Phylum Cnidaria

• Cnidarians have two morphological forms

  o   Polyps

  o   Medusae (singular: Medusa)
• Cnidarians

  • Two tissue layers - Gastrodermis and Epidermis, with Mesoglea in between
  • Gastrovascular cavity used for digestion of food and transport
  • Gastrodermis lines gastric cavity
  • Epidermis provides protection
  • Contains specialised epidermal cells (cnidocytes) used for stinging prey and predators
  • Mesoglea is a collagen fibre matrix that contains mainly water along with nerve fibres and muscle fibres
  • Nerve and muscle fibres help in swimming
  • Nerve fibres are dispersed - No CNS (primitive version of brain)
  • Oral surface contains mouth/anus and tentacles
  • Aboral surface contains nothing (opposite surface of oral surface)
• Morphology of polyps

  • Pedal disk on aboral surface – allows for attachment to surfaces via chemical/physical means
  • Tubular body
  • Oral surface faces up
  • Sessile
• Morphology of medusae

  • Umbrella body
  • Aboral surface faces up
  • Motile
  • Ability to swim, but very weak due to weak muscle fibres – relies on currents to be pushed around, cannot move with a direction
• Cnidarians have radial symmetry so that nerves, receptors, and muscles can respond equally well from all directions

  • Important since polyps cannot escape from predators and medusae don’t know where they are going (don’t move with a purpose)
• Some cnidarians exist only as polyps (anthozoa) or only as medusae (scyphozoa), others have both a polyp stage and medusa stage in life cycle (hydrozoa)
• Sexual reproduction

  • Medusae are dioecious (individual has gender and gonads that produce gametes)
  • Medusae release eggs and sperm into ocean via spawning
  • Sperms fertilise eggs via external fertilisation
  • Fertilised zygote implants itself to become larva which forms polyp
  • Polyp settles down on nice substrate and aboral surface forms pedal disk
• Asexual reproduction

  • Polyps have specialised buds for feeding and breeding
  • Feeding bud: gastrozooid (contains tentacles)
  • Breeding bud: gonozooid (has no tentacles)
  • Reproductive bud falls off when matured, and undergoes transformation to form medusa (every bud is one medusa)
• Aurelia life cycle

  • No specialised buds for feeding and breeding
  • Once polyp matures and starts reproducing, it relies on stored food to reproduce – if the food runs out, it doesn’t reproduce anymore
• Advantage of asexual reproduction by polyp form

  o   Faster reproduction
• Advantage of sexual reproduction by medusae

  • Genetic variation and diversity, adapt and evolve to changing environments
  • Movement and dispersal of gametes and larvae
• Cnidarians are predators that use their tentacles to sting and capture their prey

  o   Don’t chase after prey

  o   Polyps: sit, wait and trap

  o   Medusa: float, wait and trap
• Nematocytes/cnidocytes

  Specialised sting cells in the epidermis of cnidarian tentacles
• Cnidocytes

  • Have a special organelle called nematocyst that contains a thread to sting
  • The right combination of chemical and mechanical stimuli triggers the cnidocil (contains chemoreceptors and mechanoreceptors)
  • Once discharged, the cells cannot be used anymore – cell doesn't waste resources and only gets triggered with right combination
  • Surface of the thread has barbs to anchor and pierce prey
  • Inverted in nematocyst to keep the cell safe from damage by the barbs
  • When triggered, operculum (lid) opens up and thread is everted
  • Thread coils around prey to immobilise prey
  • Thread pierces into prey to inject venom
• Cnidocytes

  • Connected by nerve fibres to form clusters
  • When stimuli is strong, entire cluster of cnidocytes is activated for more effective response
• Medusae

  • More fragile since they are motile, thus they need to immobilise prey or threat faster, hence they are more venomous
• Two-way gut: incomplete gut

  o   Cannot eat and shit at the same time

  o   If it is digesting, then it cannot ingest until it egest
• One-way gut: complete gut

  • Can ingest digest and egest at the same time
  • Five processes: ingestion, egestion, digestion, absorption and storage
  • Specialised chambers in the gut to increase efficiency of food processing
  • Separate processing of food
• Cnidarians have a two-way gut

  • When they are digesting, they can prevent nematocyst from firing and can fold up around the surface
  • Otherwise, they catch the prey and immobilise it, only digest it upon completion of digestion of current prey inside it
• Digestion in cnidarians

  • Gland cells release digestive enzymes out of the cell into GV cavity – extracellular digestion
  • Can digest larger food items
  • Even though the gland cells secrete digestive enzymes, digestion by enzyme is only partial
  • Nutritive-muscular cells take in partially digested food into food vacuoles to take in nutrients
  • Nutrients are distributed to other cells via diffusion
• Hard corals

  • Build calcified exoskeletons
  • Each living coral is made up of a colony of coral polyps
  • Coral polyps reproduce by budding so all the polyps in the colony are genetically identical
  • Coral reefs are a marine ecosystem formed by the hard skeletons of the corals
  • Corallite is the skeleton built by a single polyp
  • Corallites of all polyps in the colony fuse to form skeleton
  • Corallite is important to provide support and protection
  • Layer of living tissue that expands out of polyp connects all the polyps so that polyps can share nutrients with other individuals in the same colony
• Just because coral is bleached doesn’t mean it’s dead – tentacles could still be out meaning it is alive

  • It cant be kept alive for long
  • Coral bleaching occurs in unfavourable conditions – insufficient nutrients in such conditions
  • It can recover if conditions become favourable again since it can acquire zooxanthellae again
• Coral bleaching means the underlying calcium carbonate skeleton is exposed

  Zooxanthellae: algal cells enclosed in vacuoles of cells in gastrodermis

  • Obtained from open waters when ZX is ingested by polyp and absorbed in via endocytosis
• ZX benefits

  • Protection from predators
  • Receive carbon dioxide for photosynthesis
  • Organic matter inside of coral helps algal cells grow

  Coral benefits

  • Receive oxygen for respiration
  • Protection from UV light
  • ZX provides 90% of nutrition for coral through photosynthesis
• Corals with ZX live in clear, shallow and tropical waters

  What happens during coral bleaching:

  • Coral expels out ZX since ZX releases harmful free radicals in unfavourable conditions
  • Coral stressed by water
  • ZX stressed
  • ZX produce free oxygen radicals
  • Radicals may react with DNA or proteins which is bad
  • ZX is expelled out