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 186 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, including round ones with tentacles to catch food, flat ones without combs, and big-mouthed ones that eat other comb jellies.

Almost all comb jellies are hunters, eating tiny creatures like larvae, rotifers, and small crustaceans. However, young ones of two species live by attaching to salps, which are their food source when they are adults. Even though their bodies are soft, scientists have found fossils that look like comb jellies from very old rock layers, dating back about 525 million years. Where comb jellies fit in the family tree of all living things has been a big question for scientists, with some thinking they are one of the earliest groups of animals to appear.

Etymology

The name Ctenophora comes from ancient words meaning "comb" and "to carry". This name refers to the special rows of tiny hairs that these sea creatures use to move through the water.

Distinguishing features

Further information: Sponge, Cnidaria, and Bilateria

Ctenophores, also known as comb jellies, have special sticky cells called colloblasts that help them catch their food, although some species do not have these cells.

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

Description

Ctenophores, also known as comb jellies, are sea animals with a unique way of moving using special rows of tiny hairs called cilia. They live in oceans all around the world, and there are 186 known species. These animals can range in size from just a few millimeters to as large as 1.5 meters, depending on the species.

Their bodies are built differently from other sea creatures. Some coastal species are tough enough to handle rough waves, while oceanic ones are very delicate. Most studies have focused on three coastal types: Pleurobrachia, Beroe, and Mnemiopsis. Their bodies are almost round, but not perfectly mirrored on both sides.

Ctenophores have a jelly-like middle layer surrounded by two layers of cells. The outer layer includes cells that can sense things, produce protective mucus, and change into other cell types. Special cells called colloblasts help catch food, and other cells have many cilia for movement. The inner layer has a nerve net and cells that act like muscles. Inside, they have a mouth, a throat, a stomach, and canals that carry fluids. These canals help with moving food and controlling balance.

Most ctenophores swim using eight rows of combs that run from near their mouth to the far end of their body. Each "comb" is made of thousands of long cilia that beat in a wave-like motion to push them through the water. Unlike other swimming animals, ctenophores can reverse their direction when needed.

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

Ctenophores can reproduce in different ways. Most can regrow lost body parts, and some can even fuse with other individuals. They can be born with both male and female parts, or develop separate sexes. Their young look similar to adults but grow into their final form over time. Some can even produce eggs and sperm before reaching full size, allowing their populations to grow quickly.

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

Ecology

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

Most ctenophores are hunters, eating tiny animals floating in the water, such as small crustaceans and even other jellyfish. They have many ways to catch their food, like setting traps or waiting quietly 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 exactly sure how many different kinds of comb jellies, or ctenophores, exist because some names might actually 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 officially.

Long ago, people grouped comb jellies with another group called cnidarians because they looked somewhat similar. Both have bodies made mostly of jelly, with layers of cells on the outside and inside. 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, and 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 as far back as the early Cambrian period, about 515 million years ago. These fossils show comb jellies that were different from the ones alive today, 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, though past estimates placed this event closer to 66 million years ago, right after a major event that changed Earth's life.