Ctenophora

Ctenophora

Ctenophora, also known as comb jellies, are a special group of animals that live in the ocean. Their scientific name comes from Ancient Greek words for "comb" and "to carry" because they move in a unique way using tiny hair-like structures called cilia, arranged in combs. These animals are found all around the world in marine habitats, and scientists have identified many different living species.

Comb jellies are interesting because they are the largest animals that swim using cilia. They can be very small, just a few millimeters, or as large as 1.5 meters (about 5 feet). Their bodies are mostly jelly, with thin layers of cells on the outside and inside. There are many different shapes of comb jellies.

Almost all comb jellies are hunters, eating tiny creatures. Even though their bodies are soft, scientists have found very old fossils that look like comb jellies. Where comb jellies fit in the family tree of all living things has been a big question for scientists.

Etymology

The name Ctenophora means "comb" and "to carry". It is because these sea creatures have special rows of tiny hairs. They use these to move through the water.

Distinguishing features

Further information: Sponge, Cnidaria, and Bilateria

Ctenophores, also called comb jellies, have special sticky cells named colloblasts that help them catch food. But some kinds of ctenophores do not have these cells.

These animals have two main layers of cells with a jelly-like layer between them, like some other simple animals. They are special because they are the largest animals that swim using tiny hair-like parts called cilia. Most comb jellies have eight rows of these cilia that look like combs along their bodies.

Description

Ctenophores, also called comb jellies, are sea animals that move with special rows of tiny hairs called cilia. They live in oceans everywhere, and there are 186 known species. They can be very small, just a few millimeters, or as large as 1.5 meters, depending on the species.

Their bodies are different from other sea creatures. Some near the coast can handle rough waves, while ocean ones are very delicate. Most studies look at three coastal types: Pleurobrachia, Beroe, and Mnemiopsis. Their bodies are almost round but not exactly the same on both sides.

Ctenophores have a jelly-like middle layer with two layers of cells around it. The outer layer has cells that can sense things, make protective mucus, and change into other cells. Special cells called colloblasts help catch food, and other cells have many cilia for moving. The inner layer has a nerve net and cells that work like muscles. Inside, they have a mouth, a throat, a stomach, and canals that move fluids. These canals help with moving food and keeping balance.

Most ctenophores swim with eight rows of combs that go from near their mouth to the end of their body. Each "comb" has thousands of long cilia that move in a wave to push them through the water. Unlike other swimmers, ctenophores can change direction when they need to.

They don’t have a brain, but they have a nerve net that connects parts of their body, especially near their combs, throat, and tentacles. This helps them sense things around them and move. A special balance sensor at the end of their body helps them stay upright.

Ctenophores can reproduce in many ways. Most can grow back lost body parts, and some can join with other individuals. They can be born with both male and female parts, or develop separate sexes. Their young look like adults but grow into their final shape over time. Some can make eggs and sperm before they are fully grown, helping their numbers grow fast.

Many ctenophores are almost see-through, but deeper ocean ones can be colorful. Their rows of combs often shine with a rainbow effect as they move. Many can also make their own light, called bioluminescence, which is usually blue or green and helps them in the dark ocean. This light comes from special cells and proteins that glow when certain things happen.

Ecology

Ctenophores, or comb jellies, live in almost all ocean areas—from cold polar waters to warm tropical seas, and from the surface to deep ocean floors. They live near coasts and far out in the open ocean. Some well-known types are Pleurobrachia, Beroe, and Mnemiopsis, often found close to shore.

Most ctenophores are hunters. They eat tiny animals floating in the water, such as small crustaceans and even other jellyfish. They catch food in many ways, like setting traps or waiting to ambush their prey. Some ctenophores can eat much more than their own body weight in one day if there is plenty of food.

In one famous case, a type of ctenophore called Mnemiopsis leidyi was accidentally carried to the Black Sea in ship water tanks. It multiplied quickly and ate many small sea animals, which hurt fish populations. Later, another ctenophore that eats Mnemiopsis helped control its numbers.

Taxonomy

Scientists aren't sure exactly how many kinds of comb jellies, or ctenophores, exist because some names might describe the same animal. One scientist thinks there are about 100 to 150 true kinds, plus another 25 that live deep in the ocean but haven’t been studied enough to name.

Long ago, people grouped comb jellies with another group called cnidarians because they looked somewhat similar. Both have bodies made mostly of jelly. But comb jellies have two layers of cells, while cnidarians have only one. Both groups use water moving through their bodies to help them digest food and breathe. Neither has a brain, just a network of nerves. Recent studies show that the nerves in comb jellies developed differently from those in other animals.

Today, comb jellies are split into two main groups: those with tentacles and those without. The group without tentacles has just one family with two types of animals. The group with tentacles has eight different orders, including egg-shaped animals with long tentacles, flat animals that live on the sea floor, and ribbon-shaped ones that are the largest comb jellies.

Evolutionary history

Fossils that look like comb jellies have been found from the early Cambrian period, about 515 million years ago. These fossils show comb jellies that were different from today's, with more rows of combs and no tentacles.

Recent studies suggest that the common ancestor of all comb jellies lived around 350 million years ago.