Phylum Cnidaria

Jellyfish, Corals, and Sea Anemones

General Characteristics• Tentacles• Hydrostatic skeleton• Nerve net• Radial symmetry• Saclike digestive system (only one opening for

mouth/anus)• Two layers of cells with mesoglea (jelly-like material)

in between.• Lack special organs for respiration, excretion, and

have no blood

• Nematocysts (stinging cells) – used for feeding, defense, and some contain toxins.

Two Body Forms of Cnidarians

• Medusa – free floating, motile, part of the plankton, tentacles and mouth point down

• Polyp – sessile, part of the benthic community, tentacles and mouth point up.

Classes of Cnidarians• Hydrozoans

– Most are colonial – Polyp body form for most of its life cycle– Some do not have a medusa stage others do not

have a polyp stage, but most have both stages in their life cycle

– Examples: Portuguese Man-o-War, Hydra

• Scyphozoans– Most are solitary– Medusa body form for most of its life cycle– Examples: true jellyfish like the Moon Jelly

(Aurelia)

• Cubozoa – Box Jellyfish (Sea Wasp)– The deadliest jellyfish in the world are a type of

box jellyfish, with the typical cube body shape– Not all species are deadly, but can cause very

painful stings

• Anthozoans– Only found in the polyp body form– Reproduce both asexually (budding) and

sexually (shed eggs and sperm into the water)– Three groups of anthozoans:

• Anemones – soft fleshy polyps, usually solitary

• Soft Corals – sea fan and sea whips

• Hard Corals – which have a calcareous skeleton (usually) and build coral reefs, usually colonial

                           

Most anthozoans have a symbiotic relationship with a dinoflagellate known as zooxanthellae living in their tentacles that produce food for the coral in exchange for a place to live and nutrients.