Jellyfish Confidential

From the elaborate species on display in Jellies Invasion to the silky-smooth moon jellies in Living Seashore, we have a thing for the sting! Listen in as we consider 500 million years of jellyfish history.

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For more than 500 million years, jellyfish have drifted through all seven oceans—there are fossils to prove it! In fact, scientists believe that many common species arose at the same time all those millennia ago and persist to this day. Their subphylum, Medusozoa, has about 3,800 known species, and probably at least that many still unknown species. There is even one species that can clone themselves from the medusa stage back into the polyp stage under stress, making them essentially immortal. Sound impossible? Let's listen in on this history of jellies and leave it to the experts.

This conversation features Dr. Allen Collins, director of the National Systematics Laboratory at NOAA Fisheries, and the National Aquarium's own Jennie Janssen, assistant curator of our Blue Wonders exhibit.

What Makes a Jellyfish a Jellyfish?

With so many varied species, one of the most important things to understand about jellies are the simple characteristics that unite them. While all jellies look generally the same, many only share distant ancestors. Jellies do not have brains or hearts, and they move by pulsing their bells. They also all have stinging cells known as nematocysts, which are used by some species to paralyze prey and protect them from predators. Nematocysts are typically located along retractable tentacles; however, some species also have nematocysts located on their bells. Other species emit a stinging mucus. More about that later.

So, are they fish? In a word, no. Because a fish's anatomy is centered around its backbone and all species of jellies are dome-shaped invertebrates, the word "fish" is a misnomer, and they are frequently referred to simply as jellies.

The Jelly Lifecycle

For simple animals, jellies have a complex lifecycle beginning in the simple polyp stage and maturing through the adult or medusa stage. Many jelly species hatch from eggs into free-swimming larvae known as planula, which eventually settle on a hard surface, like a seashell, and transition to a stationary polyp stage. Jelly polyps resemble miniature sea anemones. From here, when environmental conditions are right, the polyp will undergo "strobilation," where ephyra, resembling a stack of microscopic dinner plates, form. Tiny free-swimming ephyra are then released one by one into the water column where they eventually grow to the adult medusa stage. Polyps can also bud off to produce more polyps, so some jelly species can reproduce both sexually and asexually. In the case of box jellies, a juvenile jelly grows into a two-tentacled medusa before third and fourth tentacles grow congruently from the medusa bell (or "pedalium") and increase in size toward adulthood.

Back in 2016, Jennie Janssen noticed something peculiar in the Camouflaging exhibit in the Surviving Through Adaptation gallery. On the water's surface, she spotted a transparent, barely visible jelly about 5 millimeters in length. Considering the Camouflaging exhibit is not jelly-centric, this was a shocking find!

Over the following weeks, more jellies were spotted on exhibit and moved to the National Aquarium's Culture Lab for closer examination. Under a microscope, it was clear to Aquarium staff that these were box jellies, but determining their exact species proved to be a challenge.

When experts classify animals, species are typically described based on the appearance and characteristics of the adults, rather than those of the juveniles. Despite the existence of approximately 50 known species of box jellies, experts are only aware of the full life cycles of a few species—meaning that the box jellies discovered on exhibit could have been a known or unknown species.

To begin the process of classifying these box jellies, Aquarium staff—in collaboration with Allen Collins and his team at the Smithsonian National Museum of Natural History—sequenced their DNA and compared it to the DNA of other box jellies. The result was an 88% match with a known species with obvious physical similarities—each has only two tentacles as a juvenile. This discovery aided staff in determining the jelly's family: Alatinidae. While this discovery was a breakthrough, the question of the exact species of these box jellies will take longer to answer.

While successfully raising these jellies from polyps to a four-tentacled medusa stage, Aquarium staff continues to gradually collect more information upon which to determine whether this is in fact a brand-new species, and it is our hope that Jennie and her team will eventually be able to formally describe these jellies and give them a scientific name.

Yes, We Said Stinging Snot

Jennie and Allen have also collaborated on another important jelly project: Stinging snot. Yes, you heard right.

In 2017 during a Smithsonian team visit to the Aquarium, Jennie and Allen realized that there was nothing in current scientific literature that described the phenomenon of stinging water around jellies—a well-known and somewhat painful experience commonly shared by aquarists. The teams collaboratively studied and named the stinging-cell structures—known as "cassiosomes"—in the mucus of upside-down jellyfish on exhibit in Jellies Invasion and jointly prepared a manuscript for publication.

Closeup of the Translucent and Tan Tentacles of an Upside Down Jelly

The paper, published in Communications Biology, outlines how some species of jellyfish can sting without touching their prey, thanks to their ability to launch venom-filled mucus grenades. "I think the research resonated in part because so many people have an innate fear of being stung by jellyfish," Jennie said, "and it's unsettling to know that you can get stung without even touching them."

"Also, talking about stinging snot and mucus grenades is just too fun to pass up."

National Aquarium Culture Lab

Our aquarists have created a Culture Lab at the Institute of Marine and Environmental Technology (IMET) right next door to our main campus on Inner Harbor Pier 5 where research is conducted, and the moon jellies you see in Living Seashore are grown.

Why Jellies Matter

If it weren't enough that jellies are so scientifically and visually cool, they play a critical role in their ecosystems. That's right, jellies do not exist just to ruin your day at the beach. Jellies play an important role in ocean food webs as a crucial dietary staple for everything from bottom dwellers such as crabs to sea turtles, ocean sunfish and marine mammals. In fact, the world's largest sea turtle species, the leatherback, is a jellivore that feeds almost exclusively on jellies.

Because they don't have complete control over their momentum or movement in the ocean, jellies are known as opportunistic eaters, taking in zooplankton, or tiny crustaceans, fish eggs, larvae and other tiny matter encountered as they drift along. Jellies are known as an indicator species because changes in their populations reflect greater changes in their ecosystem. For example, when jellies are seen in large quantities in an area—known as a smack of jellies or a jelly bloom—scientists know that something within the environment is tipping off balance. This could mean that their predator populations are suffering, or that ocean temperatures or acidity are elevated. In a changing ocean environment, scientists can sometimes look to jellies to spot trouble as it begins.

Jellies at the National Aquarium

From the stunning array of species in our popular Jellies Invasion exhibit to the soft, smooth bells of the bobbing moon jellies in our Living Seashore touch experience, jellies are getting their share of the limelight at the National Aquarium. You can even spend some time with our jellies from the comfort of your own screen with our jellies live feed.

Atlantic Bay Nettle (Chrysaora chesapeakei)

The Atlantic bay nettle's bell can grow up to 7 inches wide.

Learn More

Blue Blubber Jelly (Catostylus mosaicus)

Japanese Sea Nettle (Chrysaora pacifica)

Moon Jelly (Aurelia aurita)

Pacific Sea Nettle (Chrysaora fuscescens)

Upside-Down Jelly (Cassiopea xamachana)

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