Marvelous Medusae

The first picture most people have in mind when they think of jellyfish is probably a gelatinous, transparent mass being flushed on the beach or a pink creature buzzing around in the popular animated TV series "Spongebob". Unfortunately hardly anyone knows what jellyfish really are, how they live, eat and reproduce. As some of you might know, I have chosen a lot of elective subjects this semestre in order to obtain the required amount of ECTS, and one of these subjects is zoology. I would love to give you a more scientific view on these mysterious creatures.

Let's start with the classification. Jellyfish actually aren't true fish, they are invertebrates. Like hydro polyps and corals, jellyfish are part of the phylum Cnidaria. The term for true jellyfish is "medusa". Medusae only have one single body orifice which opens into the gastrovascular cavity where digestion takes place and nutrients are absorbed. They do not need a respiratory system since their skin is thin enough that the body is oxygenated by diffusion. The only body orifice, the "mouth", which is located in the centre of the underside of the umbrella, is surrounded by stinging tentacles, which are used to catch prey and to defend themselves. Jellyfish sting their prey using nematocysts. These stinging structures are located in specialized cells called cnidocysts, which are characteristic of all Cnidaria. When a nematocyst is triggered by contact by predator or prey, pressure builds up rapidly inside it until it bursts. A lance inside the nematocyst pierces the victim's skin, and poison flows through into the victim. 

Medusae do not have a central nervous system, therefore they do not have a brain, but they have nervous cells distributed all over their body, which form a "nerve net". This "nerve net" allows the jellyfish to detect various stimuli in its environment e.g. the touch of other animals.

Jellyfish are found in every ocean, from the surface to the deep sea. Some species even live in freshwater, like the Cassiopea (upside-down jellyfish), which is found in warmer coastal regions around the world, including shallow mangrove swamps where it usually lives upside-down on the bottom. Depending on the species, the body contains between 95 and 98% water.

Reproduction

The reproduction might be the most fascinating and unusual fact about Jellyfish because it involves several different stages. In the adult, or medusa stage of a jellyfish, they can reproduce sexually by releasing sperm and eggs into the water, forming a planula. In this larval stage, the planula hooks on to the bottom of a smooth rock or other structure and grows into another stage of jellyfish life, the polyp, which resembles a miniature sea anemone. During this stage, which can last for several months or years, asexual reproduction occurs. The polyps clone themselves and bud, or strobilate, into another stage, called ephyra. This form grows into the adult medusa jellyfish.