Unit 2 Lesson 2 Cnidarians

By Kimberlea Scott

​Domian Eukarya
Kingdom Animalia
Ex: Jellyfish, Anemones, Corals

General Characteristics

• Radial Symmetry

• Have true tissues!

  • Diploblastic (“2 tissues”)

    • Endoderm = (“inside skin”) innermost tissue layer, develops into gut

    • Ectoderm = (“outside skin”) outermost tissue layer, develops into epidermis and nerve tissue

• No coelom 

• Dimorphic (“two forms”)

  • Display 2 different body forms in a lifetime

Systems Breakdown

• Digestion:

  • Gastrovascular Cavity (GVC) – large cavity that is connected to an opening that serves as the mouth and anus.  Allows for direct digestion

    • “Mouth” → GVC → “Anus”

  • Carnivorous

  • Obtain food using cnidocytes

    (stinging cells) on the tentacles

Systems Breakdown

• Nervous:

  • Contain Nerve Net – no brain, but basic abilities to send and receive signals throughout body.

  • Specialized cells – Cnidocytes

    • Stinging cells found along the exterior of the tentacles

    • Produced by ectoderm cells

    • Used in food getting, defense & attachment

  • Cnidocyte Structure

    • Cnidocil – “trigger”

    • Operculum – covering

    • Nematocyst – barb and thread containing toxin

Human Impact

• Most cnidarian toxins are paralytic to their prey.

• These toxins do not generally cause permanent damage to humans apart from pain and irritation.  This can be relieved by removing the barbs with tape or by scraping, then soaking in vinegar or hot water to dilute the toxin.

• Some cnidarian species have powerful toxins that can cause immediate cardiovascular and respiratory failure in humans. 

  • Portuguese Man O’ War, Irukandji, Sea Wasp Box Jelly (1)

Systems Breakdown

• Skeletal:

  • Hydrostatic Skeleton – Keep body shape using water pressure inside the body.

    • Can change pressure levels to move and help ingest food.

  • Calcium carbonate “skeletons” form reef foundations

• Muscular:

  • True muscle cells that allow for controlled movement

  • Gliding, somersaulting, propulsion

• Circulation:

  • GVC circulates water and dissolved gasses through the body.

Systems Breakdown

• Reproduction: 

  • Dimorphism - 2 forms

    • Polyp Stage

      • Asexual and sessile

      • Aboral end attaches to substrate, oral

        end (“mouth”) surrounded by tentacles

    • Medusa Stage

      • Sexual and mobile

      • Dioecious (separate sexes)

      • Sperm and egg released externally to create a zygote which grows into a free-swimming planula larva

    • One body form will always be dominant

    • Shows “alternation of generations”

      • 2 body forms alternate between sexual and asexual

Ecology

• Eaten by sea slugs, some starfish, butterfly and parrotfish and sea turtles.

  • Sea slugs have the ability to incorporate the cnidarian nematocysts into their own body for protection.

• Make up coral reefs, which support more species per unit area than any other ecosystem on Earth, but cover less than 1% of the Earth’s surface.

• Coral reefs are threatened due to human actions, mainly coral bleaching. 

  • Caused by pollution and increased water temperature (global warming).  “Bleached corals” are not dead yet, but in a very vulnerable state.  They can be revived if water temperatures cool.

•  Coral reefs have a high impact on tourism economies.  The Florida Keys reefs alone have an estimated asset value of $7.6 billion.