Syncytial hypothesis: came from multinucleate ciliated cells

1
Origin of multicellular animals

• Syncytial hypothesis came from multinucleate
  ciliated cells
• Colonial hypothesis came from colonial
  flagellated cells (some rRNA
• evidence)
• Polyphyletic no single
• scheme (monophyletic would indicate single
  ancestor for all animals)

2
Phylum Porifera The Sponges
"pore bearer"
3
Phylum Porifera The Sponges

• 9,000 species- mostly marine (150 fresh water)
• First appear in Paleozoic Era (600 mya)- first
  thought plants (animal kingdom- 1857)
• Simplest of all animals
• Possibility arose from protozoan similar to
  present day choanoflagellates (have choanocytes)
• Most sessile (permanently attached)

4
Characteristics

• Range from mm to 2m- many shapes (ball, vase,
  disc, shrub, tree)
• Asymmetrical or radially symmetrical
• No tissues or organs (cell aggregate with
  specialized cells)
• Diploblastic
• Central cavity (spongocoel) or branching chambers
  through which filter feeds exits out large
  opening in top (osculum)

5
Body plan

• Pinacocytes thin, flat cells that line outer
  surface
• Some porocytes (ostia or pore cells) openings
  for water movement

6
Body plan

• Mesohyl jellylike middle layer
• Mesenchyme cells (amoebocytes) specialized for
  reproduction, secreting skeletal elements,
  transporting and storing food, forming
  contractile rings

amoebocyte
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Body plan

• Choanocytes (collar cells) flagellated cells
  that beat and create current

8
Body plan

• Spicules skeleton of needlelike spikes- CaCO3
  (can get from boring holes into shells) or
  silica- used for defense (some produce poisons)
• Spongin spongy protein of collagen (do not cause
  allergic reaction like latex)

spongin
spicule
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Three forms
figure 18.7

• Ascon- vase like
• Sycon- folded wall
• Leucon- extensively branched canal system (can
  provide protection for invertebrates)

ascon
sycon
leucon
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Nutrition

• Filter feeders bacteria, algae, microorganisms
  (up to 1500L per day)
• A few carnivorous
• Important source of nutrition for some snails,
  sea stars and fish
• Respiration and excretion by diffusion

11
Reproduction

• Most hermaphroditic but do not self fertilize
  (produce eggs and sperm at different times) form
  from choanocytes or amoebocytes
• Eggs retained in mesohyl, sperm exit osculum and
  travel to other sponges

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Reproduction

• Larva develop within wall
• Larva breaks free and swims for no more than two
  days

13
Reproduction

• Asexual by budding, fragmentation (one cell can
  regenerate in some species), and creating
  gemmules (weather resistant capsules of
  ameobocytes surrounded by spicules)- released in
  winter

14
Ecological
brittle stars
home to many other species
15
Natural sponges
non allergenic
spongin spicules (collagen)
16
Antibiotic possibilities
AR5
17
Interesting fact

• Some genetically engineered bacteria produce a
  gluelike chemical similar to that produced by
  sponges (used in medicine and industry)