Frequently Asked Questions
Do freshwater jellyfish sting?
Yes. Like true jellyfish, they do have stinging cells (cnidocytes). This mechanism is designed for feeding, as the cnidocytes are utilized to paralyze macroinvertebrates and even small fish. There are conflicting reports whether these microscopic stinging barbs can penetrate human skin. Some individuals have reported that they encountered the jellyfish and felt it. Whether they were actually stung (involving a penetration of their skin) or whether the stinging barbs were released due to contact with the person and the effect was due to brushing against the released stinging barbs is unknown. The sensations reported range from mild itching, to red spots, to various levels of irritation, to a slight numbness. Others have reported handling the jellyfish or swimming among them with no adverse effects. Assuming that the jellyfish did release their barbs when handled or contacted, yet the person did not feel any effect, could be due to different individuals having different levels of sensitivity to the animal’s toxin just as folks do to the toxin or chemicals found in certain insect bites or stings.
Can I raise freshwater jellyfish in my aquarium?
Maintaining freshwater jellyfish in an aquarium is not easy. They usually live only few days to a couple of weeks in an aquarium. The filtration system must be such that the jellyfish are not sucked into the filter. Even a sponge filter has the potential to trap the jellyfish. A pump system that circulates the water, rather than bubbling air, is needed. Air bubbles could become trapped under the bell of the jellyfish, forcing the animal to the top of the water and holding it there. The pump should be adjusted so there is not a strong current in the tank. It is important to remember that freshwater jellyfish have stinging cells. They have the potential to sting and possibly kill any small fish that may be present in the aquaria. We have not been successful in keeping freshwater jellyfish alive in aquariums. Public aquaria, such as Monterey Bay, culture jellyfish in tanks with circular chambers (kriesels) with circulating water.
What is the recommended
temperature, pH, and food supply for captive jellyfish? Can they reproduce in
The water temperature should be 20°-25°C, pH should be 6-7, and food supply should be plentiful. The jellyfish require a constant supply of live food. Brine shrimp or Daphnia should be fed to the jellyfish daily. We have not been able to rear medusae from immature to mature. We have brought mature medusae from the field, which have subsequently released eggs in our tank. Several public aquaria have contacted us over the years seeking information on how to keep freshwater jellyfish in captivity, and so far, no attempts have been successful.
What shape tank, lighting,
and environment is preferable for a captive jellyfish habitat?
It is best if the tank is round (either horizontally or vertically), and the water is circulating in the tank to keep the jellies in motion. Size of the tank is determined by the number and size of jellies; too many jellies in too little volume of water can cause fouling of the water. A rule of thumb that is often used is one inch of animal per gallon of water. We have used a variety of sizes from 2 gal. up to 100 gal. with no better success in one than the other. The other problem to consider is that you want the volume to be such that the jellies do not run the risk of not encountering food organisms as they swim about in the tank. The bottom apparently could be anything: small stones, rocks, or nothing at all. Plants would be a problem, unless they are separated from the jelly chamber by a fine mesh.
How can I catch freshwater
We use nets to catch the jellyfish. Sometimes we use a long-handled, large fine mesh dip net (12-16 inches in diameter), and sometimes we use a small size aquarium net. It just depends on where we are working and how quickly we want to gather a quantity of them.
Is the appearance of the freshwater jellyfish related to water quality
or the quality of fishing in a lake?
At this point, we do not know. The freshwater jellyfish has been found in a variety of water types. We have observed them in waters ranging from crystal clear rock quarries to soupy green farm ponds. Preliminary research indicates that waters high in chloride do not favor the appearance of the freshwater jellyfish. Hopefully, further research will answer this question.
The matter of the jellyfish impacting the fishing is a difficult one to answer. The jellyfish could be having an effect on the zooplankton population on which the fish normally feed, thus affecting the growth of the fish. This, however, is dependent on several things: size of pond, size of fish, timing of the fish-zooplankton-jellyfish populations maturing and interacting, zooplankton present, etc. Another possibility is that the fish are feeding on the jellies and therefore not interested in the bait or lures that are being used for fishing. I doubt this latter idea, since we have not observed fish eating jellyfish.
How can you tell if the jellyfish is male or female?
It is impossible to tell the males from the females until they are sexually mature and ready to shed their sex cells (gametes). At that time, the gonads do look a little different, and eggs or sperm can be seen being shed. At earlier stages, the medusae may be sexed by taking a snip of tissue off from the end of the gonad and preparing a squash slide. Developing eggs or sperm may be detected depending on the developmental stage of the animal and the state of maturity of the developing sex cells in the gonad.
Can freshwater jellyfish harm small or young
Yes, young or small fish may be harmed by freshwater jellyfish if they encounter them. We have put small minnows into an aquarium with freshwater jellyfish and some have died after being stung.
How do freshwater jellyfish enter a pond?
Freshwater jellyfish could enter a pond in a variety of ways. For example, they could have entered in the polyp stage attached to plants. The plants could have been carried in by birds, on the props of boats, on the bottom of boat trailers or in the buckets of fishermen. They also could have entered in the medusa form in the bilge water of boats, in the water of bait buckets, or by flooding from another site.
It is noted that the best way to preserve
specimens for DNA analysis is in a jar of 70% isopropyl (rubbing) alcohol. What
is the best way to preserve jellyfish for species identification?
In a jar with 10% formaldehyde.
Could you provide literature on or an annotated
bibliography about freshwater jellyfish life history and distribution?
Please see the references listed on our webpage.
Where/what is the origin of
There are two schools of thought as to the origin of the freshwater jellyfish. One is that they originated in the upper Yangtze River basin in China, where populations are still found today. They then were discovered in water lily tanks in Regents Park in London in the late 1800's, presumably having arrived there as polyps on the imported plants. There is nothing specific in the literature to as to how the organisms might have gotten from either China or London to the U.S.
Another story that can be found in literature is that they originated in South America and made their way to the U.S. in bilge water of ships. About the same time (late 1800's), the polyp form was discovered in Tacony Creek near Philadelphia, PA. The records of the organism in the U.S. have been sparse over the years (perhaps leading to the belief that they were rare), although we do have sighting information from various states.
Please also see the distribution section of the website.
Is there a connection between
weather conditions and the appearance of blooms?
Although we have no data to support this statement, I would hypothesize that if there is a connection between weather conditions and the appearance of blooms, it is because of an effect on the zooplankton available when the polyps are budding off the new medusae. It takes about a month for the medusae to reach maturity after they are produced. So, if weather is having an effect, it would seem that it would have to be a month or more prior to the sighting of the bloom.
Is it thought that the
'resting stage' is polyphyletic in the Hydrozoans? That is, did C. sowerbii
evolve a separate resting stage from Hydra?
The podocyst, a multicellular capsule, is considered one of the "innovations" of the Hydroidomedusa (the main group of Hydrozoa).
What is the size and shape
of the resting stages? Do they exhibit any apparent adaptations such as burrs or
The resting stage is circular, fairly flat, no hooks or burrs (there does seem to be some adhesive property which attaches to surfaces), yellowish-whitish in color, and only a mm or so in diameter.
Is there any case known
where a hydrozoan 'resting stage' has been revived from the intestinal contents
of the animal?
I know of no case where hydrozoan 'resting stage' has been revived from the intestinal contents of the animal.
What affect do freshwater
jellyfish have on us and what affect do we have on them?
The freshwater jellyfish feeds on microorganisms in the water column. So, they are competing with other organisms which feed on the same sources. This may impact fish populations in a body of water, especially if there is a large, established population of jellies. To date, there is no research evidence to support or refute this.
Our affect on jellyfish is unknown. We have discovered them in pristine waters as well as heavily polluted waters. In a study we did of over 20 chemical and physical parameters in about 6 dozen known jellyfish sites across PA, we found that the only thing significantly impacting their presence was the level of chlorine in the water.
How do freshwater jellyfish obtain food?
The tentacles are covered with thousands of stinging cells. Smaller organisms which come in contact with the tentacles are stung and paralyzed. The paralyzed prey usually stays tangled in among the tentacles. The tentacles then sway about moving the captured prey to the mouth, which is in the middle of the underside. The bell (body or umbrella part) of the jelly goes through several contractions to move the prey into the mouth and digestive cavity.
Are freshwater jellyfish (C. sowerbii)
It is a misconception that is often found in the literature and popular reports that C. sowerbii is rare. It is found on all continents, except Antarctica. As you can see from our maps, it is has been reported from most states. In Pennsylvania, they have been reported in 50 of our 67 counties. What is true is that their appearance is sporadic and unpredictable. We are currently in the process of doing DNA fingerprints (nuclear) on specimens from all over the U.S. as well as some foreign countries. So far, we have found no differences from one geographic location to another.
base are you compiling information for?
The data base started out as a survey for Pennsylvania as part of a grant from the PA Fish and Boat Commission and has grown from there to include the rest of the United States and the world. We continue to add and maintain information for the general public who encounter freshwater jellyfish.
jellyfish move for warmer water?
The jellies typically move up and down in the water column while also being carried along by currents. Typically they do not go down below the thermocline (if one exists in the particular body of water) during vertical migrations.
How do I
get rid of jellyfish in my pond?
I know of no ways to get rid of them permanently. They will disappear naturally after a few days, but may come back next year. I have talked with folks who have even drained their pond, but it did not keep them from reappearing. They should not result in any real negative environmental impact to your pond.