Phylum Cnidaria

Jelly fish, hydra, coral, and sea anemones

General Characteristics• Cnidaria- name derived from stinging cell

cnidocyte• More highly organized than sponges but still very

simple.• Radial symmetry• No advancement above tissue level organization• Aquatic-mostly marine (few in fresh water)– Shallow warm marine habitats

• No terrestrial species• Mostly sessile some mobile• Predatory

Cnidocytes• Specialized stinging cells– 20 different types

• Important in taxonomic determinations• Formed only by Cnidarians• Located mainly on tentacles

Cnidocyte anatomy• Capsule composed of chitin like material

containing a coiled thread with barbs– Some contain toxin– Others recoil

• Capsule covered by operculum (lid)• Cnidocil- modified cilium (trigger)• Discharged by a combination of tensional

forces generated during the formation of the cnidocyte (osmotic pressure)

Cnidocyte function

• Small organic molecules from prey “tune” the mechanoreceptors, sensitizing them to the frequency of vibration caused by prey swimming

• Tactile stimulation of the cnidocil causes the cnidocyte to discharge

• Jellyfish feeding on fish http://www.youtube.com/watch?v=bcmLxsJ5SAg

Morphologically Dimorphic • Polyp

• Medusa

Polyp

• Hydroid form• Sedentary sessile life• Tubular body with a mouth directed upward at

one end surrounded by tentacles• Attached to the substrate by a pedal disc or

other device• May live single or in colonies• Usually reproduce sexually• Anthozoa

Medusa

• Free swimming• Bell or umbrella-shaped bodies• Mouth is centered on the concave side• Tentacles extend from the rim of the umbrella

Body Plan

• Outer epidermis• Inner gastrodermis which forms a gastrovascular cavity

• Has a middle “jelly filled” layer called a mesoglea

Nervous system

• Nerve net– Two interconnected nerve nets

• Diffuse nervous system• No cephalization• Neuromuscular system– Receives external stimuli– Stimulates the epitheliomuscular cells and

nematocysts

4 Classes of Cnidarians• Hydrozoa

– 2,700 species – Hydroids, fire corals, and Portuguese man-of-war

• Scyphozoa– 200 species– True jellyfishes

• Anthozoa– 6,000 species– Sea anemones, stony corals and soft corals

• Cubozoa– 15 species– Cube jellyfishes

Class Hydrozoa

• Freshwater or marine• Solitary or colonial• Life cycle with both polyp and medusa, one or the

other form may be suppressed– Asexual polyp form– Sexual medusa stage

• Simple gut cavities• Ectodermal gonads• Statocysts and ocelli (special light sensing organs)

may be present in some medusa

Class Hydrozoa: Hydra

• Freshwater solitary polyp• Habitat– Found worldwide under leaves or lily pads in cool

streams or pools.• 25-30 mm• Hydrostatic skeleton

Class Hydrozoa: Hydroid colonies (Obelia)

• Have both polyp and medusa stage in their life cycle

Obelia anatomy• Polyp

– Hydrorhiza- rootlike stalk– Hydrocauli- stalks– Coenosarc- living part of the stalk– Perisarc- protective covering (chitin)

• Thecate- have perisarc (cups)• Athecate- thin pericarc (naked)

– Hydranths- feeding polyps– Gonangium- reproductive polyp

• Medusa– Manubrium- hold mouth– Velum- margin of the bell partly closes and is used for swimming

Class Hydrozoa: Other Hydrozoans

• Physalia physalis Portuguese man-of-war• Pneumatophore -float

Class Scyphozoa: The jellies

• Large jellyfish– may exceed 2 m and have tentacles 60 to 70

meters long– Most are 2 to 40 cm in diameter

• Found in the open sea

Aurelia

Scyphozoa anatomy

• No velum• Manubrium divided into four oral arms• Gastric cavity more specialized– Four gastric pouches– Gastric filaments– Radial canals– Ring canal

Class: Cubozoa

• Primarly medusa form• Small 2 to 3 cm • Bells are almost square• Stings are often fatal• Pendalium- base of the tentacle has a

flattened tough blade

Class Anthozoa: “flower animals””

• Polyps only• All marine• Sea anemones and hard corals• Form symbiotic relationships with fish– Fish get protection– Anemone gets cleaned off (sediment) and fish

lures in other fish

Anthozoa anatomy

• Gastravascular cavity divided into six radial chamber by six pairs of septa or mesenteries

Class Anthozoa: Corals

• Live in calcareous cups they have secreted• Epidermis secretes limy skeletal cup at base

which provides protection (organism may contract back into the skeletal cup)

Reefs• 60% are threatened• Second most diverse places on the planet• Types of reefs – Fringing reef• Close to landmass with either no lagoon or a narrow

lagoon between reef and shore– Atoll• Reefs that encircle a lagoon but not an island• Slope steeply into deep water at the sea edge

– Barrier reef• Runs parallel to shore and has a wider and deeper lagoon

that a fringing reef

Economic Importance

• Food for other animals– http://www.youtube.com/watch?v=DmNOsOm0JiE

• Symbiotic relationships– Live on crab shells and provide protection

(mutualistic relationship)– Toxins may be sequestered by others (parasitic)

http://video.nationalgeographic.com/video/animals/invertebrates-animals/other-invertebrates/weirdest-nudibranch/

• Rarely food for people• Coral Reefs

Effect on Humans

• Mostly non life threatening– Exceptions• Portuguese Man-of-war• Box jelly

• Treatment– White vinegar– urine

Coral Reefs Economic Importance

• Fish and other animals associated with reefs provide food for humans

• Tourist attractions• Serve as a breaker for waves• Coral jewelry and ornaments• Building materials

Movies

• Worlds Deadliest Jellyfish– http://www.youtube.com/watch?v=zbdo2KcaG1M

• Secrets of jellyfish, killers and life savers– http://

www.youtube.com/watch?v=VHTmRwwKb1k