In Greek mythology, Cassiopeia was the beautiful and vain wife of Cepheus, an Ethiopian king. Cepheus and Cassiopeia had a daughter, Andromeda, who was to be wed to the hero Perseus. Cassiopeia, at the last minute, decided to promise Andromeda to Agenor, a son of Poseidon.
Agenor arrived at the nuptials with an army, intending to kill Perseus and take Andromeda for himself. If you remember, Perseus is the guy who slayed the Gorgon. As Agenor’s army approached, Perseus retrieved the Medusa’s head and turned the army, including Cassiopeia, into stone. Poseidon decided to place Cassiopeia, along with Cepheus, in the heavens as constellations. However, as punishment for her vanity he placed her in the night sky sitting in a chair that revolved around the North Star so that she would spend half of all eternity upside down.
Cassiopeia is also the scientific name of a common jellyfish found in shallow bays and mangrove forests of Florida, Bermuda and the Gulf of Mexico. Like its namesake, this animal spends much of its time upside down. In fact, the common name for this animal is the upside-down jellyfish.
Jellyfish, in general, feed on smaller animals and plankton, capturing and killing the prey with stinging cells on the tentacles called nematocysts. Although the sting of these animals affords it protection as a side effect, the main reason for a jellyfish’s ability to sting is for food gathering. As I mentioned in a few past diaries, tropical seas are mostly barren of planktonic life, which explains why these areas are famous for having such clear, blue waters. Animals, such as jellyfish, that rely on a steady supply of planktonic life for survival have a difficult time surviving in these seas without some help.
So Cassiopeia xamachana (which translates to "upside-down jellyfish of Jamaica") has done what many other filter feeding organisms do under these conditions: Like many species of corals, and even clams, the upside-down jelly recruits symbiotic algae to provide additional nutrients. This algae is known as zooanthellae, and live within the host’s tissues. Like all algae, zooanthellae are photosynthetic, using sunlight, carbon dioxide and water to produce oxygen and carbohydrates. When this process occurs within another animal’s tissues, the host is able to utilize these by-products for their own respiration and nutrition.
It’s pretty obvious why we only find this species of cnidarian (a group of animals that include jellyfish, sea anemones and corals) in shallow waters. Sunlight is filtered out pretty rapidly by seawater, so the closer the jellyfish is to the surface the more sunlight is available to its symbionts. Although the zooanthellae are found throughout the jelly’s body, it is most highly concentrated in the tentacles. This explains why it spends so much time upside-down. While lying on the bottom it is often mistaken for a sea anemone, which are close relatives of jellyfsh and have a body plan that is like a jellyfish turned upside-down, with the base of the body on the ventral surface and food-gathering tentacles on top.
This sea anemone is built like
an upside-down jellyfish.
Cassiopeia does have stinging cells lining the tentacles just like other jellyfish do. And these tentacles do sting, although the effect on humans usually amounts to a just a minor rash. What little plankton and fish larvae the tentacles manage to catch will supplement the nutrients provided by the symbiotic algae. Although these nematocysts, as mentioned above, are used primarily for food gathering, this species has evolved a unique defense against predators. Normally the tentacles need to be physically touched in order to trigger the stinging cell into releasing its toxin. Cassiopeia, when disturbed, will release a cloud of nematocysts into the surrounding water, actually ejecting the cells from their bodies. These cells float around in the water like little underwater mines and explode upon contact. A person walking through a colony of these animals will suddenly be surrounded by thousands of toxic little cells detonating all around them. For someone sensitive to jellyfish poison, it's not a pleasant experience.
Also like other species of jellyfish, Cassiopeia is capable of swimming by pulsating its bell (the main umbrella-shaped part of the body), but this is normally only done when the animal is disturbed. It will swim to another location and then flip itself over and settle back down to the bottom.
Cassiopeia reproduce using a process known as strobilation. Although I won’t go into this process here, I did put together a whole diary describing this process called Life Cycle of the Moon Jelly. You might want to read that one since this life cycle is fascinating. One way that the upside-down jellyfish differs from nearly all other strobilating species is that this process occurs over the summer, rather than during the winter. This brings us to another unique characteristic of this species, which is an unusual tolerance to temperature extremes. This is a requirement for any animal that lives in shallow tropical estuaries since the temperature of these waters can vary greatly, not only with the seasons but also within a twenty-four hour period. Not many other jellyfish can survive near freezing nights followed by an overheated lagoon during the day.
Cassiopeia differs from most other jellyfish in another important way. As you can see on the moon jellyfish above, the bell is convex. This shape helps to both increase the surface area of the inside of the bell to increase the amount of water held within the bell cavity during jet propulsion, and also helps to streamline the body to enable it to move through the water more efficiently. By contrast, the upside-down jelly’s bell is concave (when viewed from the top of the body). This hampers the swimming ability, but since this animal rarely swims, this isn’t such a big deal. What this shape does help the jellyfish do is stick to the bottom. The concave shape forms a mild suction cup on the sand, helping to prevent the animal from getting tipped right-side up by waves or currents.
You’ve all heard of the damage done to reef ecosystems worldwide, a phenomenon known as coral bleaching. As mentioned, corals also contain symbiotic zooanthellae within their tissues, but corals are even more dependent on this algae for nutrition than these jellyfish are. Bleaching occurs when water conditions, such as temperature or pollution, kill off the zooanthellae, leaving the coral animals without a food source. More on coral bleaching in a later diary.
Other diaries in this series can be found here.