The Weirdness of Jellyfish

Jellyfish are some of the most intriguing animals of the ocean. They are zooplankton, not fish, yet they are not microscopic. They don’t have a heart, a brain, or blood, and are still able to function normally. They are also able to reproduce asexually and sexually. In addition, they have been on Earth for 500 million years, meaning that they are older than dinosaurs. Here are some more interesting facts about jellyfish, explained in detail.

1.Jellyfish do not have brains.

First of all, how do we define the term brain? A brain is an organ that is the center of the organism’s nervous center. It is able to control all the other organs in an organism’s body. All vertebrates have brains and most invertebrates have one as well. Jellyfish are invertebrates because they don’t have a backbone.

Jellyfish, however, do not have brains, but still have a nervous system comprised of 2 nerve nets. One of the nerve nets is the motor nerve net, which controls the jellyfish’s swimming, and the other one is called diffuse nerve net; it controls every other action and relays information to pacemaker neurons that are in a sensory structure called the rhopalia. Within each nerve net there can be areas of concentrated nerves that can serve as "mini-brains". Another “mini-brain” is the the rhopalia, which has photosensitive structures to help detect light and also gravity receptors so that the jellyfish knows which way is up or down.

2. Jellyfish can reproduce asexually and sexually

A jellyfish’s life cycle is very diverse. First, jellyfish larvae are created when adult jellyfish, called medusae, release eggs and sperm into the water at once to form a planula, or a larvae. The larvae sinks to the bottom of the ocean floor and grows on a rock to form a sea anemone shape.

Above: the lifecycle of a jellyfish

While it is on the bottom of the ocean floor, the polyp divides asexually by forming clones of itself. After a few months or a few years, the small larvae are able to grow into an adult jellyfish and continue the life cycle.

3. One species of jellyfish is “immortal”

The species of the jellyfish is turritopsis dohrnii and it is able to be “immortal” by reverting itself back to its larvae stage when it faces trouble such as physical damage or starvation. It is the only living creature capable of this and is able to do it because of a process called transdifferentiation, when a specialized adult cell is able to revert back to a different type of specialized cell.

Because of this unique ability, turritopsis dohrnii is called the “immortal jellyfish”. It is only “immortal” in a sense because what really happens is that the larvae has the exact same DNA as the previous adult jellyfish; however, it is impossible to know whether the jellyfish’s consciousness is preserved, or if it is a new organism that happens to have the same DNA. By biological standards, it can be considered immortal.

Above: the immortal jellyfish

Turritopsis dohrnii are extremely small (about 4.5 mm when they are an adult) and are bright red at their center because they are transparent and the red is the color of their stomach.

4. Jellyfish can glow using bioluminescence

Left: bioluminescent jellyfish

Bioluminescence is light produced by a chemical process within a living organism. Light is created when luciferin, a compound, is combined with oxygen to produce energy in the form of light. The reaction is also aided with luciferase, an enzyme. To be able to glow continually, animals must acquire luciferin or make it themselves. Jellyfish do not make their own luciferin; they get it from eating crustaceans, which means that jellyfish must have gotten their bioluminescence abilities after they started consuming crustaceans.

Animals use bioluminescence for many reasons, such as luring prey, camouflaging with the surface of the water, escaping from predators, and even to attract mates. It is very common in deep sea creatures, without about 90% of all them being able to use it.

Jellyfish use bioluminescence to defend themselves from predators. They may flash to blind their predator, or release small particles in the water to confuse the predator. About 50% of jellyfish are bioluminescent.

Overall, jellyfish are fascinating creatures and are quite capable, despite having very different anatomy compared to other animals.


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