26-3 Cnidarians

Cnidarian overview (7)

• Old phylum: Coclenterata

• Live in ocean

• Soft-bodied

• Have stinging tentacles around mouth

• Live as individuals or in colonies/polyps

• Most alternate between medusa & polyp stages

• Sessile polyps & motile medusa

Form & Function (4)

• Have radial symmetry (no left or right)

• Have specialized cells & tissues

• Has a body wall surrounding hollow gastro-vascular cavity (stomach): used for digestion

• 3 layers of the body wall:

  • Epidermis: outside layer

  • Gastroderm: inside layer: lines gastrovascular cavity

  • Mesoglea: middle layer: has the multi-purpose moveable amebocyte cells

How cnidarians get food (9)

• Cnidoblast: a stinging cell containing the nematocyst or "spears"

• Nematocysts: cells with poison tipped stinging darts that capture small animals (located on tentacles)

• Paralyzed prey is moved into mouth and then into the gastrovascular cavity

  • Where it is engulfed by amoebocytes into the gastroderm and then digested

  • Undigested food back goes back out through mouth (only 1 opening, no anus)

• Cnidarians = carnivores, some have symbionts: photosynthetic protists live inside gastroderm cells

  • Protist produces O2 & organic compounds (protein, carbos)

  • Cnidaria produces CO2, metabolic wastes (nutrients)

• respiration & excretion: diffusion

Nervous system (5) & Movement (1)

• Don’t have a CNS (central nervous system: no brain or spinal column)

• Has a nerve net: simple nervous system that controls the tentacles and mouth area

• Has sensor cells in epidermis: for locating food & touch

• The medusa has statocysts for balance (upside down or right side up)

• Have ocelli: eyespots to light

• Has no muscle cells (has special shape changing epidermal cells)

Sexual & Asexual Reproduction (4)

• Asexual: Polyps make clones: budding

• Sexual:

  • Polyps become medusa by asexual budding

  • Medusa releases eggs & sperm into water

  • Fertilization occurs, a zygote (2N) → larva → new polyp

Class: Hydrozoa (9)

• Most of lifecycle as polyps

• Tiny, short lived medusa for sexual reproduction

• Some can form colonies of specialized polyps:

  • Ex: Obelia feeding polyps, defense polyps, reproductive polyps

• Some are freshwater polyps such as hydra

• Portuguese man-of-war (unusual) by window

  • Several floating specialized polyp colonies:

  • Ex. 1 floating balloon

    • Stinging polyp: danger for humans, Digestive polyp, Reproductive polyp

Class: Scyphozoa (5)

• “True jellyfish”

• Large, most of life as a long-lived medusa

• Small, short part of life as a polyp larva

• 2-3.6m long + 30m tentacles

• Australian jellyfish die in 3-20 min

Class: Anthozoa (4)

• Ex. Sea anemones, corals, sea fans, sea pens

• Only exist as polyps, no medusa stage

• Sexual reproduction: Sperm & eggs are released into the water → fertilization occurs → zygote → ciliated larva swims away → new polyp

• Asexual reproduction: by budding

Sea anemones (Class: Anthozoa) (4)

• Are solitary polyps

• Live in low-tide areas where the sun reaches (photosynthetic symbionts) to deep water where there is no light

• Up to 1m diameter

Corals (Class: Anthozoa) (9)

• Live in shallow tropical water

• Produce skeletons of CaCO3 (limestone) which can be destroyed by ocean acidification

• Are colonial polyps living together

• Polyps grow on top of secreted limestone skeleton

  • They keep building on top of older calcium carbonate skeletons secreted by previous polyps

• Grow very slowly, if destroyed can take years to rebuild

• Can live 100-1000s of years

• Colonies of polyps produce coral reefs

  • Most famous = Great Barrier Reef (Australia) (2000 km by 80 km)

Ecology (Ex. Sea anemone) (4)

• Many symbiotic relationships with many different animals

  • Ex. sea anemones

    • small organisms live in tentacles

    • Anemone provides protection & food scraps

    • Symbiont (nemo) cleans & protects the anemone

Ecology of Corals & Reefs (8)

• Provide tunnels, caves, deep channels that act as many habitats for organisms, produces complex ecosystems

• Greatest oceanic diversity (different kinds of species), called the tropical rainforests of the ocean

• Protects coastal shoreline from wave erosion

• Being destroyed by global warming which causes coral bleaching

• Illegal to buy or sell coral

• Indicator species: 1st organisms to die during environmental change: like poriferans

• Medicine: have some unusual protective chemicals/properties

• Don’t get cancer, protect themselves by making nerve toxins