Cnidarians

What is a cnidarian?What is a cnidarian?•invertebrates

•more than 9000 species

•jellyfishes, corals, sea anemones, and hydras.•worldwide

•all but a few cnidarians are marine

• variety of colors, shapes and sizes

- can be as small as the tip of a pencil.

• two distinct body forms during their life cycles

What are cnidarians?

Polyp

Medusa

• polyp = sessile form of a cnidarian. Its mouth is surrounded by tentacles.

- sea anemones, corals, and hydras.

What are cnidarians?

• medusa = free-swimming form of a cnidarian.

- jellyfish

The Medusa

It possesses an umbrella-shaped, floating body, called a

bell, with the mouth on its underside.

Tentacles

Body FormBody Form

The Polyp

Attached to substrate, the mouth is on the top surrounded by tentacles.

mouth

Body FormBody Form

Body formBody form

In cnidarians, one body form may be more observable than the other.

The polyp is the familiar body form of hydras.

In jellyfishes, the medusa is the body form usually observed.

http://www.masla.com/invert/moon-jellyfish.html

http://www.microscope-microscope.org/gallery/Mark-Simmons/pages/hydra2.htm

Body structureBody structure

• radially symmetrical

• one body opening

• two cell layers

How is this similar to sponges?

Mouth

Cavity

Bud

Disc Outer cell layer

Jellylike layer

Inner cell layer

Tentacle

Body structureBody structure

• cell layers are organized into tissues with specific functions.

- inner layer mainly assists in digestion

It surrounds the GASTROVASCULAR CAVITY

Mouth

Tentacle

Cavity

Bud

Disc Outer cell layer

Jellylike layer

Inner cell layer

Body structureBody structure

• two cells layers allows easy diffusion of:

- Oxygen

dissolved in water, it can diffuse directly into body cells. - Carbon dioxide /other

wastes

moves out of the body cells directly into the surrounding water.

Body formBody form

Most cnidarians undergo a change in body form during their life cycles.

Polyp

Medusa

At some point, most Cnidarians exist as both a polyp and a medusa.

Reproduction in cnidariansReproduction in cnidarians

• sexual and asexual reproduction

- Sexual reproduction occurs in only one phase of the life cycle – the Medusa

IF there is no medusa stage, then the polyp can reproduce sexually.

Male Female

Eggs

Fertilization

Blastula

Larva

Polyp

Asexual Reproduction

Bud

Sexual Reproduction in CnidariansSexual Reproduction in Cnidarians

Both the medusae

and polyps are

diploid animals.

Digestion in cnidariansDigestion in cnidarians

• predators

- capture or poison prey using nematocystsA nematocyst is a capsule that contains a

coiled, threadlike tube. The tube may be sticky or barbed, and it may contain toxic substances.

Nematocysts are located in stinging cells that are on tentacles.

Digestion in cnidariansDigestion in cnidarians

Nematocyst before discharge

Nematocyst after discharge

Digestion in cnidariansDigestion in cnidarians

Once captured by nematocysts, prey is brought to the mouth by contraction of the tentacles.

Mouth

Medusa

Mouth

Gastrovascular cavity

Polyp

Digestion in cnidariansDigestion in cnidarians

• Food enters gastrovascular cavity

• digestive cells release enzymes to break down prey

•undigested materials are ejected back out through the mouth.

Diversity of CnidariansDiversity of Cnidarians

There are four classes of cnidarians:

• Hydrozoa

•Scyphozoa

•Cubozoa

•Anthozoa

HydrozoaHydrozoa

• two groups

- hydroids (hydra)

- siphonophores (Portuguese man-of-war)

• marine animals

• branching polyp colonies formed by budding

•found attached to pilings, shells, and other surfaces.

The siphonophores are floating colonies that drift about on the ocean’s surface.

The Portuguese man-of-war, Physalia, is an

example of a siphonophore hydrozoan

colony.

Each individual in a Physalia colony has a function that helps the entire organism survive.

HydrozoaHydrozoa

AnthozoaAnthozoa

• exhibit only the polyp form.

• Corals

• Sea anemones

• Sea fans

• live in colonies of polyps in warm ocean waters around the world.

• secrete protective, cuplike calcium carbonate shelters around their soft bodies.

Colonies of many coral species build the beautiful coral reefs that provide food and shelter for many other marine species.

Corals

Corals that form reefs are known as hard corals.

• soft corals do not build calcium carbonate structures – not reef builders.

Corals

The living portion of a coral reef is a thin, fragile layer that grows on top of the shelters left behind by previous generations.

Although corals are often found in relatively shallow, nutrient-poor waters, they thrive because of their symbiotic relationship with microscopic, photosynthetic protists called zooxanthellae.

Corals

The zooxanthellae produce oxygen and food that the corals use, while using carbon dioxide and waste materials produced by the corals.

These protists are primarily responsible for the bright colors found in coral reefs.

Corals

Because the zooxanthellae are free-swimming, they sometimes leave the corals.

Corals without these protists often die.

Corals

Corals and Global Warming

• Corals are being threatened by rising ocean temperatures and increasing acidity

• Increasing temperatures stress the zooxanthellae and they leave the corals – this results in coral bleaching

• Without the colorful zooxanthellae, the corals appear white.

• Coral bleaching is reversible, but often does not happen.

• At current rate, 70% of coral reefs will be bleached in the next 20-30 years.

• At present 35 million acres of reef have been destroyed.

Corals and Global Warming

http://www.wri.org/map/coral-reefs-world-classified-potential-threat-human-activities

• Corals provide habitat for 25% of marine fish

• About 1 billion people rely on fish as their primary food source

Corals and Global Warming

• Tourism – over a billion dollars is spent every year in the Caribbean, Australia (3.9 billion/year), and the Pacific Islands

• 1.2 Billion in Florida each year

Corals and Global Warming

• Corals are used for pharmaceuticals

• Chemical extracts from corals have helped create drugs to treat AIDS, Cancer, Arthritis, Inflammatory disorders, and pain killers

• They are beneficial for patients with heart, kidney, and liver transplants.

Corals and Global Warming

• Coral Reefs are beautiful!

• Most diverse ecosystem – “Ocean Rainforest”

• 4000 species of fish, 800 species of coral

Corals and Global Warming

ScyphozoaScyphozoa

The fragile and sometimes luminescent bodies of jellyfishes can be beautiful.

Some jellyfishes are transparent, but others are pink, blue, or orange.

The medusa form is the dominant stage in this class.

ScyphozoaScyphozoa

• musclelike cells in their outer cell layer that can contract.

• When these cells contract together, the bell contracts, which propels the animal through the water.

Cubozoa

• Until recently, box jellyfish were included in Class Scyphozoa.

• They differ from Scyphozoans in several ways

Cubozoa

• Their bells are square-shaped, instead of round

• They have primitive brains

• They have eyes

• They swim, not float

• They sleep

Cubozoa

• The Irukandji in Australia is one of the most dangerous animals

• It is very small, only 2.5 cm from bell to tentacles

Giant Jellyfish

• While Irukandji is very small, Japan has been invaded by Nomura’s jellyfish – the giant jellyfish.

Research:• What are Nomura’s Jellyfish?

• Where are they found? Just Japan?

• What causes them to get so big?

• Are they dangerous?

• Are they new? Or are they only now getting noticed?

Giant Jellyfish

• 6.5 ft wide and 450 lbs

• More common in China and Korea

• Only recently in Japan

Giant Jellyfish

• Not much is known, Japan studying mating/migration habits

• Choking fishing nets

• Possible warmer seas (global warming) causes increase in population

• http://video.google.com/videoplay?docid=2208948115892996006&q=giant+jellyfish&total=103&start=0&num=10&so=0&type=search&plindex=0

The earliest known cnidarians also date to the Precambrian, about 630 million years ago.

The earliest coral species were not reef builders, so reefs cannot be used to date early cnidarians.

The larval form of cnidarians resembles protists, and because of this, scientists consider cnidarians to have evolved from protists.

Where did they come from?