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Cnidaria and Ctenophora Study Notes

Diploblasts

  • Diploblasts are animals with two embryonic germ layers: ectoderm and endoderm (give rise to epidermis and gastroderm).
  • The middle layer, mesoglea, is non-cellular and contains no organs.
  • Diploblastic organization is a major step up from Porifera (sponges) in tissue organization.
  • Radial symmetry is typical; this imposes constraints on locomotion and body plan in aquatic, mostly sedentary or freely moving (pelagic) lifestyles.
  • Cnidaria and Ctenophora are part of the Radiata, with ancestral lineages in the animal kingdom; the slide outline shows a broader taxonomic placement (Parazoa Porifera; Cnidaria; Ctenophora; etc.), and contrasts Protostomia and Deuterostomia within Bilateria, as well as broader clades like Ecdysozoa and Lophotrochozoa; noted terms include Radiata, Eumetazoa, Bilateria, Deuterostomia, Protostomia.
  • Notable ecological and evolutionary context: diploblastic animals include key marine taxa; their tissue organization underpins complex feeding and ecological roles (e.g., corals, jellyfish).

Phylum Cnidaria

  • Diverse, familiar phylum with over 11{,}000 species, including jellyfish, sea anemones, and corals.
  • Diversity largely due to life history features: capable of forming colonies via asexual reproduction; dimorphic lifestyle with two adult forms (polyp and medusa).
  • Diploblastic organization with true tissues; relatively few cell types (fewer than many groups except sponges and mesozoans).
  • Body plan features: radial symmetry; cnidocytes (cnidae); gut; gelatinous mesoglea.
  • Lacks a centralized nervous system, and lacks specialized organs for respiration, excretion, and circulation.
  • Most cnidarians are marine and carnivorous; some are suspension feeders or engage in mutualisms for food.
  • Extremely important in marine ecosystems; coral reefs are a key starting point for understanding their ecological significance.

Morphology and embryology: two germ layers and the mesoglea

  • Embryology: two primary germ layers (ectoderm, endoderm) and a middle non-cellular mesoglea; this is a defining feature of diploblastic cnidarians.
  • Radial symmetry imposes inherent constraints on orientation and movement in aquatic environments.
  • Life cycle often includes two morphs: polyp (sessile) and medusa (free-swimming), providing ecological flexibility and enabling exploitation of different habitats.
  • The mesoglea is a gelatinous middle layer that supports the body plan but contains no organs.

Body plans in Cnidaria: Polyp and Medusa

  • Polyp: oral end up, tentacles surrounding mouth, gastrovascular cavity, mesoglea, epidermis; typically sessile.
  • Medusa: bell-shaped, capable of swimming; mouth/anus oriented downward; gastrovascular system extended via radial canals; epidermis on outer surface; gastrodermis lines the gut; mesoglea between layers.
  • Both forms share a basic plan but differ ecologically:
    • Polyp tends to be sedentary, often colonial, with varying polyp types within colonies (e.g., hydranths, gastrozooids, gonangia).
    • Medusa is generally free-swimming with a gelatinous body and a central gastrovascular system; some classes show only one form predominating in their life cycle.

Classification of Cnidaria

  • Three subphyla and eight classes; representative members:
    • Hydrozoa (hydroids): ~3{,}200 spp in 5 orders; includes freshwater hydras (O: Hydroida) and colonial forms like Portuguese man o' war (O: Siphonophora).
    • Anthozoa (sea anemones, corals): ~6{,}200 spp, all polyps; includes:
    • Hexacorallia (Anthozoa): O: Actiniaria (true anemones); O: Scleractinia (true corals)
    • Octocorallia (Anthozoa): O: Gorgonacea (sea fans, sea whips); O: Pennatulacea (sea pens, sea pansies)
    • Cubozoa (box jellyfish): ~20 spp
    • Scyphozoa (true jellyfish): ~200 spp in 4 orders
  • These taxonomic groupings reflect both morphology and life-history traits and are used to distinguish major cnidarian lineages.

Basic Cnidarian phylogeny

  • Cnidaria comprises several major lineages including Cubozoa, Scyphozoa, Anthozoa, and Hydrozoa.
  • Example taxa to illustrate diversity:
    • Coral (Scleractinia)
    • Hydra (Hydrozoa)
    • Box jelly (Cubozoa)
    • Moon jelly (Scyphozoa)
    • Anemone (Anthozoa)
    • Portuguese man of war (Hydrozoa, siphonophore-like colonial life)

Medusas

  • Medusa is the free-swimming form observed in three cnidarian classes (Hydrozoa, Scyphozoa, Cubozoa).
  • General morphology: bell or umbrella-shaped; gut occupies central region and extends outward via radial canals; outer surface is epidermis; gut is lined by gastrodermis; mesoglea forms the jelly-like middle layer.
  • Most medusae possess quadriradial symmetry and swim by contractions of coronal muscles that move the bell; some species cannot change direction easily, while others respond to light cues to modulate movement.
  • In many jellyfish, the jelly itself is largely acellular mesoglea; in some forms, partially cellular mesenchyme contributes to structure.
  • Medusae can form local populations that reflect prey availability and ecological conditions.

Polyps

  • Polyp stage is often more diverse and ecologically influential due to asexual reproduction and colony formation.
  • Basic polyp anatomy: tubular body, epidermis, mesoglea, blind gut; radial symmetry; aboral end may have a pedal disc for attachment or a burrowing function.
  • Oral end may be elevated as a hypostome or form a flat oral disc.
  • The gut digests and circulates nutrients; divided by mesenteries; tentacles surround the mouth, with variations among classes.
  • Tentacle morphology varies and can be branched; certain colonies contain multiple polyp types: hydranths (gastrozooids), dactylozooids, gonangia.

Movement

  • Movement is achieved via contractile movements of epithelial cells; coupled with a water-filled gut, cnidarians form a hydrostatic skeleton.
  • Water volume in the gut can be altered to enable movement and shape changes.
  • Polyps are primarily sessile or slow-moving, moving mainly to feed or withdrawing when threatened.
  • In polyps, musculature is largely gastrodermal, with contributions from the oral disc, tentacles, and mesenchyme; retractor muscles shorten the column, circular muscles aid contraction and closing.
  • In medusae, the major muscles are in the epidermis and subepidermis; gastrodermal muscles are reduced; contraction patterns are concentrated around the bell margin and the subumbrellar surface; coronal muscles embedded in the mesoglea drive pulsing movements; some species also have radial muscles to assist opening the bell.

Cnidae (Nematocysts)

  • A defining cnidarian feature: the cnidae (nematocysts) are specialized organelles used for prey capture, defense, locomotion, and attachment.
  • Produced in epidermally derived cnidoblast cells (cnidocytes).
  • When stimulated, the cnidae discharge a hollow tube outward toward prey; the discharge is typically one-shot per cell.
  • Most abundant on the ends of tentacles and around the oral region.
  • There is evidence that firing thresholds are regulated by internal hunger signals and water chemistry; the exact control mechanisms can vary across taxa.

Feeding and Digestion

  • Most cnidarians are predatory, using cnidae to incapacitate prey.
  • Ingested prey are taken into the gut and digested extracellularly with enzymes; the gut distributes partially digested material.
  • Nutrients are taken up by nutritive-muscular cells; digestion is completed intracellularly in food vacuoles.
  • Wastes are expelled through the mouth via a two-way gut.
  • Many scyphozoans capture prey with tentacles and nematocysts; some anthozoans use the oral disc as a net to capture prey; some corals catch suspended items by mucous nets; coral mucus is nutrient-rich and can be consumed by other organisms.
  • Cnidarians are keystone components of ecosystems and food webs, influencing coral reef structure and broader marine communities; jellyfish blooms can affect fish hatcheries and zooplankton communities by predation and competition for resources.

Nervous systems

  • Cnidarians possess a diffuse nervous system (nerve net) with two arrays of neurons; neurons are generally non-polar and lack centralization.
  • Subepidermal nerve nets are typically better developed than subgastrodermal nets; signals move bidirectionally due to non-polar neurons.
  • Polyps typically have fewer sensory structures; medusae possess more sophisticated nerve nets and, in some taxa, rings around the margin (epidermal nerve nets with interconnecting fibers).
  • Some medusae have ocelli (eye spots) for light detection and statoliths for proprioception.

Reproduction in Cnidarians

  • Reproduction is tied to alternation of generations: asexually reproducing polyp phase and sexually reproducing medusa phase.
  • Planula larvae are produced in sexual reproduction and give rise to new polyps.
  • Variations exist among classes:
    • Hydrozoa: some hydromedusae reproduce asexually via budding or binary fission; solitary hydroids may lack a medusoid stage; planula progeny of medusae may become actinula larvae that bypass the polyp stage and develop directly into medusae.
    • Anthozoa: dedicated polyps, lack medusoid generation; eggs and sperm released to form planula larvae; planula can be brooded by the parent or fertilized in gut; some species copulate; planula can become a new polyp; some planula are benthic; asexual budding from planula or eggs can form polyps.
    • Scyphozoa: the polyp stage (scyphistoma) may undergo strobilation to produce new medusae (ephyrae); monodisc or polydisc strobilation; medusa-dominant life histories; planula becomes medusa directly; cubozoan biology is less well known, with polyps metamorphosing into single medusae; some medusae exhibit direct sperm transfer.

Cnidarians in deep and cold waters

  • Recent discoveries expand our understanding of cnidarian diversity in Arctic and Antarctic waters (e.g., Crossota sp. hydromedusa in the Marianas Trench; Chrysaora melanaster in the Arctic; Edwardsiella andrillae – an Antarctic anemone attached inverted to the underside of ice).
  • Diversity of Antarctic seafloor cnidarians highlights the ecological breadth of the group.

Notable examples for context

  • Dofleinia armata (striped/armed anemone):

    • In family Actiniidae; monotypic genus Dofleinia; large anemone, up to about a foot in diameter, tentacles up to about 20 inches.
    • Found in SE Asia to western Australia, depths to ~20{,}m.
    • Can change shape due to the hydrostatic system (muscles + gut); color variation is high.
    • Noted for potent stinging nematocysts; pain can be prolonged.

  • Chironex fleckeri (Australian box jellyfish / sea wasp):

    • Considered among the most venomous cnidarians; >60 deaths attributed in Australia (1884–2021).
    • Most stings are non-lethal: from 225 stings (1991–2004), only about 8 ext{\%} required hospitalization.
    • Box jelly; range includes northern Australia, Indonesia, Singapore, and the Philippines.
    • Largest known cubozoan; bell up to $$14