Jellyfish aquariums certainly seem to be a growing trend, and in light of that, I’d like to take a closer look at these gelatinous animals to hopefully provide a bit of insight into their life history and what goes into caring for them.
We already keep corals and anemones in our home aquariums, which are close relatives of jellies, so it isn’t much of a stretch to take that last step toward maintaining jellyfish as well. Public aquariums and some dedicated hobbyists have already been successfully keeping jellies in captivity for decades, but until now, they have not been a mainstream option for marine home aquariums. Before we discuss their care, let’s first consider their natural history.
Corals, jellies, and anemones all belong to the phylum Cnidaria of which there are about 10,000 species, and they have some basic traits in common. This phylum is broken down further into four classes: Hydrozoa, Anthozoa, Scyphozoa, and Cubozoa. Hydrozoans include fire corals, the Portuguese man-of-war, and the only freshwater cnidarian, the hydra. Corals, anemones, and sea pens are anthozoans, jellyfish are scyphozoans, and box jellyfish are cubozoans. All cnidarians possess tentacles with stinging cells called cnidocytes that are primarily used to capture prey items. Cnidarians don’t have a circulatory system, central nervous system, or a conventional digestive tract. Their bodies are made up of two primary cell layers that hold in a jelly-like middle layer called mesoglea that functions like a skeleton.
Jellyfish in particular tend to display four-part symmetry and are almost entirely composed of water. Throughout their lives, jellyfish exist in two forms: medusa and polyp. We’re used to seeing jellies in their free-swimming planktonic form, the medusa, but they actually spend a larger percentage of their lives in the sessile polyp phase anchored in place until the conditions are right to transition to the medusa phase. Jellies are typically either male or female and most are broadcast spawners, meaning that gametes are released en masse into the water where they are fertilized and begin their lives in the open ocean. Once an egg is fertilized it becomes a mobile larva called a planula that soon descends to the sea floor to anchor onto a stable substrate. It quickly develops into the polyp stage and when environmental conditions are favorable it transitions to the intermediate strobila form that resembles a bunch of graduated bowls stacked on top of each other. This is a form of asexual reproduction that generates layers of tiny, identical medusae (called ephyra at this stage) that are released one at a time to begin the mobile phase of life that we recognize as a jellyfish.
Jellyfish range drastically in size from one inch in diameter up to more than six feet across in the case of the Lion’s Mane jellyfish (Cyanea capillata), which can have tentacles in excess of 100 feet in length. The diet of a jellyfish consists primarily of zooplankton, small fish, crustaceans, and other pelagic creatures small enough to be eaten, though many marine animals also strategically use them for shelter.
Now, onto the part where we try to keep them in aquariums. As mentioned at the beginning, public aquariums have been successfully rearing and keeping jellies for decades, but the move from a floor to ceiling display to a desktop kreisel is something else entirely. A kreisel is basically just a circular tank with low laminar water flow that keeps the jellies gently moving around the tank; the lack of right angles helps keep them from getting stuck in corners and hurting themselves. Moon jellies (Aurelia aurita), which inhabit waters around the world with a huge range of temperatures and salinities, are the species of choice in this new age of nano jellyfish systems due to their relatively robust nature. But they can grow to an adult size of more than a foot in diameter. So how can we possibly keep these animals in a desktop aquarium? The factual answer lies within a combination of stellar water quality, low stocking density, and just the right amount of food. The ethical answer is a bit less clear. The only way to keep a gelatinous animal from growing is to restrict its food intake. Whether this translates to starvation or maintenance feeding is a matter of semantics and opinion that I won’t be tackling here.
There are quite a few jellyfish aquariums on the market, but after doing some research it appears that one company has this novelty cornered in both aesthetic appeal and functionality. Jellyfish Art has a comprehensive website that addresses all the common questions you would want answered, save of course from delving into the ethics of feeding jellies just enough food to keep them alive but make sure they don’t grow. They sell the whole system, all replacement parts, the jellies, food for the jellies, and provide buyers with enough instructions and information to minimize the likelihood of mishaps as much as possible. A six-gallon aquarium can supposedly handle three to five animals depending on their size, and the jellies should be fed twice daily with either prepared frozen foods or live brine shrimp nauplii. The natural lifecycle of a moon jelly takes about a year to complete in the wild, and according to the website these jellies should live for about the same length of time at which point you will be left with an empty tank in need of replenishment.
As intriguing as these nano jellyfish tanks are, I am still on the fence about the overall validity of the concept. The aquariums are beautiful and are absolutely stunning when viewed in a dimly light room. The color changing LEDs perfectly complement the ethereal movement of the moon jellies and the system is a striking centerpiece and certain talking point. The market is there, the demand is growing, the price is reasonable, the maintenance isn’t ridiculous, the result is gorgeous, but I’m still not convinced. Jellies might just be one of those animals whose captive care is better left to the experts.