sponges and cnidarians

Learning Outcomes and Universal Biological Challenges

• Primary Learning Objectives:
    - Identify members of the phyla Porifera and Cnidaria and describe their fundamental characteristics.
    - Detail key biological innovations in sponges relative to their protist ancestors.
    - Detail key biological innovations in cnidarians relative to their sponge ancestors.

• Universal Animal Challenges:
    1. Finding and digesting food.
    2. Finding a mate and successfully reproducing.
    3. Avoiding predation while accomplishing the first two tasks.

• Phylogenetic Context and Biodiversity:
    - The evolutionary lineage progresses from an ancestral "protist" to Sponges, then Cnidarians, followed by Flatworms, Mollusks, Annelids, Roundworms, Arthropods, Echinoderms, and Chordates.

• Phylum Porifera (Sponges):
    - Consists of approximately $10,000$ species.

Phylum Porifera: Sponges

• General Characteristics:
    - All members are aquatic; the majority are marine, though freshwater sponges exist.
    - Most sponges lack body symmetry entirely. Some species are nearly radial.

• Anatomy and Specialization:
    - Sponges are truly multicellular because they possess specialized cells.
    - They do not have cells organized into true tissues.

• Cellular Components and Feeding:
    - Choanocytes (Collar Cells):
      - Specialized feeding cells.
      - Similar in structure to colonial choanoflagellates and unicellular choanoflagellates.
      - Use flagella to create water currents to filter organisms for food.
    - Amoebocytes:
      - Multipurpose cells involved in secreting the skeleton and reproductive processes.

• Regeneration:
    - Sponges possess the ability to regenerate from fragmented parts.

• The Sponge "Skeleton":
    - Spicules:
      - Found in hard sponges (e.g., the Venus’ basket sponge).
      - Needles composed of calcium carbonate ($CaCO ext{ }_3$) or silica ($SiO ext{ }_2$), secreted by amoebocytes.
    - Spongin:
      - Found in soft, spongy sponges.
      - Fibers made of a tough protein, also secreted by amoebocytes.

• Adaptations for a Sessile Lifestyle:
    - Adults are sessile (cannot move).
    - Key adaptations include:
      - Filtering small organisms from water (filter feeding).
      - Secreting chemicals to prevent other organisms from growing over them.
      - Having a motile larval stage that uses cilia to swim and find a substrate before metamorphosing into an adult.

• Reproduction:
    - Sponges are "sperm casters."
    - Sperm is released into the water (spawning).
    - Amoebocytes "fish" the sperm from the water and carry it to the egg for fertilization.

Phylum Cnidaria

• General Characteristics:
    - All are aquatic; nearly all are marine, with a few freshwater species (e.g., Hydra).

• Tissue Organization:
    - Cnidarians possess distinct tissues but lack organs.
    - They are diploblastic:
      - Ectoderm: Forms the outer covering and nerve cells.
      - Endoderm: Lines the digestive cavity.   - Mesoglea: A jelly-like "filling" located between the ectoderm and endoderm that contains no cells.

• Symmetry:
    - All cnidarians exhibit radial symmetry.

• Body Forms:
    - Polyp: Usually sessile and cylindrical (e.g., Sea Anemones, corals, Hydra).
    - Medusa: More motile and umbrella-shaped (e.g., Jellyfishes).

• Feeding and Digestion:
    - Cnidarians are carnivores, not filter feeders.
    - Nematocysts:
      - Specialized cells on tentacles that contain a discharge mechanism.
      - Used to penetrate, stick to, wrap around, or deliver venom to prey.
    - Innovation in Digestion:
      - Cnidarians utilize extracellular digestion inside a gastrovascular cavity.   - Comparison:
      - Sponges use intracellular digestion (inside amoebocytes); Cnidarians release digestive enzymes into the cavity to break down food.
    - Advantage:
      - Extracellular digestion allows cnidarians to consume food items larger than a single cell.

• Colonial Polyps: Corals:
    - Corals are colonies of polyps that frequently secrete calcium carbonate ($CaCO ext{ }_3$) exoskeletons.
    - Harbor symbiotic photosynthetic protists known as dinoflagellates (e.g., Ceratium).

• Habitat:
    - Most coral reefs are found in shallow ocean waters to ensure symbiotic protists have access to light for photosynthesis.

• Coral Bleaching:
    - Occurs when warming ocean temperatures cause corals to expel their symbiotic protists.

• Reproduction:
    - Many variations exist.
    - Asexual Reproduction:
      - Seen as "budding" in Hydra.

Questions & Discussion

• Q1: Similarity between Sponges and Cnidarians:
    - Both are aquatic, multicellular, heterotrophic, and have specialized cells.
    - Major difference is symmetry (Sponges lack it; Cnidarians have radial symmetry).

• Q2: Comparison Accuracy:
    - Sponges generally lack symmetry (Cnidarians are radial).
    - Sponges do not have tissues (Cnidarians do).
    - Sponges are sessile as adults (many Cnidarians can move).
    - Note: It is inaccurate to say Sponges do not have specialized cells; they do (choanocytes).

• Q3: Sessile Adaptation Exceptions:
    - Sponges feed by filtering, release swimming gametes, have motile larvae, and secrete defensive chemicals.
    - They do not have a complex nervous system.

• Q4: Polyp vs. Medusa:
    - Polyps are typically sessile, whereas medusas are more motile.

• Q5: Coral Reef Location:
    - Expected in shallow ocean waters due to light requirements for photosynthesis.

• Q6 & Q7: Digestion Mechanics:
    - Sponges digest food inside cells (intracellular); Cnidarians digest food inside a digestive cavity (extracellular).
    - Primary advantage of extracellular digestion is the ability to eat prey items larger than a cell.

Administrative Reminders

• Next Class Topics: Flatworms and Roundworms.

• Assignments: Check Canvas for readings and videos. WC10 is due on Monday.