearance, jellyfish play a key role in the Salish Sea’s complex food web, both predator and prey. Their presence supports a range of species, many of which rely on these gelatinous drifters more than most people realize.

Unlike animals with hearts and blood vessels, jellyfish lack a true circulatory system. Instead, they rely on the simple structure of their bodies to move nutrients. Through a combination of internal canals and diffusion, the passive movement of molecules from areas of high concentration to low, nutrients are spread throughout the jellyfish's tissues. This efficient, low-energy system works because jellyfish are composed mainly of water and have thin, soft bodies, allowing nutrients and gases to move freely without blood or vessels.

Jellyfish lack a brain but rely on a decentralized nerve net to detect touch, orientation, and light. Around the bell’s rim are specialized sensory structures called rhopalia, which contain balance organs (statocysts) and light-sensitive spots (ocelli). These help jellyfish maintain their position in the water column and respond to environmental changes. In many species, like the Moon Jelly, you can see the four gonads (reproductive organs) through the bell. These are typically arranged symmetrically and give the jelly its iconic pattern. Jellyfish are usually gonochoric, meaning individuals are either male or female, though some species can change sex depending on environmental conditions.

Though they may appear delicate, jellyfish anatomy is an evolutionary success story, fine-tuned over half a billion years. Every structure, from the trailing tentacle to the light-detecting rhopalium, keeps these animals alive in a constantly shifting marine world.

Born of Water

Jellyfish have one of the most complex and fascinating life cycles in the animal kingdom, cycling between two distinct forms: the polyp and the medusa. Most of the jellyfish we see while diving are medusae—the free-swimming, bell-shaped adults. But before reaching this stage, each jellyfish begins life as a microscopic larva called a planula, which settles onto a hard surface and develops into a polyp. These polyps can remain dormant for years, cloning themselves and waiting for the right environmental cues—usually a change in temperature or light—to begin a process called strobilation.

During strobilation, the polyp stacks tiny disc-like ephyrae that break off and develop into juvenile jellies. This ability to clone and reproduce asexually means jellyfish can quickly take advantage of favorable conditions, leading to the massive blooms divers witness during Salish Sea summers. Some polyps may produce genetically identical clones, while others undergo sexual reproduction to introduce new genetic material.

In particular, Moon Jellies are known for their resilience and adaptability. They can tolerate low-oxygen conditions better than many other marine organisms, allowing them to thrive in increasingly stressed ecosystems. Crystal Jellies exhibit a unique form of reproduction in which males and females release gametes into the water column, a method known as broadcast spawning—one of the most ancient reproductive strategies in the ocean.

Hanging beneath the bell, a jellyfish’s tentacles and oral arms serve as its defense system and means of feeding. Along the tentacles are thousands of specialized stinging cells called cnidocytes, each containing a microscopic harpoon-like structure known as a nematocyst. When triggered by touch or the presence of prey, these structures fire, delivering venom that can stun or immobilize small organisms. Moon jellies don’t chase their meals—they drift with open arms through clouds of plankton, letting the current deliver each bite. Their tentacles sweep plankton, tiny crustaceans, and larval fish toward their fringe of short, frilly oral arms, transporting the food to the central mouth.

The fried egg jelly takes a slightly different approach. Its long tentacles snare zooplankton and smaller jellyfish, which are slowly drawn to its oral arms for digestion. Large and dramatic, it feeds with quiet efficiency, drifting through the current, letting the ocean deliver each meal. But while they drift and feed with effortless grace, jellyfish are also food for a surprising array of marine life. Sea anemones, rooted to rocks and pilings, often snatch passing jellies with their sticky tentacles. If a moon jelly drifts too close or becomes weakened, an anemone will pull it in, undeterred by the jelly’s stinging cells. Whenever I run across this it always looks like a child sucking in a plate of spaghetti.

Conservation

Although jellyfish are often seen as survivors—creatures that thrive where others struggle—they are also indicators of broader changes in marine ecosystems. In the Salish Sea, shifts in jellyfish populations can reflect changes in water temperature, nutrient levels, and even human impacts such as coastal development and pollution.

For instance, increased runoff from agriculture and urban development introduces nutrients that can intensify plankton blooms, indirectly supporting larger jellyfish populations. While this may seem like a success story for jellies, it often signals ecosystem imbalance. In extreme cases, jellyfish blooms can disrupt fisheries by clogging nets, consuming fish larvae, and competing with commercial species for food. Plastic pollution also poses a risk. Jellyfish sometimes ingest microplastics, mistaking them for prey, and may in turn be eaten by larger animals like salmon or sea turtles, passing toxins up the food chain. Interestingly, the translucent structure of jellyfish makes them particularly difficult to spot when entangled in discarded fishing gear, meaning they can suffer unnoticed in ghost nets.

Despite these concerns, jellyfish have become unlikely ambassadors for science, conservation, and discovery. One of the most important breakthroughs in modern biology came from the Crystal Jelly, found off the coast of Friday Harbor. Scientists discovered green fluorescent protein (GFP) in this species—a molecule that glows green under ultraviolet light. GFP acts like a molecular flashlight when inserted into other organisms, allowing researchers to track genes and observe biological processes in real time. This discovery revolutionized genetic and medical research, earning a Nobel Prize and highlighting the hidden value of marine life.

Conclusion

Encountering jellyfish while diving in the Salish Sea is never ordinary. Whether it’s a Moon's hypnotic pulse or a Crystal's rare glow, these animals embody a unique blend of grace, resilience, and ecological importance. They are living filters, slow-motion drifters, and seasonal storytellers, revealing much about the world beneath the waves. While some may still associate jellyfish with stings or beach closures, divers know better. To hover beside a jelly in its element is to witness nature in its purest form—unhurried, mysterious, and vital. In a time when ocean health is at risk, jellyfish remind us that beauty and science are not separate but intertwined. Their presence is more than a summer spectacle—it’s a quiet signal that even the oldest life forms still move among us, pulsing with stories the sea has never stopped telling. To witness one is to glimpse time suspended—fragile, pulsing, and older than memory