Jellyfish

Jellyfish, also known as sea jellies or simply jellies, are the medusa-phase of certain gelatinous members of the subphylum Medusozoa, which is a major part of the phylum Cnidaria. They are mainly free-swimming marine animals. Jellyfish have an umbrella-shaped body called the bell, made of mesoglea, and trailing tentacles with tiny stinging cells.

These stinging cells help jellyfish catch food and stay safe from other animals. They move through the water by pulsing their bell. Jellyfish have a complex life cycle, starting as tiny planula that grow into polyps before becoming adults.

Jellyfish live everywhere in the world's oceans, from the surface to the deep sea. Most types grow fast, become adults in a few months, and die after having babies, but their younger stage can live much longer. They have been around for at least 500 million years.

In some cultures, jellyfish are eaten by humans, especially in parts of Asia. They are also important in molecular biology research, particularly for the green fluorescent protein they use for bioluminescence. While jellyfish stings can sometimes hurt people, they are usually more interesting than dangerous.

Names

The name "jellyfish" has been used since 1796 to describe medusae and similar animals, including the comb jellies (ctenophores). The words "jellies" or "sea jellies" were added later by public aquaria. They did this to avoid using the word "fish," because people often think "fish" means an animal with a backbone. Even though shellfish, cuttlefish, and starfish do not have backbones, they are still called "fish" in everyday language. In science, "jelly" and "jellyfish" are often used to mean the same thing, but some experts only call scyphozoans "true jellyfish."

A group of jellyfish is called a "smack" or a "smuck."

Mapping to taxonomic groups

The term jellyfish refers to a life stage in certain sea creatures called medusae, which are part of a group known as Medusozoa. These jellyfish usually swim freely in the ocean and are known for their umbrella-shaped bodies and trailing tentacles.

Jellyfish belong to a larger group of animals called Cnidaria, which includes creatures like corals and sea anemones. The main groups of jellyfish are Scyphozoa (often called true jellyfish), Cubozoa (box jellyfish), Hydrozoa (small jellyfish), and Staurozoa (stalked jellyfish). Each of these groups has special features that help them survive in the ocean. There are many different species of jellyfish.

Anatomy

The main feature of a true jellyfish is the umbrella-shaped bell, made of a jelly-like substance called mesoglea. This bell is covered by the epidermis on the outside and the gastrodermis on the inside. The edge of the bell often has rounded parts called lappets and dangling sense organs known as rhopalia.

Under the bell is a stalk-like part called the manubrium, with a mouth at the tip. The mouth opens into a gastrovascular cavity where digestion happens. This cavity is divided into sections, and there are gonads attached to these sections. Some parts have tiny stingers called nematocysts to help catch food.

Box jellyfish have a box-shaped bell with tentacles attached to stalks called pedalia. They can swim faster than true jellyfish due to a folded rim called a velarium. Stalked jellyfish are attached to surfaces by a basal disk and look like small plants.

Most jellyfish do not have special systems for osmoregulation, breathing, or circulation, and they lack a central nervous system. They have a nerve net in the outer layer that helps them detect things and move. Some jellyfish can see light and darkness with simple eyes called ocelli, while box jellyfish have more advanced eyes that can see color. These eyes help them move and avoid things.

Largest and smallest

Jellyfish come in many different sizes. Some of the smallest jellyfish, like those in the genera Staurocladia and Eleutheria, have bell disks only a few millimeters across. These tiny jellyfish move across seaweed or rocky surfaces and can often only be seen with a magnifying glass.

On the other hand, some jellyfish are very large. The lion's mane jellyfish, Cyanea capillata, has tentacles that can stretch up to 36.5 meters long. Another giant is the Nomura's jellyfish, Nemopilema nomurai, found in the waters of Japan, Korea, and China. These can have a bell diameter of up to 2 meters and weigh as much as 200 kilograms.

Life history and behavior

Jellyfish have a special life cycle with different stages. They begin as eggs joined by sperm, which grow into tiny creatures called planulae. These planulae attach to surfaces and turn into polyps, small stalk-like shapes. Polyps can make new polyps or tiny jellyfish called ephyrae that swim away and become adults.

Jellyfish are good swimmers, moving by pushing their bell-shaped bodies. This motion sends water behind them, letting them float through the ocean easily. They can go far with each push, making them great travelers of the sea.

Ecology

Jellyfish mainly eat small fish, plankton, and other jellyfish. They catch their food with their tentacles, which have special cells that can stun or grab prey. Some jellyfish can also get food from tiny plants living inside them.

Jellyfish sometimes become very numerous, forming large groups called blooms. These blooms can change life in the ocean by eating many small fish and competing with other animals for food. They are also important because they provide food for other sea creatures. Most jellyfish live in saltwater, but a few types can be found in freshwater.

Relation to humans

Jellyfish are eaten in many parts of the world, like China, Japan, and Korea. They are often dried and prepared in different ways before eating.

Jellyfish are also important for science. A glowing protein from jellyfish helps scientists study genes. Many aquariums show jellyfish because they are beautiful and fun to watch. Some jellyfish can sting people, but most stings are not serious.