Notes on Porifera and Cnidaria

Porifera (Sponges)

• Overview:

  • Sponges determined to be animals around 200 years ago.
  • Approximately 8,000-10,000 species.
  • Habitat: All aquatic; mostly marine.
  • Characteristics:
  • Size ranges from 1-2 cm to over 1 meter.
  • Symmetry: Most sponges lack symmetry; some exhibit radial symmetry.
  • An ancient group; earliest in fossil record, do not give rise to other lineages.
  • Considered a "dead-end" phylum, but successful with a large number of living species.

• General Characteristics:

  • Classification: Metazoa at cellular grade of construction.
  • All sponges are aquatic; 99% marine; found mainly in warmer waters with some bizarre deep-sea varieties.
  • Unique flagellated cells called choanocytes (collar cells) for feeding.
  • Adults are sessile and filter feeders reliant on water:
  • For food, gas exchange, and gamete dispersion.
  • Body structure consists of:
  • Outer layer of cells
  • Inner layer (choanoderm)
  • Mesohyl: a gelatinous region in between.
  • Structural elements include spicules (calcium carbonate or silica).

• Cell Types in Sponges:

  • Choanocytes: Flagellated cells that engulf bacteria and food particles via phagocytosis.
  • Amoebocytes: Totipotent cells that digest food, transport nutrients, and produce skeletal fibers (e.g., spicules).

• Ancestry:

  • Porifera is a basal lineage diverged early from protists.
  • Choanocytes are similar to the sister group of animals, the choanoflagellates.

• Evolutionary Timeline:

  • Porifera likely first appeared in the Cryogenian period (720-635 million years ago).
  • Fossil evidence dates back to the Ediacaran period (635 - 545 million years ago).

• Development in Sponges:

  • No gastrulation, lack of gastrula formation.
  • No symmetry or germ layers; mainly organized in layers.
  • Nurturing their animal classification by:
  • Being multicellular eukaryotes.
  • Functioning as heterotrophs.
  • Exhibit motility at least during larval stages.
  • Possessing flagellated sperm.

• Reproduction:

  • Sponges reproduce both sexually (using gametes) and asexually (budding).
  • Hermaphroditic; individuals produce one kind of gamete at a time.
  • Asexual methods include:
  • External Budding: Part of the sponge breaks off and regenerates.
  • Internal Budding (Gemmules): Enclosed clusters of cells that form under adverse conditions.

• Body Forms:

  • Asconoid: One-cell thick structure (example: early-stage calcareous sponges).
  • Syconoid: Folded structure (example: many calcareous sponges).
  • Leuconoid: Highly subdivided into flagellated chambers (example: most calcareous sponges and all Demosponges).

• Anatomy:

  • No gastrovascular cavity.
  • Spongocoel: Large central cavity for feeding, water enters through ostia (pores) and exits through osculum.
  • Pinacocytes: Form the outermost layer (pinacoderm).
  • Mesohyl: Jelly-like matrix providing structure.
  • Amoebocytes within mesohyl maintain nutrient transport and structural functions.

Cnidaria (Jellyfish, Corals, Anemones)

• Overview:

  • Includes corals, sea jellies, anemones, and hydrozoans.
  • Approximately 11,000 extant species (and about 5000 fossil species).
  • Ancient group dating back to the Ediacaran; existing classes date to the Ordovician period.
  • Exhibit diploblastic development with radial symmetry, many exist in colonies.

• General Characteristics:

  • All cnidarians are aquatic.
  • Have ectoderm and endoderm (diploblastic).
  • Possess stinging structures called cnidae.
  • Muscular movement via epitheliomuscular cells.
  • Alternation of life stages: Asexual (polyp) and sexual (medusa).
  • Gastrovascular cavity functions as a single opening for digestion (mouth/anus).
  • Simple nerve net present in body.
  • Mesoglea: a gelatinous layer between tissue layers.

• Life Cycle:

  • Polyp Stage:
  • Sessile phase attaching to a substrate.
  • Buds to form medusae.
  • Medusa Stage:
  • Free-swimming; produces gametes.
  • Fertilized eggs form ciliated planula larvae that settle to form new polyps.

• Body Forms:

  • Polyp: Cylindrical, adheres to substrate; can move when threatened.
  • Medusa: Mouth-down with a flattened structure, capable of drifting or movement through contraction.

• Nematocysts:

  • Unique stinging cells defining cnidarians, located on tentacles or around the mouth.
  • Utilized for predation, using toxins to capture prey.

• Nervous System:

  • Non-centralized nerve net for sensing and movement.
  • Simple contractile tissues without a brain.

• Major Clades:

  • Medusozoa: Include all Cnidarians producing a medusa form.
  • Examples:
    • Scyphozoans: True jellyfish.
    • Cubozoans: Box jellyfish.
    • Hydrozoans: Small and predatory.
  • Anthozoa: Always exist as polyps (e.g., sea anemones, corals).
  • Corals can be solitary or colonial; important for reef formation.
  • Threats include pollution, overharvesting, ocean acidification, and climate change.