Lecture 4: 5 February

Text book Chapter 12

• Gould, S. J., 1984. A most ingenious paradox. Natural History 12/84, 20-on.
  • Web resources for this lecture:
  • University of California, Berkeley web page on relations between animal (Metazoan) Phyla
  • University of California, Berkeley web page on Porifera
  • University of California, Berkeley web page on Cnidaria

• Origin of multicellular Animals, Plants, Fungi: colonies of unicellular eukaryotes; originated independently
• Most commonly shown example of colony of eukaryotic unicellular organisms: alga Volvox
• Cells have to cooperate, specialize in roles in multicellular animals (Metazoa)
  • Slime molds: cooperating cells
  • Porifera (sponges): between individual and cooperating cells
  • Cnidaria: Siphonophores (jellyfish family) and reef corals - cooperating individuals

Eukaryote animals

• types of tissues, organs
• body cavities
• embryological development
• symmetry

Subdivision of Animals (Eukarya)

• Phylum Porifera (Sponges): not much more than colonies of cells; no symmetry
• Phylum Cnidaria (corals, jellyfish): two layers of cells in embryo, tissues (ectoderm, endoderm); radial symmetry
• Bilateria: three layers of cells (endoderm, ectoderm, mesoderm); organs; bilateral symmetry
  • No body cavities: Acoelomates
    • Phylum Platyhelminthes (flatworms)
  • Body cavities: Eucoelomates
    • Deuterostomates:
      • Phylum Chordates
      • Phylum Echinodermata
    • Protostomates
      • Ecdysozoa: molters
        • Phylum Arthropoda
      • Lophotrochozoa
        • Lophophorates: feeding apparatus ‚ lophophore
          • Phylum Brachiopoda
          • Phylum Bryozoa
        • Trochozoa: larvae ‚Trochophores‚
          • Phylum Mollusca: snails, clams, octopus, Nautilus, ammonites

Protostomes - Deuterostomes: Very important and old split in lineage of animals. Difference originates very early in embryonic development. When hollow ball of cells forms (gastrula stage), the first invagination becomes the mouth in the protostomes; it becomes the anus (and another mouth opening forms) in the deuterostomes.

• Slime molds: cooperating cells
• Siphonophores (Cnidaria): cooperating individuals

• Originally classified as Fungi (mushrooms); now placed in large group of 'protists', which is changing in the classification from one group into many different ones at the same hierarchical levels as 'animals', 'plants', 'mushrooms'.
• Have complex life: partially as unicellular organisms (amoeba-like), partially as multicellular animal-look-alikes, partially looking like Fungi
• Produce spores like Fungi

• Spore lands in good habitat (rich in bacteria, moist soil)
• Hatches into amoeba-like being
• Reproduces asexually (splitting): many amoebae
• Food shortage: chemical released (Adenosine Mono Phosphate, AMP): amoebae get together
• Form mound-like object; topples; changes into slug-like shape
• Migrates into light; forms discoid anchor, grows into stalk, spore-holding know at end: mushroom-like.
• During life time: changes from unicellular, to moving, multicellular, to stationary spore-former.

Even in present world: Transitions between unicellular colonies and individuals exist

• Components are individuals part of their life cycle, function as 'organs' part of their life cycle
• In between multicellular animals, colony of unicellular organisms
• We think multicellularity (and colonialism) has evolved many times independently. Can not be used to trace descent (not homologous). 

• No symmetry; 'Animal' is close to 'colony of cells'
• Cells have few specializations; no organ system
• Collar cells (choanocytes) gather food, pump water
• Archaeocytes: undifferentiated cells
• Some forms have skeleton: spongine; some have needles (spicules) of SiO₂ or CaCO₃.

• Few cell types: epithelial (cover), connective, muscular, nervous, reproductive
• Outer layer (ectoderm), inner layer (endoderm); in between jelly-like material
• Cnidaria name after Greek for stinging nettle: all Cnidarians sting (knidos). They sting using nettle cells (nematocysts)

  Cnidaria life cycle: alternating generations. Jellyfish-> fertilized eggs -> larva called Planaria -> polyp -> splits of medusae -> grow into adult jellyfish. There is a ring of muscle and nerve tissue around the mouth.

Modern Cnidaria: 3 groups

• Anthozoa: Many corals, anemones: lost jellyfish stage
• Scyphozoa plus Cubozoa: true jellyfish, lost polyp stage (split by some people in two, Scyphozoa and Cubozoa). A Cubozoan jellyfish is the most deadly jellyfish in the world, Chironex fleckeri. A Scyphozoan is the largest (and deadly) jellyfish in the world, Cyanea capillata, also called the 'Lion's mane'.
• Hydrozoa (mixed bag): hydras, some corals, siphonophorans

Siphonophoran: many individuals ('persons'): some float, some swim, some catch food, some digest food, some reproduce. Many are poisonous, including the Portuguese Man-o-War.

• Components are individuals by history, but function as organs (not each component does all life functions, such as eat or reproduce or walk)
• In between colony of animals and individual animal, in effect forming a kind of 'super-organism'

• Symbiotic algae: zooxanthellae (dinoflagellates)
• Life in very low-food (oligotrophic) conditions: nutrients continuously recycled
• Reef corals: endosymbiosis
• Scleractinian (stony) corals are now (and were the last 65 myr) the dominant reef-builders of shallow tropical seas
• Scleractinians date back to the mid Triassic (220 Ma).
• Molecular, stable isotopic, ecological evidence suggests scleractinian corals formed symbioses soon after their appearance in the fossil record.
• Corals were not ALWAYS the dominant reef builders!

• Up to 95% of the carbon fixed by zooxanthellae is translocated to the coral host (glycerol)
• Opportunistic heterotrophic feeding:
  • Phagotrophy (tentacles grab passing food items and pass to the mouth, where they are ingested)
  • Ciliary feeding (the mucus sheath covering the coral traps tiny organic particles which are wafted to the mouth by cilia).
  • Uptake of dissolved organic matter from seawater