Strictly speaking, jellyfish are not fish. But they live in the sea like fish and look like jelly. So these two words are stuck in a single name. Also, jellyfish don’t have brains. They also have no heart or lungs.
People often search strange animals in the depths of the oceans or in the most remote corners of the earth. But jellyfish, those strange creatures, anyone can come across. How can they live when they don’t have any of the organs that are most important to us humans?
From a zoological point of view, jellyfish Related to hydra, sea anemones, sea pens and corals. They are free-swimming marine animals that have a distinct umbrella-shaped upper body, followed by tentacles following. Some of them also live anchored on the seabed.
But why do jellyfish have this surprisingly simple anatomy?
Modern jellyfish have distinct organs. Beneath their umbrella-shaped bell that appears hollow to an outside observer is a transparent jelly-like substance known as mesoglea. This structure is the basis of the hydrostatic skeleton of this animal. 95% and even many mesogleas consist of water. The remaining 5% is mainly composed of protein.
Jellyfish fossils from the Jurassic period, found in Germany. Image credit: James St. John over WikimediaCC-BY-2.0
In fact, jellyfish are one of the oldest groups of animals. Scientists have found fossils of jellyfish-like animals that are between 635 and 577 million years old. It is not known exactly whether these animals are the true ancestors of modern jellyfish. However, it is thought that jellyfish have existed for at least 500 million years and possibly even 700 million years or more.
Since then, jellyfish have not changed much in appearance and they still look very similar to their ancient relatives with a very simple anatomical structure.
So where are the jellyfish’s organs located?
It is true that jellyfish have no brains or hearts. But they have a simple nervous system that forms a radial network through the body, in different layers. Although relatively simple in appearance, these nerves perform a variety of functions: they can sense light, gravity, temperature, and various chemical compounds.
Structure of a jellyfish. Image credit: Mariana Ruiz Villarreal via Wikimedia, Public Domain
On the underside of the bell, there is a central protruding structure that functions as a mouth. Interestingly, the jellyfish’s mouth also functions as an anus. Above the mouth is a structure called the gastric cavity – here, digestion takes place and nutrients are absorbed.
Jellyfish also have roundworms – blast cells contain a specialized organelle. This organelle contains a coiled hollow tubular structure filled with venom. When activated, this tube is ejected and pierced through the prey. Nematode cells are mainly located on the tentacles.
This nematode is from the pale anemone species. But in jellyfish, its structure is very similar. The red staining shows the polyanionic venom proteins found within. Image credit: David D. Brand via Wikimedia, Public Domain
Jellyfish do not have a respiratory system. And they don’t need it, because a sufficient amount of oxygen is absorbed through their top layer called the epidermis.
Although they have no brain and no central nervous system, the neural network inside their bodies can be thought of as the basis of a rudimentary nervous system. This mesh controls the jellyfish’s bodily functions, such as detecting stimuli (eg, presence of light, salinity levels), signaling, and controlling swimming mechanics.
