6.2 Cnidarians: jellyfish, corals, and sea anemones

Cnidarians, from jellyfish to corals, are fascinating marine creatures with unique features. They have two body layers, radial symmetry, and special stinging cells called cnidocytes. These animals come in two main forms: sessile polyps and free-swimming medusae.

Cnidarians play crucial roles in marine ecosystems, especially coral reefs. These underwater oases support incredible biodiversity but face threats from climate change and human activities. Cnidarians also form important symbiotic relationships, like corals with algae and clownfish with sea anemones.

Cnidarian Characteristics and Body Plan

Defining features of cnidarians

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• Diploblastic organisms have two germ layers (ectoderm and endoderm)
  • Separated by mesoglea, a non-living jelly-like layer between the germ layers
• Exhibit radial symmetry
  • Body parts arranged around a central axis like spokes on a wheel
• Possess a gastrovascular cavity
  • Functions as a "gut" for digestion and circulation of nutrients
  • Single opening serves as both mouth and anus for ingestion and waste removal
• Equipped with cnidocytes, specialized cells
  • Contain nematocysts, stinging organelles used for prey capture and defense (jellyfish tentacles)
• Exist in two main body forms: polyp and medusa
  • Polyp: sessile, cylindrical body with mouth and tentacles facing upward (sea anemones, corals)
  • Medusa: free-swimming, bell-shaped body with mouth and tentacles facing downward (jellyfish)

Cnidarian Life Cycles and Reproduction

Life cycles of cnidarian species

• Jellyfish (Scyphozoans) undergo complex life cycle with alternation of generations
  1. Planula larva settles and develops into a polyp (scyphistoma)
  2. Polyp undergoes strobilation, producing ephyrae
  3. Ephyrae develop into adult medusae
  • Medusae reproduce sexually by releasing gametes into the water column for external fertilization
• Corals (Anthozoans) are mostly hermaphroditic, producing both eggs and sperm
  • External fertilization occurs in the water column
  • Planula larvae settle and metamorphose into polyps
  • Polyps can reproduce asexually through budding or fragmentation to form colonies (brain coral)
• Sea Anemones (Anthozoans) lack a medusa stage and exist only as polyps
  • Can reproduce sexually by releasing gametes into the water for external fertilization
  • Asexual reproduction occurs through pedal laceration or longitudinal fission (cloning)

Coral Reefs: Ecology and Threats

Ecological importance of coral reefs

• Harbor high biodiversity and support a wide range of marine species (Great Barrier Reef)
• Provide habitat, food, and shelter for many organisms (clownfish, parrotfish)
• Serve as nurseries for juvenile fish and other marine life
• Protect coastlines from erosion and storm damage by absorbing wave energy
• Face numerous threats that jeopardize their survival
  • Climate change and ocean warming cause coral bleaching when corals expel symbiotic algae (zooxanthellae)
  • Ocean acidification reduces availability of calcium carbonate for coral skeleton formation
  • Pollution and nutrient runoff promote algal growth that can smother corals
  • Overfishing and destructive fishing practices (dynamite fishing) disrupt ecological balance and damage coral structures
  • Coastal development and sedimentation smother corals and reduce water quality

Cnidarian Symbiotic Relationships

Symbiosis in cnidarian ecosystems

• Coral-zooxanthellae symbiosis: a mutualistic relationship
  • Zooxanthellae are dinoflagellate algae living within coral tissues
  • Provide corals with energy through photosynthesis
  • Receive protection and nutrients from the coral host
• Clownfish-sea anemone mutualism: both species benefit
  • Clownfish protected from predators by sea anemone's stinging tentacles
  • Clownfish have mucus coating preventing them from being stung
  • Clownfish provide nutrients to anemone through waste and help clean the anemone
• Jellyfish-fish symbiosis: fish gain protection
  • Some fish (juvenile jack mackerel) associate with jellyfish for protection
  • Fish may feed on jellyfish tentacles or parasites
• Coral-crustacean symbiosis: crustaceans gain shelter
  • Some crabs and shrimp live among coral branches for shelter
  • Crustaceans may clean coral surface and defend against predators