Do Jellyfish Have Eyes?

Although we cannot attribute eyes to them in the human sense, certain types of jellyfish have specialized structures called rhopalia. These are made up of two types of organs known as statocysts and ocelli:

• Ocelli: small photosensory organs that play a key role in regulating the depth at which jellyfish will swim. These organs are sensitive to light intensity and allow them to adjust their position in the water. Ocelli appear as brown, black or reddish spots. Although they are less complex than vertebrate eyes, they have a similar function in light perception. In most cases, the ocelli consist of a mass of pigmented cells associated with nerve cells that are generally located in the basal part of the tentacles. In a more advanced stage of differentiation, the ocelli may present lenses. It should be noted that not all jellyfish have eyes.
  
  
• Statocysts: fluid-filled balance organs that provide jellyfish with information about their spatial orientation in the water. These organs are crucial for them to maintain their proper position and movement in the aquatic environment. It is a more basic visual system than many other animals, but it still constitutes jellyfish vision.

Although jellyfish do not have eyes like those we know of in other animals with more complex morphologies, their rhopalia with statocysts and ocelli constitute specialized structures that allow them to regulate their position in the water. Since they perceive the intensity of light, it means these jellyfish have vision. Despite their simplicity, these structures demonstrate the amazing adaptation of jellyfish to the marine environment.

The number of eyes in jellyfish varies depending on the species and the type of jellyfish. In particular, box jellyfish have 24 eyes. These are grouped into 4 structures in rhopalia organs which are located on the lower edge of the bell (the main part of the jellyfish's body). In each rhopalium are two complex eyes with lens, cornea and retina, as well as two pairs of simpler eyes known as ocelli.

The vision of cubozoans or box jellyfish has been the subject of scientific study. Research indicates they are capable of forming images and reacting to shadows, colors and shapes. It is believed this helps box jellyfish to inhabit complex environments such as coastal areas, kelp forests, coral reefs and mangroves. Some species exhibit complex courtship and mating behaviors and attraction to light. However, the ecology of Cubozoans is still largely unknown due to their high spatiotemporal variability, their rarity and the difficulty of their detection.

Research has revealed that a species of box jellyfish known as Tripedalia cystophora contains a single type of opsin. This is an optical pigment, meaning its vision is monochromatic. T. cystophora uses its lower eyes to locate obstacles in order to swim more freely. In addition, it can perceive what swims under the foliage of the mangroves in which it usually lives, detecting the small crustaceans on which it feeds.

Now let's find out which types of jellyfish have eyes.

We have already explained that not all types of jellyfish have eyes. Those that do include:

• Bougainvillia fulva: common in coastal waters of the tropical Pacific and Indian Oceans. It has small, elongated ocelli and is located at the base of the marginal tentacles.
  
  
• Halitholus intermedius: it is a subantarctic species observed in the Malvinas Islands, in South Africa and in Chile that has a bell-shaped body and whose ocelli are located in the basal bulbs of its tentacles.
  
  
• Halitholus pauper: it is an arctic species that has a barrel-shaped body and whose ocelli are located at the base of the tentacles.
  
  
• Leuckartiara zacae: commonly observed in the Gulf of Panama, the Galapagos Islands, Chile and on the Mexican coast, it is a species that is dome-shaped and whose entire tentacles have ocelli.
  
  
• Dipleurosoma pacificum: a flattened umbel jellyfish that has been observed north of Tahiti, an island in French Polynesia, in the south of the Pacific Ocean, and in the northwest of the Galapagos Islands. Their ocelli are located at the base of most of their tentacles.

Now that you know whether jellyfish have eyes, don't miss these our related article on the different types of Mediterranean jellyfish.

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