Sponges, Cnidaria, and Animal Characteristics

Sponges and Cnidaria

  • Jellyfish populations are expanding globally due to:
    • Over-fishing of their predator species.
    • Surface run-off.
    • Climate change, as warm waters favor their growth and proliferation.

Animal General Characteristics

  • All animal cells:
    • Lack cell walls.
    • Are multicellular.
    • Most form tissues.
  • Cell membrane allows for flexibility.
  • Held together by extracellular matrix proteins:
    • Collagen.
    • Proteoglycan complex.
    • Carbohydrates.
    • Protein.

Body Plan Innovations

  • Diversity of body plans.
  • Four key innovations identify types of animals:
    1. Symmetry.
    2. Tissues.
    3. Body cavity.
    4. Embryonic development.

Symmetry

  • Organization of body form.

  • Three types:

    1. Asymmetry - No pattern in growth or body shape.
    2. Radial symmetry – Arranged in a radial pattern, where any longitudinal plane may divide an organism into mirror images.
    3. Bilateral symmetry – Body may be divided into mirror images only into right and left halves.
    • Allows directional movement.
    • Important for the evolution of a nervous system grouped at the anterior end à Cephalization.

Tissues

  • Zygote gives rise to all tissue types.
  • Specialization is irreversible in all animals except Porifera à Lack tissues!
  • Embryonic development leads to two germ layers in diploblasts and three germ layers in triploblasts:
    • Ectoderm à Integument and nervous systems.
    • Mesoderm à Skeletal and muscular systems.
    • Endoderm à Digestive system.
  • Cnidaria are diploblasts, and most Bilateria are triploblasts.

Body Cavity

  • Germ layers form a fluid-filled cavity.
  • Three types found in animals:
    • Acoelomate à No body cavity. Organism is completely filled with cells and connective tissue from endoderm to ectoderm.
    • Pseudocoelomate à Body cavity forms between the mesoderm and the endoderm.
    • Coelomate à Body cavity forms entirely within the mesoderm.

Embryonic Development

  • All animal zygotes begin development through mitotic divisions called cleavage.
  • Blastopore à Hole connecting internal cavity with external environment.

Protostomes

  • Most bilaterians, share a few traits:
    1. Blastopore forms the mouth.
    2. Many exhibit spiral cleavage.
    3. Determinate development à Tissue type is determined early in development. If a cell separates, the embryo dies.

Deuterostomes

  • Only echinoderms and chordates
    1. Blastopore forms the anus.
    2. Exhibits radial cleavage.
    3. Indeterminate development à Tissue type is determined later in development. Any cell is capable of forming an individual.

Metazoan Groups

  • Protostomes:
    • Blastopore forms mouth.
    • Spiralia:
      • Spiral cleavage.
      • Platyzoa: Platyhelminthes or flatworms.
      • Lophotrochozoa: Mollusca and Annelida.
    • Ecdysozoa: Molting animals, including arthropods and nematodes.
  • Deuterostomes: Blastopore forms the anus, including echinoderms and chordates.
  • Metazoa (acoelomate)

Porifera

  • Animals without specialized tissues and mostly asymmetric body plans – Sponges!
  • Most sponges are marine, but there are a few freshwater species.
  • Free-living larval stage.
  • Sessile adult stage.
  • Key characteristics:
    1. Specialization in cells is reversible.
    2. Filters water for nutrition.
    3. Can undergo asexual or sexual reproduction.

Porifera Anatomy

  • Three distinct cell layers:

    1. Choanocytes
    2. Mesohyl
    3. Outer epithelial

  • Osculum: Typically a large, singular pore that water travels out or sponge.

  • Ostia: Typically many, small pores that water travels into the sponge.

  • Choanocytes

    • Forms inner layer of sponge.
    • Cell with singular flagella that creates current to bring in oxygen and food, expel waste à Feeds on single celled organisms.

  • Mesohyl

    • Middle layer of sponge.
    • Contains eggs, spicules, and spongin.
      • Spicules: Skeleton, formed from carbonate or silica.
      • Spongin: Fibrous protein skeleton.

Porifera Reproduction

  • Can undergo asexual or sexual reproduction:
    • Asexual: Via fragmentation à Each fragment may form a new individual.
    • Sexual: Sperm forms from choanocyte. Sperm captured by another choanocyte and fertilizes egg within mesohyl.

Eumetazoa

  • Animals with true tissues. Includes Cnidaria and all other animals except Porifera.
  • Irreversibly differentiated tissues, exhibit either radial or bilateral symmetry.

Cnidaria

  • Key characteristics:
    1. Diploblastic, but lack true organs.
    2. No cephalization. Nervous system throughout body.
    3. Capture prey using nematocysts (stinging cells).

Cnidaria Body Forms

  • Radially symmetrical, but has two distinct forms:
    • Polyp: Cylindrical body with mouth surrounded by tentacles. Usually sessile.
    • Medusa: Umbrella-shaped, with mouth surrounded by tentacles. Usually free-living.

Cnidaria Anatomy

  • Mouth opens into gastrovascular cavity à Site for digestion, gas exchange, waste discharge, gamete formation, etc.
  • Three layers:
    1. Epidermis: Outer layer.
    2. Mesoglea: Middle layer, can be acellular or cellular.
    3. Gastrodermis: Inner layer of gastrovascular cavity.

Gastrovascular Cavity

  • Extracellular digestion occurs within the gastrovascular cavity à Enzymes released, food particles absorbed.

Nematocyst

  • Stinging cell, unique to Cnidaria.
  • Used to capture prey items and as defense.
  • Usually located on tentacles, but not always.
  • Some inject venom, which may be deadly to humans.

Anthozoa

  • Includes sea anemones and corals.
  • Usually polyp body form, can be solitary or colonial.
  • Corals form reefs, where they form symbiotic relationships with dinoflagellates to utilize photosynthesis as a food resource.

Cubozoa

  • Box jellies, where tentacles form from corners. Mainly medusa stage.
  • Some of the most deadly cnidarians to humans! Some have rudimentary eyes.

Hydrozoa

  • Hydroids, consists of both polyps and medusa forms. Only freshwater cnidarian Hydra.

Schyphozoa

  • Jellyfish, mostly medusa form.
  • Portuguese man-of-war is a floating colony of polyps and medusas.
  • Contains ring of muscle cells that propel water out through gastrovascular cavity for propulsion.

Key Points

  1. Four body plan innovations in animals:
    • Symmetry
    • Tissues
    • Body cavity
    • Development
  2. Know basic clades and divisions within Animalia
  3. Porifera lack specialized tissues and filter water for food
  4. Cnidaria are diploblastic and capture larger prey using nematocysts