An Introduction to Aquarium Jellyfish

Jellyfish, a gelatinous invertebrate that is found all over the world’s oceans, are mesmerizing to observe and can be a wonderful addition to your home aquarium. However, caring for jellyfish is also complicated because of their unusual life cycle and complex anatomy. The goal of this article is to provide a basic introduction to these amazing creatures and some tips for keeping them happy at home.

Unlike fish, which are vertebrae-based, all jellyfish have no backbones. Their anatomy consists of a bell-shaped membranous body with tentacles and an oral arm, or sucker. They have no brains or hearts and move by pulsing their bells. Some species have stinging cells, called nematocysts, located along their retractable tentacles that can paralyze prey or deter predators. Jellyfish spawn asexually, producing many identical offspring (ephyrae) that can grow into adults.

Jellyfish can survive in a wide range of temperatures, from tropical heat to the cold northern Atlantic. They are often seen in captivity in aquariums with a water temperature between 49 degrees Fahrenheit and 70 degrees Fahrenheit. They thrive in these conditions and don’t require a heater or chiller for their tanks. Jellyfish can also tolerate low oxygen environments and do not require aeration systems.

After a jellyfish sperm and egg meet, the fertilized embryo becomes a free-swimming larva called a planula. It resembles a fuzzy ciliated tic tac and whizzes around before landing on a sponge or other seafloor fixture, which determines its eventual fate. Some species of jellyfish are pelagic and move about freely in the ocean, but others are sessile and attach to solid substrates for stability.

Once a planula finds a suitable spot, it settles and begins to grow larger. As it grows, its tentacles and arms elongate into the specific shapes of its species. At the same time, it clones itself, making the new jellyfish male or female. (Jellyfish only mate when conditions are ideal, and hermaphroditism is rare in the wild.)

Some specialized species of jellyfish are often found in aquariums. For example, the Pacific Mastigias papua, which houses its symbiotic zooxanthellae in its bell, is easy to keep alive in the home. However, other eupeid species, such as Chrysaora fuscescens, the sea nettle, require more elaborate setups to be successful in captivity.

Until the right conditions for asexual reproduction arise, most jellyfish spend years in their polyp phase. Jellyfish aquarists like to cultivate polyps because the process of strobilation is much less time-consuming than growing a new jellyfish from an egg or an ephyra. In addition, the polyps are a continuous source of jellyfish and can provide a steady supply to customers (Raskoff et al., 2003; Pierce & Schaadt, 2005; Helm et al., 2018; Duarte & Helm, 2021).

When a polyp grows to the point of being ready to bud off new jellies, it will look like a stack of little discs. When the top one is pulled up, a whole row of the smaller ones will fly off, much like clay pigeons at a shooting range. Sometimes dozens of discs will break off, each one containing a baby jellyfish.