Jellyfish. Are they jelly? Are they fish? Actually, they’re neither! Jellies are part of a phylum called Cnidaria which includes other creatures such as corals, anemones, and hydroids. These strange creatures are also considered the largest type of plankton.
When you think of plankton you might think of small microscopic organisms that float around in the water, but the definition of plankton is- an organism that cannot swim against a current. Jellies can move, but they cannot locomote well enough to fight a current, making them plankton!
When I go to an aquarium, the jellyfish exhibit is always something I look forward to viewing. I have been caring for jellies for over 7 years, and I could still watch them for hours. A common species of jelly, both in aquariums and in the wild, is a moon jellyfish. I’ll be focusing on this species during this article so I can get into more specific details about husbandry and their life cycles.
In aquaria, moon jellies need good water quality, a consistent water temperature (usually cold), perfect flow, and a sufficient amount of live microscopic brine shrimp nauplii.
As an aquarist, I spend a lot of time cultivating this food. This consists of hatching the brine shrimp from eggs, sorting out the eggshells from the animals, and feeding them a high-quality diet that will help keep the jellies healthy too.
I keep the exhibit windows as clean as can be, I adjust the flow of the exhibit to keep the jellies from sitting on the bottom, and I maintain good water quality with water changes and lab tests. If the water temperature changes too drastically, the jellies can experience buoyancy issues and it’s bad for their tissues.
When you look at the bell of a jelly, also called a medusa, you can easily see its many body parts. All the “jelly” in the bell is called mesoglea. Around the edge of the bell are the tentacles; those tiny hair-like structures grab small food particles out of the water column and can also sting prey items.
As we move inward you will see many canals; although they don’t have blood, those canals help move nutrients around to all the cells. At the very center of the bell, you will find the mouth which is surrounded by oral arms hanging down. The oral arms help move food into the mouth. The four lobes that look like a four leaf clover are actually the stomachs. When a jelly has full bellies these will often be orange, the color of their food.
The life cycle of a moon jelly is more complicated than these simple-looking creatures would have you believe. Let’s start with the medusa stage, that’s the stage of life that everyone thinks of when they picture a jellyfish.
These medusa can be male and female. The males will release sperm into the water column, and the females will consume it, similar to how they would eat their food out of the water, then move the sperm near to the stomachs where the gonads are also located. Once the eggs are fertilized the female jellyfish will put them on her oral arms in brood pouches and hold them to let them grow.
Once they are fully developed they will pop off and be free swimming for a short time, called a planula, until they settle on a surface. Once settled they are now in the polyp phase, which looks a lot like a small anemone. Here is the wild part! The polyps will reproduce a-sexually by a process called strobilation.
Using transverse division the polyp will replicate body segments to create a stack of replicas on its crown. These segments will develop and pop off to become the ephyra phase of our jelly lifecycle. These tiny ephyra will grow and develop until they become medusa, the jelly phase we all know and love.
The current jellyfish exhibit that I am caring for is definitely my favorite of all time. Using mirrors, the Aquarium at the Boardwalk exhibits “infinite” jellies. If there is anything a jelly aquarist dreams about, it’s infinite jellies. Hopefully, you will have a new appreciation for jellyfish and can stare at these mesmerizing creatures with more knowledge under your belt next time you visit your local aquarium.
