Freshwater Jellyfish

Quick Take

• The most common jellyfish in the United States doesn't live anywhere near the ocean, and the story of how it ended up in landlocked ponds is stranger than you'd expect. Explore its habitat origins →
• This jellyfish traveled from China to nearly every continent without ocean currents or fins, and the method it used is almost unbelievable. How podocysts enable dispersal →
• A bloom of these jellyfish is almost never mixed-sex, and the reason behind this reveals something deeply strange about how the animal reproduces. Single-sex blooms explained →
• Fish consistently refuse to eat this jellyfish, yet other freshwater predators don't hesitate. This distinction points to something unexpected about its biology. See which predators bite →

Many people might be shocked to learn that the tiny, harmless Craspedacusta sowerbyi, a hydromedusan jellyfish, is the most commonly found jellyfish in the United States. Moreover, it’s not found in the waters off the coasts, but in quiet bodies of freshwater. Where did it come from, and how on earth did it get into that flooded gravel pit in your neighborhood? Read on to find out.

Four Amazing Facts About the Freshwater Jellyfish

• A bloom of freshwater jellyfish is usually either all male or all female.
• The polyp of this freshwater jellyfish lacks tentacles, which is unusual.
• During times of cold weather or drought, the jellyfish turns into a podocyst. This is probably what allowed it to be carried all over the world from China.
• The jellyfish is so small that the stinging cells in its tentacles can’t penetrate human skin, so it is harmless to humans.

Classification and Scientific Name

The scientific name of the freshwater jellyfish is Craspedacusta sowerbyi. Craspedacusta comes from craspedon, an irregular New Latin word that means “velum” and kystis, a Greek word for “bladder.” The species name is probably in honor of the Sowerby family of British naturalists. C. sowerbyi is the only known species of freshwater jellyfish, and no subspecies are recognized.

Appearance

There are two mature forms of the C. sowerbyi, a polyp or a medusa. The medusa is shaped like a bell, with a structure called a velum attached to its underside. It is rarely more than an inch in diameter, translucent with a tinge of white or green, and about 99 percent water. Inside the bell is the equivalent of a stomach, one circular canal, and four canals that radiate out from the stomach. The stomach has a mouth with four lips, and the radial canals each bear a reproductive organ.

The polyp is shaped like a vase. Only about 0.04 to 0.08 inches (1 to 2 mm) tall, it doesn’t have tentacles but a cap that holds nematocysts. It also has a mouth and holds on to a surface with a round base. It can contract to half its height or stretch to twice its height, and can capture prey many times bigger than it is. Sometimes, a colony of polyps can be seen eating the same prey.

Tentacles

The edge of the medusa’s bell bears as many as 400 tentacles of various lengths. The longer ones help the animal swim, while the shorter ones help it feed. These tentacles have cnidocytes, which in turn hold nematocysts, which paralyze prey and pull it into the mouth. After the food is digested, the waste is expelled from the mouth. The tentacles also have eyespots at their tips.

Distribution, Population, and Habitat

Native to China’s Yangtze River, this little jellyfish has managed to spread to bodies of freshwater on every continent on earth save Antarctica. It was first reported in Michigan in 1933 and in Kentucky as early as 1916. It is now established throughout most of the United States, having been recorded in at least 44 states, including both eastern and western regions. The only real requirement for this jellyfish is that the waters be calm.

The number of C. sowerbyi in any body of water increases depending on the temperature and the abundance of food. Still, it is difficult to determine the overall population, because newsworthy blooms can happen one year and not the next, even when conditions appear similar. Maybe because of this, C. sowerbyi’s conservation status is not evaluated.

Just how these jellyfish are distributed is the subject of a study by Indiana University in Pennsylvania.

Predators and Prey

Freshwater jellyfish eat zooplankton, which are tiny animals found in the water. All the jellyfish have to do is extend their tentacles and wait for one of these animals to brush against them. The prey is then poisoned, paralyzed, and drawn into the jellyfish’s mouth. C. sowerbyi prefers Daphnia and copepods as prey. In turn, the jellyfish is eaten by crayfish and turtles. Fish tend to avoid it.

Reproduction and Lifespan

The most fascinating thing about C. sowerbyi is its reproduction. Though there are male and female jellyfish, it is unusual for them to reproduce sexually. The medusa form, which looks like a “real” jellyfish, is only one of the forms the animal can take and is the most studied.

C. sowerbyi usually reproduces through budding. A polyp can grow a type of bud called a frustule. The frustule breaks off, then turns into another polyp over time. Or, the polyp can grow a polyp bud, which can mature on the parent polyp until a colony is formed. The colony can also form a medusa bud. Eventually, the medusa bud breaks off and forms the familiar, mature jellyfish. If the jellyfish reproduces sexually, it simply releases eggs or sperm into the water. The fertilized egg develops into a blastula, which develops into a planula larva, which settles on a surface and over time becomes a polyp.

Scientists aren’t sure how long C. sowerbyi lives, but if it is like most other jellyfish, it lives one to two years. During harsh periods of winter or drought, C. sowerbyi curls up into a nearly microscopic podocyst, which is just a few cells encased in a hard shell. These podocysts stick to aquatic plants and the feet of waterbirds and are carried from one body of water to another.

Fishing and Cooking

C. sowerbyi is not deliberately fished nor is it used in cooking. Though other types of jellyfish are used in cooking, C. sowerbyi is too small and fragile.

Population

It’s probably not wrong to believe there are millions of these freshwater jellyfish at any one time, but determining precise population numbers remains an inexact science. In some years, there will be abundant blooms of jellyfish, and in other years, they are scarce.