Module 2 - Phylum Cnidaria

• Cnidarians exhibit two body plans during their life cycle: medusa and polyp
• Radial symmetry in cnidarians allows them to be divided into similar halves by passing a plane at any angle along a central axis
• Oral and aboral sides in cnidarians have 1 opening and 2 ends
• Cnidarians are diploblastic organisms with no specialized cells, featuring:

  • Epidermis covering the body
  • Gastrodermis lining the gastrovascular cavity
  • Mesoglea with ameobocytes for digestion, nutrient transport and storage, wound repair, and antibacterial defense
• Cnidarians have different types of cells including:

  • Epitheliomuscular cells, acting in contraction
  • Nutritive muscular cells, responsible for water intake into the cavity
  • Interstitial cells, transforming into other cell types
  • Gland cells, secreting digestive enzymes
• Cnidarians possess cnidae (nematocysts) which are complex intracellular organelles used for defense and prey capture
• Cubozoan jellyfish stings evolve through stages, from tentacle marks to wound healing over 120 days
• Cnidarians like jellyfish and lampreys swim by jet propulsion for low energy usage
• Cnidae in C. adhaesivum are discharged for wrapping around small objects, sticking surfaces, penetrating surfaces, and secreting proteinaceous toxins, and they are discharged only once
• Cnidarians have no brains but possess decentralized nerve nets with sensory, motor, and intermediate neurons for coordination
• Cnidarians like Aurelia sp. have bipolar neurons running through conduction tracts for faster impulse sending, with rhopalia serving as receptors for light, balance, touch, and chemical detection
• Statocysts in cnidarians sense tilt and acceleration, with organs lined with hairs detecting the movements of internal mineral grains called statoliths
• Cubozoans have simple eyes called ocelli that detect sources of light and can distinguish the direction from which light is coming
• Cubozoans have four sets of eyes, each with retinas, corneas, and lenses, allowing them to clearly distinguish the direction from which light is coming as well as negotiate around solid-colored objects
• Feeding habits of Cnidarians include predation, absorption of dissolved organic matter, filter feeding, and having endosymbionts
• In digestion and excretion, gland cells release enzymes, gastroderm cells absorb nutrients, intracellular digestion occurs, nutrient transport happens by diffusion, indigestible remains are expelled through the mouth, and wastes (ammonia) are removed by external and internal water currents
• Metagenesis in Cnidarians involves a succession of differently organized generations, including one asexually reproducing, sessile polyp, and one sexually reproducing, free-swimming medusa or sessile polyp
• Spawning in Cnidarians is generally driven by environmental factors like changes in water temperature and is triggered by lighting conditions such as sunrise, sunset, or the phase of the moon
• Larval formation in Cnidarians involves fertilized eggs developing into larvae by dividing until there are enough cells to form a hollow sphere (blastula), then a depression forms at one end (gastrulation) and eventually becomes the digestive cavity
• All known Cnidarians can reproduce asexually by various means, in addition to regenerating after being fragmented
• Cnidarians have no respiratory organs; both cell layers absorb oxygen and excrete carbon dioxide, with some having ciliated grooves on their tentacles to pump water out and into the digestive cavity
• Taxonomic classification of Cnidarians includes Subphylum Medusozoa with classes like Scyphozoa, Cubozoa, Polypodiozoa, Hydrozoa, Staurozoa, and Hydrozoa with subclasses like Hyrdoidolina and Trachylinae
• Subphylum Medusozoa contains all cnidarians except anemones and corals (Anthozoans) and possibly parasitic myxozoans
• Mitochondrial genome in Medusozoa is linear, unlike other metazoans studied so far
• Class Scyphozoa characteristics:

  • Asexual replication by strobilation
  • Separate male and female sexes
  • Marine habitat
  • Mesoglea layer has consistency of firm gelatin
  • Medusa dominant, with inverted cup (bell) and nematocyst-studded tentacles
  • Locomotion through active swimming by contracting muscle of the bell and exploiting mechanical properties of mesoglea
• Class Cubozoa characteristics:

  • Medusa with boxlike bodies
  • Rhopalia bear complex, lensed eyes
  • Cube animals, with cuboidal swimming bell and 4 tentacles emerging from the corners
  • Mostly eat small fish, annelids, and arthropods, including crustaceans
  • Can kill individuals larger than themselves, including humans
• Class Hydrozoa characteristics:

  • Water animals with both polyp and medusa stages
  • Subclass Hydroidolina includes marine and freshwater species with medusa in adults
  • Most hydrozoans are colonial, unlike the solitary Hydra
  • Reproduction through medusoids produced by asexual budding from various regions of the polyp colony
• Hydrozoans remain attached to the hydrozoan colony, with no free-swimming medusa stage in some species
• In some hydrozoan species, medusa eventually break off the colony, while in a few species, medusa can bud off
• Like scyphozoans, medusa in hydrozoans can "reverse" to polyp
• Defense mechanisms in hydrozoans include dactylozooids, which are finger-like modules specialized for defense and have a lot of nematocysts
• All modules in a hydrozoan colony, regardless of polymorphism, are originally derived from a single planula larva, serving as the founding member of a new genet
• Symbiotic associations of hydrozoans include species-specific relationships with other animals such as fish, sea urchins, sea squirts, polychaetes, annelids, gastropods, bivalves, crustaceans, bryozoans, sponges, sea anemones, and other hydrozoans
• Order Siphonophora, epitome of hydrozoan polymorphism, includes free-floating colonies with medusoid and polyploid morphs present simultaneously
• Nectophores in Siphonophora are for jet propulsion and lack mouth and tentacles, while bracts/phyllozooids are for defense with nematocysts
• Pneumatophores in Siphonophora are gas-filled floats with reduced or absent mesoglea
• Polyp morph in Siphonophora is represented by gastrozooids, gonozooids, dactylozooids, and cormidia, with one or more morphs possibly absent in some groups
• Hydrocorals, also known as Anthomedusae, form massive colonies resembling true corals, secrete a calcareous skeleton, and are restricted to warm waters with abundant and potent dactylozooids
• Myxozoans are extracellular, spore-forming parasites that mostly infect aquatic annelids and freshwater bryozoans, with fishes as intermediate hosts
• Class Staurozoa, the medusoid sister group, mostly found in cold shallow waters, feed on small crustaceans, have a trumpet-shaped body with a stalk and branches or arms with tentacles, and practice asexual reproduction by splitting their body into new individuals