Physics in the animal world: cubomedusa and their eyes

Tripedalia cystophora. Source: DOI : 10.1371 / journal.pone.0098870

Many of us consider jellyfish to be the most primitive creatures that are not too different in structural complexity from the mucus that they look like. But it is not so. Jellyfish are well-organized creatures, there are many types of them, among which there are simply amazing creatures. For example, a Portuguese boat - this species has evolved for a long time, before taking on a modern form. True, a boat is not exactly a jellyfish, but a species of colonial hydroid from the siphonophore squad, whose colony consists of polypoid and jellyfish. Scientists also know giant jellyfish, the appearance of which nearby causes a thrill in humans.

But there is an even more amazing view of jellyfish, which, at first glance, is no different. We are talking about cubo-jellyfish. It is enough to take a closer look, and it will immediately become clear that even the behavior of this jellyfish is different from the behavior of other cnidarians . And one of the organs is completely radically different from everything that other species of these living organisms can boast of.



Cubomedusa, or box-shaped jellyfish swim in a special way. In water, their behavior is very active, it is more reminiscent of the behavior of fish than jellyfish. It is both a purposeful movement in a certain direction, and the manifestation of a quick reaction to certain stimuli. This is a small group of Cnidaria, which includes about 20 species inhabiting tropical and warm temperate seas with oceanic salinity.

Bell structure

Upon closer examination, it becomes clear what is the matter. They are distinguished by the shape of the bell, which has a rectangular shape in cross section. The inner space is limited only by the muscular fold of the edge of the bell. The movement of the jellyfish is determined by the rapid contraction of the musculature of the bell, as a result of which quite a lot of water is ejected from under the dome by a powerful jet, which pushes the jellyfish in the opposite direction. It is interesting that the musculature of the bell allows not only to strengthen the jet stream, but also to direct it in different directions.

Jellyfish eyes

But the most interesting thing is not even the principle of jet propulsion of these jellyfish, but their sensory apparatus. The fact is that each of the four sides of the dome is “equipped” with a sensory organ called “ropalia”. This system looks like a bunch of eyes of different sizes. And these are actually eyes, not just eye-like outgrowths. Two types of eyes look especially similar to the eyes of higher animals in cube jellyfish . Previously, scientists did not understand what the purpose of these organs was, although they tried to clarify the function of each. Not so long ago, experts found out that these are really eyes, although somewhat specific. The fact is that the image obtained by the eye is not fixed accurately on the retina.

And this is hardly a flaw, an omission of nature. The fact is that at the eyes of the cube jellyfish it has an accurate gradient of the refractive index, which allows the jellyfish to obtain a practically non-aberrational image. According to some experts, this once again confirms the idea that some “inventions” in nature appear in very different species as a result of parallel evolution, without any kindred relationship.

The eyes of jellyfish have a number of characteristics that make them look like the eyes of much more complex animals and humans. So, here you can highlight the cornea, lenses, retina, pigment layer and iris. The lenses are very small, their diameter is about 0.1 mm, they are symmetrical. They allow jellyfish to get a good “picture” of the surrounding space, although, as mentioned above, there is a certain gradient of the refractive index. This feature of jellyfish is due to the unusual packing density of special crystalline proteins.

Experts believe that the feature of focusing the eyes of jellyfish allows them to remove small image elements from the retina. Such a mechanism, at times, is found in the visual apparatus of insects. Possibly, box-shaped jellyfish see large fixed or inactive structures, but they do not see small particles passing by them over the course. Unfortunately, now it is not known for certain how the image obtained with the help of such eyes is processed in jellyfish.


Source: Nature

It is possible, scientists say, that information is processed in the eye, so the animal does not need a complex brain. In box-shaped jellyfish, complex information processing is performed earlier than in most visual systems. In such animals - four different types of eyes, most likely, each type performs a certain function. This, according to scientists, is one of the earliest examples of the evolution of the visual systems of animals. The eyes of jellyfish, despite the complexity, are not universal, but perform specific tasks, these animals do not need high visual activity, as in higher animals.

Susan Milius(Susan Milius) in one of his works published in the journal Science News says that the eyes of box-shaped jellyfish, cephalopods, and vertebrates, as far as can be judged, appeared independently of each other. As mentioned above, this is a great example of parallel evolution.