Yes, There Really Are Jellyfish in Lake Keowee

Imagine floating in Lake Keowee on a warm late-summer afternoon when something small and almost completely transparent pulses through the water beside you. It has a round bell, tiny tentacles, and moves exactly like a jellyfish.

Your first thought would probably be that it cannot actually be a jellyfish. Lake Keowee is a freshwater reservoir tucked against the Blue Ridge Mountains, hundreds of miles from the ocean.

But this time, the unbelievable answer is true.

These Are Real Freshwater Jellyfish

The species found in Lake Keowee is Craspedacusta sowerbii, commonly called the freshwater jellyfish. It is a hydrozoan belonging to Cnidaria, the same major group of animals that includes marine jellyfish, corals, and sea anemones.

They are not ocean jellyfish that somehow became trapped in Lake Keowee. This species is adapted to freshwater and is believed to have originated in East Asia, probably around China’s Yangtze River basin. It has since spread around much of the world.

Reservoirs, ponds, flooded quarries, gravel pits, and other relatively calm bodies of freshwater provide particularly suitable habitat. Lake Keowee fits that description nicely.

How Big Are Lake Keowee’s Jellyfish?

Do not picture a dinner-plate-sized ocean jellyfish drifting beneath your kayak. These things are tiny.

The visible swimming stage, known as the medusa, is generally around 5 to 25 millimeters across, with mature individuals sometimes reaching roughly 1 to 3 centimeters. In everyday terms, a good-sized freshwater jellyfish is only around an inch across.

Their small size and nearly transparent bodies help explain why thousands of people can swim, paddle, fish, and boat on Lake Keowee without ever realizing they are there.

The Jellyfish You See Is Only One Part of Its Life

This is where these little animals become much more interesting.

The transparent creature pulsing through the water is called a medusa. After sexual reproduction, a fertilized egg develops into a microscopic swimming larva called a planula. The planula eventually settles onto an underwater surface and develops into a tiny polyp.

That polyp may only be around a millimeter or two tall. It can live attached to rocks, submerged wood, vegetation, docks, and other underwater surfaces without anyone noticing it.

The polyps do not necessarily need sexual reproduction to spread. They can reproduce asexually and produce small mobile structures called frustules. A frustule can move away, settle somewhere else, and develop into another polyp.

Then there is another survival trick.

They Can Essentially Disappear

Freshwater jellyfish can produce a dormant stage called a podocyst. This tiny protected structure can survive periods when conditions are unfavorable and later give rise to another polyp.

Recent laboratory research suggests that cold conditions may actually help preserve these dormant stages. That means winter does not necessarily eliminate a population.

The visible jellyfish can disappear. Cold weather can arrive. Months or even years can pass without another obvious bloom.

Meanwhile, the population may still be sitting beneath the water in forms almost nobody would recognize.

Why Do They Suddenly Appear?

Warm water clearly plays a role. Visible medusae are most often reported during warmer portions of the year, particularly late summer and early fall.

But temperature does not completely explain a bloom.

Researchers have experimented with temperature, feeding schedules, and water conditions while trying to determine exactly what causes polyps to begin producing medusae. Surprisingly, scientists have not been able to reliably flip that biological switch.

Some laboratory populations have even continued producing medusae seasonally while being maintained under relatively stable temperatures.

So when dozens or hundreds of freshwater jellyfish suddenly appear in a cove, they probably did not suddenly arrive in Lake Keowee.

They were likely there all along.

What Do Freshwater Jellyfish Eat?

Despite their delicate appearance, these are predators.

The tentacles contain microscopic stinging structures called nematocysts that help capture tiny prey. Their diet consists largely of zooplankton, including small crustaceans such as copepods and cladocerans.

The polyp stage is predatory as well, which means the animal participates in the lake’s food web even when nobody can see a recognizable jellyfish.

Can They Sting You?

Yes, they possess genuine stinging cells. That is how they capture microscopic prey.

For people, however, freshwater jellyfish have historically been regarded as essentially harmless. Government wildlife sources describe their nematocysts as unlikely to penetrate human skin, and they are nothing like the larger marine jellyfish responsible for painful stings at the beach.

There is a little scientific uncertainty worth mentioning. Recent research has investigated possible skin reactions associated with freshwater jellyfish and identified biologically active compounds. That does not suddenly make Lake Keowee dangerous, but it is a good reason not to make the absolute claim that a freshwater jellyfish could never affect human skin.

Do Freshwater Jellyfish Hurt Fish?

This question is more interesting than it sounds.

Freshwater jellyfish can capture extremely small organisms, and laboratory observations have shown the polyp stage feeding on fish larvae. Researchers generally do not consider them major predators of fish, however.

The potentially bigger effect is competition.

Young fish eat zooplankton. Freshwater jellyfish eat zooplankton too. That makes their place in the food web particularly interesting in an Upstate where fishing is such a major part of lake and stream recreation. If you are looking for places to fish beyond Keowee, see our Ultimate Fishing Guide to the Upstate Mountains of South Carolina.

Other research has found considerable ecological overlap between freshwater jellyfish and juvenile fish, suggesting that they can compete for some of the same food.

None of that proves that Lake Keowee’s jellyfish are reducing its bass, crappie, bluegill, or other fish populations. I found no evidence demonstrating a measurable jellyfish-related decline in the Keowee fishery. For anglers interested in the colder waters and streams of the region, we also have a guide to seven easy-access trout fishing spots in Upstate South Carolina.

What the newer research does show is that these tiny animals are not necessarily ecologically meaningless. During a large bloom, they may temporarily influence the plankton community around them.

Does Anything Eat Them?

Crayfish are among the better documented predators of freshwater jellyfish, particularly when medusae are close to the bottom. Other aquatic animals may occasionally consume them as well.

Interestingly, freshwater jellyfish do not appear to be a particularly desirable food for many fish. Their role as prey is still being studied, and newer research is challenging the old idea that jellyfish are essentially a dead end in aquatic food webs.

Can Freshwater Jellyfish Be Killed?

An individual jellyfish is extremely delicate and can certainly be killed. Eliminating an established population is an entirely different problem.

Imagine removing every visible jellyfish from one Keowee cove. You would have removed the swimming medusae, but you would not necessarily have touched the microscopic polyps attached to underwater surfaces or dormant stages waiting for better conditions.

It is a little like picking every mushroom in a yard and assuming you have eliminated the fungus underground.

USGS notes that surprisingly little research exists on controlling Craspedacusta sowerbii. For a reservoir the size of Lake Keowee, eradication does not currently appear realistic.

Can You Go Jellyfishing on Lake Keowee?

Finding them is probably going to be harder than catching them.

During a bloom, the medusae are visible to the naked eye and move slowly enough that a fine-mesh net could physically scoop one from the water. But removing a handful of visible jellyfish would accomplish almost nothing toward controlling an established population.

There are also regulations governing the collection, possession, transport, and release of aquatic organisms. Nobody should move freshwater jellyfish, lake water containing them, or potentially contaminated aquatic material into another body of water.

If you encounter a bloom, there may be considerably more value in documenting it. Take photographs or video, record the date and approximate location, and note the water temperature and approximate number of jellyfish if possible.

And yes, we are still calling that jellyfishing.

Can You Eat Them?

Some large marine jellyfish are eaten in parts of Asia after specialized processing. Lake Keowee’s freshwater jellyfish is not one of the commonly harvested edible species.

I could not find reliable food-safety guidance establishing Craspedacusta sowerbii as safe for human consumption. Considering that a large specimen is only about an inch across and consists overwhelmingly of water, there would not be much of a meal anyway.

So photograph them, study them, and tell your friends that Lake Keowee has jellyfish.

Maybe skip the jellyfish sandwich.

Where Else Have They Been Found in the Upstate?

Lake Keowee is not the only Upstate location with documented freshwater jellyfish.

South Carolina records extend back decades. That means the Keowee record should not be interpreted as evidence that freshwater jellyfish first arrived there in 2018. It simply establishes that they were present by that date.

What About Lake Jocassee and Lake Hartwell?

This is where we need to distinguish between what seems possible and what has actually been documented.

I found firm documentation for Lake Keowee and several other Upstate locations, but I did not find equally strong records specifically establishing freshwater-jellyfish populations in Lake Jocassee or Lake Hartwell during this research.

That is especially interesting because both lakes are major parts of the Upstate’s freshwater landscape. Lake Jocassee is known for its unusually clear, cool water and mountain setting. If you have never explored it, our guide to swimming at Lake Jocassee is a good introduction to the lake.

The history of these waters is connected as well. Long before today’s reservoirs existed, rivers ran through the valleys that eventually became Keowee and Jocassee. You can read more in Before Lake Jocassee: The Wild Rivers That Created the Keowee Valley.

Lake Hartwell is another interesting candidate. We already explored one considerably less believable aquatic mystery in Are There Sharks in Lake Hartwell? Unlike the shark story, freshwater jellyfish are very real Upstate residents. I just have not found documentation strong enough to say that an established population has been confirmed specifically in Hartwell.

If you want to explore Hartwell from shore, Mountain View Park on Lake Hartwell has trails, disc golf, lake access, and a boat launch.

A lack of visible jellyfish sightings does not necessarily prove that they are absent. Research elsewhere has found freshwater-jellyfish polyps in lakes where the visible medusa had never previously been reported. An animal capable of spending most of its existence as an almost microscopic polyp is exceptionally easy to overlook.

If you have photographed freshwater jellyfish in Jocassee, Hartwell, or another Upstate lake, that observation could be worth documenting.

How Did They Get Into Lake Keowee?

Nobody appears to know exactly how the Keowee population arrived.

The USGS occurrence record lists aquatic plants, stocked fish, and small watercraft as potential pathways. Those should be treated as possibilities rather than proof of how the species reached the lake.

Because polyps and dormant stages are so small, freshwater jellyfish can potentially travel unnoticed with wet aquatic material.

This is another reason cleaning, draining, and drying boats and equipment between bodies of water matters. The organism hitching a ride may be too small to see.

What Should You Do If You Find One?

First, take a good look. You have encountered one of the stranger animals living in an Upstate lake.

Photograph or video it if possible. Record the date and approximate location. If you can safely determine the water temperature, write that down too. If there are dozens of jellyfish, an estimate of their concentration would make the observation even more useful.

Most importantly, do not transport jellyfish, aquatic vegetation, lake water, or wet equipment containing aquatic organisms to another body of water.

Lake Keowee’s freshwater-jellyfish population appears to have received relatively little scientific attention. A well-documented modern bloom could therefore be more interesting than simply knowing that someone spotted one.

Frequently Asked Questions

One of Lake Keowee’s Strangest Residents

The most remarkable thing about Lake Keowee’s freshwater jellyfish may not be that a jellyfish can live in a mountain reservoir.

It is how easily an established population can live there without anyone knowing.

For much of its existence, the animal can remain almost invisible. Tiny polyps cling to submerged surfaces. Dormant stages wait through unfavorable conditions. Then, when some combination of environmental factors lines up just right, transparent little bells begin pulsing through the water.

Someone swimming nearby notices one.

A photograph gets posted online.

And suddenly everyone asks the same question:

Yes.

This time, the strange Lake Keowee creature story is true.

And before anyone asks about the sharks again, we already investigated that one.