Corals, Jellyfish and Sea Anemones

Corals, Jellyfish and Sea Anemones

Corals, Jellyfish, Sea Anemones and their relatives are belongs to a large group called the cnidarians. Jellyfish, Sea anemones and hydra- like animals called hydrozoans also belong to this diverse group of mostly marine invertebrates, which includes more than 9,000 species.

If we talk about hydras, then they are a group of very simple aquatic animals that are related to corals, sea anemones and jellyfish. They are usually just a few millimetres in length and are best studied under a microscope. It is interesting to know that, this disparate group is unites by special cells, called cnidocytes and these special cnidocyte cells play an important role to capture their prey. Cnidarians are either sessile (non-moving) organism, known as polyps, or free- swimming medusa, which are comprised of a mouth surrounded by a ring of tentacles that bear the cnidocytes. They use these stinging cells to trap a wide range of prey, from microscopic plankton to small fish. Cnidarians are prey to a similarly wide- range of predators, from starfish and sea slugs to reef fish and turtles.

The classification of the group is open to debate, however, and many biologists recognize separate groups, such as the staurozoa (stalked jellyfish). 

There are four main groups of cnidarians:-

1. The Anthozoa: which includes corals, sea anemones and sea pens.

2. The Scyphozoa: Jellyfish.

3. The Cubozoa: Box Jellyfish.

4. The Hydrozoa: Hydra and relatives such as the infamous Portuguese man of war.

Cnidarians have been around for millions of years. The oldest fossils date back to more than 580 million years ago, during a period in Earth`s history called the Precambrian era. Experts think that corals first appeared around 490 million years ago. Current thinking suggests that they developed from calcareous sponges from the group calcarea, although they are also linked to comb jellies (called thus because they are covered with ‘combs’ of hair-like projections called cilia that beat synchronously and help the animals swim) from the group Ctenophora. What distinguishes them from most other animals is the dact that they are radially symmetrical, which means they have a top and bottom, but no front and back.

Corals

Corals are exclusively marine animals that live in colonies of small, identical units called polyps. The coral you can see develops from one polyp, which divides again and again to form the colony and the visible coral. Corals come in two main types:

1. Hard: Hard corals are made up of a hard limestone skeleton, which they use to build up enormous reefs in tropical waters, such as the Great Barrier Reef off the Coast of North- Eastern Australia. Reef building corals reproduce in a process called ‘broadcast spawning’ when all the coral polyps in a reef simultaneously release eggs and sperm into the water. Zoologists have noticed that the time when corals spawn broadly coincides with the time of the full moon. No one really knows why this happens, but it may something to do with the tides or light conditions.

2. Soft: Soft corals are more plant like in appearance and lack the limestone skeleton of hard corals. Each soft coral polyp has eight tentacles (as opposed to the 12 tentacles of hard corals) that surround the human mouth and the colonies form an amazing range of different shapes and sizes. For example, the polyps of the grooved brain coral form a wrinkled mass that resembles the human brain.

Jellyfish

Around 300 species of jellyfish form a group of similarly simple invertebrates that drift along in ocean currents in search of food. The main body, or bell, of these animals consists of a mass of jelly like material that surrounds the opening into gut. They use their stinging tentacles to paralyze prey and draw it into the gut. Any undigested food passes back out through the same opening. Jellyfish have developed a rather novel way of getting around- Jet Propulsion. When a jellyfish wants to move, it relaxes the opening into the gut and fills with water. When the body is full, muscles in the bell contract and push the water through the opening to the gut. The energy of the water jet then pushes the jellyfish forward in the opposite direction.

Sea anemones

Sea anemones are often called ‘sea flowers’ because they resemble tiny plants on the rocks of tidal pools or shallow waters. Indeed, these exclusively marine invertebrates are named after anemones, a group of around 120 species of flowering plants. Like plants, sea anemones do not move too much. Instead, the sea anemone uses its sticky foot to anchor its soft body onto a rock or other hard surface. The cylindrical body opens up into a central disk, or mouth, which is surrounded by stinging tentacles. Stinging cells in the tentacles shoot out when passing prey touches the trigger hair. The cells inject venom into the unfortunate victim and the immobilized victim is then drawn into the mouth. Some animals, notably the clownfish, have developed immunity to the anemone`s stinging tentacles. Clownfish make their homes in the tentacles, protected by slimy mucus that covers their bodies.

Reference

1. Britannica Reference Encyclopaedia 

2. Knowledge Encyclopedia Animal!: The Animal Kingdom as you've Never Seen it Before

3. Images are taken from Zoo-Que and Google.

4. In Defense of Plants: An Exploration Into the Wonder of Plants

5. A Naturalist’s Guide to the Birds of India

6. Earth: A Sustainable Planet

Also See

• Migration & Navigation of Birds

• Archaeopteryx – The First Bird`s Fossil

• Parrots 

• Bats

• Bears

• Woodpecker 

• Birds

• Flightless Birds 

• Corals, Jellyfish and Sea Anemones

• Butterflies, Bees and Ants 

• Starfish and member of Echinoderm 

• Jawless Fish

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