What is Jellyfish?
The Jellyfish, also known as ‘Sea Jellies’, are beautiful organisms with fascinating colours, structures, and shapes. They are soft-bodied bizarre blobs which keep drifting eerily in the waters.
The anatomy of Jellyfish is quite simple, yet they are one of the most interesting, intimidating and intriguing marine life creatures. As beautiful as the Jellyfish is, it is also on the living species to unleash one of the deadliest venoms on the earth. This makes them savage predators underwater.
It might surprise you that jellyfish inhabited the oceans even before the dinosaurs existed. They have been exited for at least 500 million years and are still gracefully flowing around.
They are so old and primitive that they don’t even possess those organs which animals like humans might consider indispensable. For instance, they don’t have a brain to regulate their body. Also, they are devoid of heart.
This makes it surprising how a sea jelly can live, eat, reproduce, sting and survive for so long if they don’t even have a brain in the first place.
Let us find out the secrets behind the mysterious life of Jellyfish.
Content: Do Jellyfish have Brain?
Do Jellyfish have Brain?
Yes, it’s absolutely true that the sea jellies are deprived of their brain and still, they perform all the necessary tasks which helped them survive for so long. Now the question arises how do they do these things without a brain?
Although they don’t have brains, jellyfishes still possess a bundle of neurons known as ‘Nerve Nets’. They are responsible for sending signals throughout their body. In easy words, you say that instead of a single centralised brain, sea jellies comprise a net of nerves.
And since they lack a centralised nervous system, they accomplish many tasks with the help of touch receptors. These receptors are present beside their mouth and tentacles.
The neurons help them to notice small changes, which enable them to catch their prey effectively. This may include:
- Shift in water’s chemistry (Change in salt concentration)
- Smells
- Motion of prey
The nerve nets serve the role of sensory organs that recognise salinity, touch, salinity, temperature, gravity and light. For instance, they have light-sensitive cells called ‘Rhopalia’ present along the ridge of their bells. In some sea jellies, there are eyespots within Rhopalia that detect light.
Jellyfish Phylum
These organisms come under the same phylum, “Cnidaria”, as that of corals and sea anemones. They belong to the class Scyphozoa and Cubozoa, around 200 and 20 invertebrate species, respectively.
- Kingdom: Animalia
- Phylum: Cnidaria
- Subphylum: Medusozoa
Characteristics of Jellyfish
Is Jellyfish a Fish?
Contrary to their name, they aren’t really fish. Fishes are vertebrates whose anatomy relies on their backbone.
Whereas, despite their name, the jellyfishes are not a type of fish as they lack backbone or any vertebral structure. And for this reason, we consider them ‘Dome-shaped invertebrates‘.
Instead, they are part of a diverse group of gelatinous zooplanktons. And thus, being zooplanktons, they keep swimming aimlessly.
Jellyfish Sting
The sea jellies don’t mean to sting you, but the stings automatically get activated as soon as something brushes against their skin, including us.
They have stinging cells inside their tentacles known as nematocysts or cnidocytes. A single tentacle may possess thousands of microscopic barbed nematocyst stingers.
Nematocyst contains tiny bulb that holds venom, which helps them capture their prey and protect themselves from danger. Along with the venom bulbs, the stingers also possess coiled sharp, tipped tubes. Any kind of surface contact triggers the release of stingers.
As soon as you come in contact with the stinging cells, you might feel a sudden electric shock and burning sensation accompanied by acute pain. Later after 30-40 minutes, this pain becomes severe, and the affected person might also suffer from the following:
- Nausea
- Dizziness
- Stomach cramp
- Vertigo
- Headache
- Palpitation
- Muscles cramp
- Breathlessness
In addition to the defence mechanism, the sea jellies also use their tentacles as powerful weapons to catch the prey. No sooner the victim comes in contact with the tentacle than the nematocyst release the venom-containing harpoons. The harpoons penetrate the victim’s skin, and the venom paralyses the prey.
There are some jellyfish that can be lethal for humans. For instance, Box jellyfish of northern Australia and the Indo-Pacific can significantly harm humans.
How fast is a Jellyfish?
Even without a developed brain, the jellyfish can produce one of the fastest attacks noted in biology. Even though it looks like. When stinging cells get triggered to strike, hundreds of cnidocytes pop out.
These stinging cells are so quick that they only take just 700 nanoseconds to get fired. That means their acceleration speed is around 5 million g, producing enough force to crush a crustacean shell at its weak point.
What is Jellyfish made of?
It is considered that the simple structure of a jellyfish is the ultimate trick behind such a neat evolution. And it is the reason why they have existed for more than 500 million years by surviving mass extinction. Its structure not only supports them to grow big in size but also supports them to eat more without even paying the cost of high metabolism.
There is nothing much to a jellyfish as its body is 90-95 % water. If you leave it on a beach for some time, it will disappear as soon as the water in it evaporates.
Mostly they are gelatinous with a translucent bell-like head. This bell which is the main body structure of Jellyfish comprises two thin cell layers. This head possesses a soft and delicate material called Mesoglea. Mesoglea is actually 95% water that remains put with the help of protein fragments. It remains sandwiched between two subsequent layers of the epidermis and gastrodermis.
How does a Jellyfish Eat?
The anatomy of the Jellyfish is so simple that it only has a single opening, both as a mouth and anus. They don’t have a well-defined digestive system; instead, a simple digestive serves the role of both the stomach and intestine.
The food is taken in via the hole present underside of the bell. Once the prey enters its mouth and stomach, the enzymes start digesting this food. The nutrients extracted from the digested food get absorbed by the inner layer. And the remaining waste gets excreted through the same single opening.
What does a Jellyfish Eat?
The sea jellies are ‘Passive Feeders’. That means when they eat, whatever happens, comes their way while freely floating in the water.
These gelatinous carnivores rely on small sea creatures like plankton, shrimp, small plants and tiny fishes for their food.
How does a Jellyfish Move?
The Jellyfish lack fins, yet they are the most energy-efficient swimmers. This is because they are capable of contracting and relaxing their umbrella-shaped bell to propel them. This movement of Jellyfish is known as “Jet Propulsion”.
Certainly, they use their muscles to propel up to a shorter distance but still, most of their movement occurs without using muscles. Much of their travelling is passive as it is governed by the water currents. They possess several gravity sensors that help them to orient and navigate in water.
Do Jellyfish Sleep?
Jellyfish might not have the brains to tell them they are tired but apparently, they do have a bedtime. Yes, you read it right; Jellyfish do sleep.
As much as sleep is important for survival, nobody knows when and where it evolved. Since sea jellies are listed very low on the animal family tree, we can conclude that sleep evolved quite early.
Many researchers worked to determine whether the jellyfishes sleep and if they do, then how. For this, they took some upside-down species of Jellyfish. These jellyfishes tend to hang out motionless, where their tentacles face upwards and not downwards.
They researched over 23 jellyfish for almost a week. They were observed to pulse their bells once per second, i.e. 60 times per minute during the daytime. But this pulse slowed down up to approx. 39 pulse per min during the night.
And when they deliberately tried to bring them upwards in the night, they swam back to the bottom within a few seconds. Also, those sea jellies were groggy the next morning due to a lack of sleep. From this, the researchers concluded that the sea jellies enter the sleep state at night.
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