Jellyfish, from their translucent bodies pulsating through the ocean currents to their mesmerizing bioluminescent displays, these fascinating creatures seem to possess an otherworldly beauty. However, amidst all the wonder they inspire, one question remains: do jellyfish have eyes?
While it may be easy to assume that such delicate organisms lack this crucial sensory organ, recent scientific discoveries challenge our understanding of these mysterious creatures. In this article, we will unravel the truth behind their visual capabilities and shed light on a subject as captivating as the creatures themselves.
Do Jellyfish Have Eyes?
Most species of jellyfish possess rhopalia, which serve as eyes and aid them in navigating properly. While most species rely on these rhopalias and can only see blurry images and changes in light, box jellyfish, on the hand, have eyes, exactly like other animals, so they have better vision.
Jellyfish are one of the ancient creatures on this planet that still exist; hence they are not fully developed to have a specific organ for each biological function, as in other animals. Instead, they have a strong bundle of nerve chains that allows several activities like sensing light.
Can Jellyfish See?
Most jellyfish species do not have actual eyes or mouths; they only have one opening in their body that serves feeding and excretion. But then comes the question, if they do not have eyes, how can they predate and hunt other small species effectively?
Despite lacking dedicated sensory organs, the jellyfish possesses a remarkable system that enables it to detect the objects near it and even perceive their shapes. They do it with the help of “Rhopalium.”
Rhopalia (singular: rhopalium) are sensory structures that are a combination of several nerves and proteins (opsins) that lies at the edge of a jellyfish’s bell (the main body) and are able to detect light and any changes in the light, like the movement of the light with respect to the gravity.
Interesting Fact: Jellyfish are members of the Cnidaria phylum. So, they can’t scientifically be classified as fish (which belong to the phylum Chordata).
How Many Eyes Do Box Jellyfish Have?
Jellyfish do not have eyes in the traditional sense but have light-sensitive cells called ocelli that can detect the presence of light.
Box Jellyfish have 24 eyes that are divided into 4 types. While two of them are similar to vertebrates’ eyes (like ours), the remaining are primitive in nature and help them perform complex tasks such as navigation, avoiding obstacles, fast directional swimming, and responding to light.
The Box Jellyfish (Tripedalia Cystophora) has 4 rhopalia, each with 6 eyes: 2 lens eyes (like ours and have the capability to form images) and 4 simple eyes (more primitive); for a total of 24 eyes. They are categorized as the upper and lower eyes, the pit eyes, and the slit eyes.
The upper eye is always pointed up, even when the box rotates in different directions. And also one eye at the bottom, enabling jellyfish with an extreme fish-eye view to see almost the entire underwater world.
Interesting Fact: Scientists say that the top eye is used to recognize terrestrial environments like the mangrove canopy to help with direction.
Also Read: Do Box Jellyfish Have Teeth?
How Do Box Jellyfish Eyes Function?
Jellyfish, generally known as carnivorous, feed on fish larvae, fish eggs, crustaceans, tiny fishes, small plants, etc. These creatures do not have a heart or brain, but scientists have discovered that some species have eyes.
Box jellyfish’s (Tripedalia Cystophora) eyes are located on cup-like structures hanging from their cube-shaped bodies (bells). While we have one pair of multi-purpose eyes that sense color, size, shape, and light intensity, box jellyfish have 4 different types of special-purpose eyes.
Whilst the most primitive eyes detect only light levels, some are more sophisticated and can detect the color and size of objects. With these eyes, they can see, but the vision might be a little blurry.
Generally, for eyes, the retina will take an image and transform it into electrical impulses, which are carried to the brain and processed. But as jellyfish do not have a brain, the eyes communicate with each other and with the rest of the jellyfish around a nerve ring that follows the perimeter.
In the presence of light, opsins in jellyfish undergo a sequence of biochemical reactions, culminating in the activation of neurons that enable the jellyfish to react to the light stimulus appropriately.
Scientists have observed that the box jellyfish use their sense of vision while identifying habitats they prefer, such as among the roots of mangroves or while finding prey.
Do Jellyfish Swim?
If jellyfish can’t seem normal, how can they swim? How do jellyfish move?
Jellyfish do not have the complex visual systems as in most animals. Their optical system provides them with very low resolution and slow vision, so they won’t see the details of objects like other animals but rather catch glimpses of moving shadows.
There are different types of species in jellyfish, and they are different. Actually, most species of jellies do not swim. Instead, they float along the ocean’s currents. This is why so many of them wash up on shore.
Unlike normal jellyfish, which drift in the ocean current, Box jellyfish are active swimmers and can rapidly make 180-degree turns. Behaviorally, they are very different from typical jellyfish.
As box jellyfish live in shallow waters where there will be many barriers and obstacles, they need their eyes to navigate in the shallow waters and to stay in areas where they can find food.
A jellyfish’s tentacles are used to drift, float, swim, and even prey on other animals.
Do Moon Jellyfish Have Eyes?
Moon jellyfish do not have eyes, but they have pretty rudimentary ocelli (rhopalia), so it can only detect light and its intensity, but nothing more than that.
Also Read: Do Orcas Have Eyes?
Wrap Up:
Most jellyfish do not have traditional eyes like we see in many animals. They have simple light-sensitive structures called “ocelli” or “rhopalia.” that are located around the edge of the jellyfish’s bell (the main body) and can detect changes in light intensity.
These aren’t capable of forming detailed images as their primary function is to help jellyfish sense changes in light and shadows so that they can respond to their environment. So they are more like light-sensing spots rather than eyes with retina and lens.
But Box jellyfish have a complex set of 24 eyes, some of which are almost similar to the eyes of other vertebrates (normal eyes).
