Without a skeleton, without lungs and blood, the jellyfish is a soft being without tail or head, without right or left, ranked at the beginning of the zoological classification, just after the sponges.
They have neither heart nor brain, and their nervous system is a simple network of cells. The edge of the umbrella carries organs of balance, the rhopalies, and ocelli sensitive to the light. The most evolved species even have eyes, with cornea, lens and retina!
The most evolved jellyfish have 24 very powerful eyes, but not the nervous system that would allow it to use them fully. Its sophisticated lenses only give it a blurred image of its environment
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How do jellyfish see?
Light and shape reach some jellyfish. Simple pigment spots sensitive to light, their rudimentary eyes are located at the base of the tentacles or around the perimeter of the jellyfish’s parasol. This primitive sense is one of the sensory organs of the jellyfish. First of all the organ of balance.
During the movements of the jellyfish, a microscopic grain of limestone moves by weighing on cilia connected to neurons, which thus signal the changes of orientation of the animal.
How many eyes do jellyfish have
The jellyfish has 24 visual sensors including 2 eyes capable of seeing color. Dangerous, it is one of the rare creatures of the sea bed to have a 360° vision, so if jellyfish glide through the sea currents, they have a perfect vision and relentless assets to attract its preys.
Some of the jellyfish’s eyes are relatively simple and serve mainly to capture light. Others, on the contrary, are particularly powerful. They are equipped with crystalline lenses comparable to those of fish and can give a precise image. On the other hand, the image is formed very far behind the retina. The jellyfish has, therefore, a blurred image of its environment.
Moreover, the eyes of this jellyfish offer a wide angle vision, a function usually linked to specific areas of the brain in mammals. However, the brain of jellyfish is primitive. It is a simple nervous network that coordinates the animal’s movements. Researchers must now understand what purpose such sophisticated eyes serve, since the brain does not have the capacity to fully relay information.
How do jellyfish eyes work without a brain?
In jellyfish, photosensitive cells are located at the bottom of small cups and their differentiated excitation allows the animal to define the direction of the light beam or the change of direction, thus a movement and its direction if several receptors are successively stimulated.
However, the movement must be made at a certain speed. The perception is better if the cup can diaphragm the light by closing a little, but then the quantity of light perceived is less and may not be sufficient.
