15.3 Cnidarians

Objectives

• Describe the characteristics of cnidarians.• Explain how cnidarians reproduce.• Compare and contrast the two body forms of cnidarians.

Characteristics of Cnidarians

• Cnidarians are common in most ocean habitats, and a few live in freshwater habitats.

• Some cnidarians live attached to a surface and look like flowers or tiny trees.

• Others, like the jellyfish, swim freely in the ocean. • Yet in spite of this diversity, all cnidarians have features in

common.

• Cnidarians have two main body forms, the polyp (PAHLihp) and the medusa (muh DOO suh).

• Medusas swim freely, and polyps usually live attached to a surface.

• Some cnidarians exist only as polyps and some only as medusas.

• Some are medusas for most of their lives after being polyps for only a short time.

• In other cnidarians the two forms are equally important in the life cycle.

• Both cnidarian body,forms have radial symmetry. • They have a top and bottom but no back or front. • This body plan helps cnidarians sense in any direction.

• Cnidarians have several features that make them differentfrom sponges.

• First, cnidarians have a mouth that leads to a digestive cavity. • The mouth is the cavity's only opening.

• Cnidarians also have tissues. • The cells covering the outside of a cnidarian make up one kind

of tissue. • The cells lining the inside are a different kind.• Both contain simple muscle fibers and nerves. • The two layers are separated by jellylike material.

• Cnidarians' most striking adaptation is their tentacles, which are lined with stinging cells.

• Stinging cells help cnidarians defend themselves and catch food.

• When prey is stunned or killed by the stinging cells, the tentacles carry the food to the mouth.

Reproduction of Cnidarians• Cnidarians can reproduce both sexually and asexually.• When it occurs, asexual reproduction is carried out by polyps. • A small piece of the animal breaks off and grows into a new

polyp. • This process is called budding.

• To reproduce sexually, cnidarians release eggs and sperm into the water.

• Each fertilized egg forms a larva.• The larva grows into a new polyp or medusa.

• Many cnidarians have a life cycle that has both a medusa stage and a polyp stage.

• In these cnidarians, it is the medusa that reproduces sexually. • In cnidarians without a medusa stage, the polyp reproduces

sexually.

Diversity of Cnidarians• Cnidarians are divided into three main groups. • Each has a different kind of life cycle. • Within each group there are many shapes and sizes, and

differences in habitat.

• The Hydra is one of the few freshwater cnidarians.

Sea Anemones• One group of cnidarians includes the sea anemones (uh

NEHM uh NEES). • Their scientific name means "flower animals." • They are polyps all their lives.

• Some fish can live safely inside the tentacles of sea anemones.

• The anemones protect the fish. • The fish, in turn, lure other fish that the anemone might eat.

Hydrozoans• This sea fan belongs to a second group called the hydrozoans

(HY droh ZOH uhnz). • A single fan contains many polyps. • Most hydrozoans have both polyp and medusa stages.

Reef-building Corals• Tropical reefs are formed by tiny cnidarians called corals. • Corals are in the same group as sea anemones.

• Reef corals spend their whole lives as tiny polyps living together in colonies.

• They make hard skeletons of calcium carbonate.

Jellyfish• Jellyfish make up the third cnidarian group.• They spend most of their lives as medusas.

Staghorn Coral

• The staghorn coral has a shape very different from that of the reef coral.

• Yet it, too, is made of tiny polyps and a hard skeleton.

Coral Reefs

• Coral reefs are found in warm ocean waters around the world.• They provide homes for a greater variety of living things than

any other place in the ocean.

• The Great Barrier Reef off the coast of Australia is over 2,000 km long.

• Many tropical islands were once coral reefs that have been left dry by changing sea levels.

• The surface of all living reefs is coated with coral polyps, each with its own hard skeleton.

• Below the surface are layers and layers of skeletons left by coral polyps that are now dead.

• The reef grows as new live corals replace ones that die on the surface.

• Algae live inside the polyps and make part of the polyps' food.• The algae also give the coral its color. • Coral must therefore live in shallow water so that their algae

can get enough sunlight.

• In many places where there are coral reefs, scientists are worried that the coral polyps are dying off.

• They are not being replaced by new living corals. • Scientists are not sure why this is happening, but some blame

pollutants in the ocean water.

Check and Explain

• What features do cnidarians have that sponges do not?• 2. In which cnidarian body form does asexual reproduction

occur?• 3. Compare and Contrast - How are the structures of polyps

and medusas alike and different?• 4. Infer - How do you think each of the following help a

jellyfish survive: being transparent, having stinging cells, having radial symmetry, being able to swim?