Jonathan Bird’s Blue World: Cnidarians

Introduction to Cnidarians

• Definition: Cnidarians are squishy invertebrates, not true fish.
• Phylum: Phylum Cnidaria, meaning "stinging animal".
• Species: Approximately 11,000 species worldwide.
• Complexity: More complex than sponges but less so than sea stars.

Characteristics of Cnidarians

• Symmetry: Radially symmetrical, body parts extend outward from the center.
• Classes: Includes hydroids, jellies, anemones, and corals.
• Tentacles: All have tentacles with stinging cells (cnidocytes) used for capturing prey.
  • Nematocyst: A coiled thread-like stinger within cnidocytes that fires to puncture prey.
  • Many contain toxins that help disable prey.
• Human Interaction:
  • Most are harmless to humans.
  • Some, like certain jellies and hydroids, can deliver painful or fatal stings.

Body Types

• Polypoid:
  • Tentacles and mouth face upward, fixed to a substrate (e.g., corals, anemones).
  • Example: Tealia Anemone in the North Atlantic, solitary and waits for prey.
• Medusoid:
  • Free-swimming form, mouth and tentacles point down.

Feeding Mechanism

• Blind Gut:
  • Cnidarians have a single opening for food intake and waste expulsion.
  • Prey is drawn into the mouth and digested in a blind gut.
• Hydroids: Small and flower-like, can sting effectively despite their delicate appearance.
  • Fire Coral: Tropical hydroid resembling peach fuzz, causes painful stings.

Coral Polyps

• Colonial Nature: Live in groups, share food resources, and can create limestone skeletons.
• Coral Heads: Structures formed by many individual coral polyps, can be seen from space.

Respiration and Metabolism

• Cnidarians lack specialized organs such as gills.
• They absorb oxygen directly through their skin, aided by their tentacles providing a large surface area.

Jellies (Medusoid Cnidarians)

• Drift in water with mouth and tentacles downwards to capture prey.
• Some, like box jellies, are highly venomous and can be deadly, while many produce only mild discomfort.

Reproduction

• Asexual Reproduction: Budding off clones or dividing.
• Sexual Reproduction: Involves releasing eggs and sperm into the water for fertilization.
  • Corals: Utilize broadcast spawning to spread to new habitats.

Ecological Impact

• Cnidarians are crucial as predators, reef-builders, and contribute to the ocean ecosystem.
• Integral to the diversity and function of marine environments.

Conclusion

• The significance of Cnidarians in the marine ecosystem and their fascinating characteristics.

Extras

• Reminder to check out the BlueWorld_plus channel for additional content!