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, with some anchored to the seabed. Jellyfish have an umbrella-shaped body called the bell, made of mesoglea, and trailing tentacles armed with stinging cells.

These stinging cells help jellyfish capture prey and defend against predators. They move through the water by pulsating their bell. Jellyfish have a complex life cycle, starting as planula larvae that settle as polyps before becoming adult medusae.

Jellyfish live worldwide, from surface waters to the deep sea. Most species grow quickly, mature in a few months, and then die after breeding, but their polyp stage can live much longer. They have existed for at least 500 million years, making them one of the oldest multi-organ animal groups.

In some cultures, jellyfish are eaten by humans, especially in parts of Asia, where they are prepared as a delicacy. They are also important in molecular biology research, particularly for the green fluorescent protein they use for bioluminescence. While jellyfish stings can sometimes injure people, they are generally more of a curiosity than a danger.

Names

The name "jellyfish" has been used since 1796 to describe medusae and similar animals, including the comb jellies (ctenophores). The terms "jellies" or "sea jellies" were introduced later by public aquaria to avoid using the word "fish," which people often think 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, with over 200 known types of Scyphozoa alone.

Anatomy

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

Underneath the bell is a stalk-like structure called the manubrium, with a mouth at its tip that also functions as an anus. The mouth opens into a gastrovascular cavity where digestion happens. This cavity is divided into sections by thick septa, and there are gonads attached to these septa. Gastric filaments armed with nematocysts help subdue and digest prey.

Box jellyfish have a squarish, box-like 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 resemble polyps.

Most jellyfish do not have specialized systems for osmoregulation, respiration, or circulation, and they lack a central nervous system. They have a nerve net in the epidermis that helps them detect stimuli and transmit impulses. Some jellyfish can see light and darkness with simple eyes called ocelli, while box jellyfish have more advanced vision with 24 eyes, including two that can see color. These eyes help them navigate their environment and avoid obstacles.

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 an interesting life cycle that includes both sexual and asexual stages. They start as eggs fertilized by sperm, which develop into tiny larvae called planulae. These larvae settle on surfaces and grow into polyps, small stalk-like forms. Polyps can reproduce asexually by budding, creating new polyps or tiny jellyfish called ephyrae that swim away and grow into adults.

Jellyfish are very efficient swimmers, moving by pulsing their bell-shaped bodies. This motion pushes water behind them, helping them glide through the ocean with minimal energy. They can travel farther on each pulse than many other animals, making them superb travelers of the sea.

Ecology

Jellyfish are mainly carnivorous, eating small fish, plankton, and other jellyfish. They catch their food using their tentacles, which have special cells that can stun or capture 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 the balance of 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 have been eaten in many parts of the world for a long time. Some places, like China, Japan, and Korea, enjoy eating jellyfish as a special food. They are usually dried and prepared in different ways before eating.

Jellyfish are also important for science. A special glowing protein from jellyfish helped scientists study genes in cells and even whole animals, earning some researchers a Nobel Prize. Many public aquariums display jellyfish because they are beautiful and interesting to watch. However, some jellyfish can sting people, and while most stings are not serious, a few can be very dangerous.