WOW: Jellyfish!

Hi there! My name is Adele, and I am one of the other aquarists in the World of Water (aka "WOW"). The jellyfish is one of my favorite animals. What normally comes to mind when you think of a jellyfish? A dangerous predator, or perhaps a memory of a bad sting? Well, while jellyfish do sting, there is actually much more to them than that. They are beautiful, graceful animals and have a fascinating life cycle.

Jellyfish, along with corals and anemones, are found in the Phylum Cnidaria. Jellyfish are not really fish, they are actually invertebrates, like corals and anemones.The name Cnidaria comes from the Greek word “cnidos,” which means stinging nettle. While all jellies have the ability to sting, there are very few species that can actually harm us. All jellyfish have stinging cells known as nematocysts which are found in the tentacles and outer surface of the jellyfish bell. These nematocysts are harpoon-like in structure and contain a neurotoxin. When something brushes up against the tentacles of the jellyfish, the nematocyst is triggered and neurotoxin is injected into the jelly’s prey. Luckily, the stinging cells of most jellyfish often do not have the ability to penetrate through our skin, since these types of jellies use their stingers to feed on very small invertebrates and other soft bodied animals. Jellies with powerful nematocysts, strong neurotoxins, and long tentacles, such as the Portuguese Man-of -War, are the ones we need to look out for. Their prey consists of adult fish and crustaceans, so it would make sense that they need a sharper “harpoon” to penetrate these animals. Unfortunately, however, these animals can not see who or what they are stinging- they do not have eyes.

Jellies have a very simple body type - they do not have circulatory, respiratory, or nervous systems. What passes for a nervous system consists of a bundle of nervous on the outside of their bodies known as a “nerve net,” and only a few jellies have light sensitive organs, known as ocelli, which can detect light. Their digestive system is what scientists describe as an “incomplete gut,” meaning that they have a mouth but no anus. So, any food that the jellyfish eats that is not digested is actually expelled back through the mouth. Yuck! The jellyfish’s reproductive system, however, is fairly well developed.

Jellyfish are dioecious, meaning that there are male and female jellyfish. The jellyfish’s gonads, or reproductive organs, are located inside the bell of the jellyfish, next to the baglike stomach. When mature, the eggs or sperm break into the stomach cavity and exit the jellyfish through the mouth. If the jellyfish is female, she will store her eggs on the outside of her body until they are fertilized. The males will release their sperm into the water. After the eggs are fertilized, they will start their unique life cycle.

The life cycle is fairly complicated. In the first stage after fertilization, the free swimming larva is small, oblong, and fuzzy. This is known as a planula. It then settles onto a hard surface on the ocean floor and becomes a polyp. The polyp is non-swimming and begins a process of growing into a stack of small disks. This process is known as strobilation, and each of these small disks will become a jellyfish. Each of these small disks will break off and swim away. The newly released jelly is known as an ephyra, and takes the form of the jellyfish that we are the most familiar with.

If you have visited McWane Science Center recently, you have probably noticed our display of jellies in the World of Water. These are moon jellies, or Aurelia aurita. They are found all over the world, from the North Pole to the South Pole, and are quite numerous in the Gulf of Mexico. If you have been to the beach and saw a jellyfish, it was probably this species. It’s most distinguishable characteristic is the four horseshoe shaped rings that are on top of the bell. These are gonads, and they even change color depending on what type of food the jellyfish is eating. A moon jelly’s diet consists of plankton, which are tiny animals found in the water column. We feed our moon jellies a type of plankton known as brine shrimp, and occasionally they will also receive Cyclopeez, a red frozen plankton. And believe it or not, other aquatic creatures such as sea turtles, the ocean sunfish and even other jellyfish consider the moon jelly a delicious meal! This species is almost completely harmless to us. Contact with one of these guys will cause a mild itchy rash that will go away in a couple of hours.

So what does it take to keep jellyfish in captivity? Jellyfish require specialized equipment and excellent water quality. In the simplest jelly system, there is a reservoir for holding water, known as a sump, and then a display tank for the jellies. In the wild, jellyfish require water currents for feeding, movement, and oxygen intake. We are able to replicate this environment in captivity by using a U-shaped tank known as a kriesel, which allows a constant flow of current throughout the tank. Water re-enters the display tank through two spray bars found at the very top of the tank, and flows downward creating a circular current. The speed of the current must be very carefully adjusted- jellies need a “lava lamp” speed current, not a “washing machine” speed!

When water circulates out of the display tank, it leaves through a hole at the top that is covered by a screen, and then pumped into the sump. Water in the sump is then sent to various parts of the life support system just like a heart pumps blood to different parts of the body. Some of the water is sent to a chiller, which is used to cool the water, and some is also returned to the jellyfish display tank. And still some is sent through mechanical, chemical, and biological filtration, and then re-circulated back to the sump. All of these components together help not only to maintain excellent water quality, but also make our work much easier. We carry out small water changes and scrub the tank for algae on a weekly basis, in addition to maintaining our brine shrimp colony to feed to the jellies. As you can see, keeping jellyfish is a lot of work! If you would like to learn more about jellyfish husbandry, there is a wonderful book written by the senior jellyfish aquarist at Monterey Bay Aquarium. How to Keep Jellyfish in Aquariums: An Introductory Guide for Maintaining Healthy Jellies is written by Chad L. Widmer.

So the next time you visit McWane Science Center, be sure to make a stop in the World of Water to check out our jellyfish tank. Keep an eye out for the next blog from the World of Water- all about Buddy the moray eel!

Contributor Profile:
Loretta and Adele's backgrounds are in the life sciences. They both love their jobs as well as all animals. Adele grew up here in Birmingham and is into yoga and gourmet cooking. Loretta grew up in Florida and just had her first child YESTERDAY!