The keeping of aquarium jellyfish is rather complicated and rarely successful.
J. Charles Delbeek
A story was carried by Associated Press that received wide play in newspapers and on television concerning the latest pet fad in Japan — the keeping of jellyfish, especially by single Japanese women. I am not sure how widespread this fad actually is in Japan, but when I was there in January, I visited a number of pet shops and saw only one small jellyfish tank on display, and only one other store had a single jellyfish specimen for sale. According to the story, the hypnotic swimming movements of the jellyfish were felt to have a calming effect on their owners.
One quote in the story stated that they were easy to keep and could be left alone for a week without needing to be fed, making them the "perfect pet." Unfortunately, nothing could be further from the truth. It is very unlikely that these animals live more than a few short months in these "home systems" now being sold in Japan. What is even more unfortunate — for a number of reasons — is that, perhaps due to that story, we are now seeing jellies appearing in pet shops and wholesalers here in North America.
Sea jellies, of which there are thousands of species in temperate and tropical waters, have many special requirements — some of which are known, many of which are not. This includes such things as water movement, the effects of animal population density on growth, the effects of photoperiod, food (types, sizes, frequency of feeding and amounts are all variables that need to be investigated), the role of temperature fluctuations in reproduction, their natural life spans (many species live less than a year) and the role of trace elements.
Some jellyfish, such as Cassiopea and Mastigias species, contain symbiotic algae and require high light levels of the proper spectrum, just as live corals do. Most jellyfish also require frequent feedings of live food, so culturing systems for brine shrimp, rotifers and other microscopic foods must be constantly maintained.
Keeping jellyfish is not as simple as the story noted above made it appear, and they are certainly not for beginners. As with so much of the aquatic life kept in aquariums today, some species of jellies can grow quite large and require larger systems as they grow. Growth is often rapid, especially in species with life spans under two years, and system designs need to take this into account. As with any organism we keep in captivity, we should strive to do so for the length of its natural life span. Keeping jellies alive for a few months does not mesh with this goal, and I would urge people not to succumb to the urge to purchase these animals unless they have access to the necessary expertise, funds, time and energy required to keep the few available species alive and well in captivity.
At present, only public aquariums have had the resources to properly display and breed some species of jellyfish, but I expect in the future dedicated advanced hobbyists will be able to keep some of the smaller, more easily maintained species. Some, such as Aurelia aurelia, breed quite readily in captivity, and sustainable populations could be maintained in aquaculture facilities for the pet trade. But until the costs for the systems necessary to maintain and feed these animals properly comes down, they will limited to individuals and/or institutions with the financial resources and dedication required. For now, admire these animals at your local public aquarium or on video, and do not succumb to the temptation to purchase and keep them in 20-gallon "jelly tanks."