Phylum Porifera

• Sponges are metazoan – multicellular organisms that do not have any specialised tissue or organs
• Sponges are motile in larval stage, and then mobile or sessile in adult form

  o   Motile: Can move without any help

  o   Mobile: Can move with or without any help

  o   Sessile: Cannot move
• Why are sponges considered animals

  • Multicellular
  • Cells connected by collagen, which is made only by animals
  • Contain membrane-bound organelles found only in eukaryotes
  • No cell wall or chloroplasts
  • Motile in the larval stage (As long as animal is motile for part of its life cycle, it is considered an animal)
• General body structure of sponges

  • Loose aggregation of cells bound by collagen
  • Two layers of cells make up body wall
• Outer layer

  Pinacocyte (flattened and tightly packed to form protective layer and maintain body shape)
• Inner layer

  Choanocytes (with flagella)
• Middle layer

  Mesohyl (consists of two layers of cells separated by a gelatinous matrix)
• Cells called

  Porocytes (form the pores from which water and nutrients are taken in)
• Water flow

  1. Flows in from smaller pores called ostia (singular: ostium) into a central cavity called spongocoel

  2. Flows out from large openings called oscula (singular: osculum)
• Amoebocytes


  Cells in the mesohyl that transport nutrients to the other cells of the sponge body, produce spicules, or become any type of sponge cell as needed (totipotent)
• Most sponges are hermaphrodites (monoecious), meaning that each individual functions as both a male and female in sexual reproduction by forming both male and female gametes
• Sponge endoskeleton is made of spicules and spongin
• Spicules can exist either as siliceous (made of silicon dioxide) or calcareous (made of calcium carbonate), but not both in one organism

  • Some species may not have any spicules – sponge skeleton is a firm rubbery mesh
  • Spicules are either fused or interlocked to form a lattice – sponge skeleton is a rigid, brittle lattice
• Spongin is made of collagen fibres

  • Keeps the spicules in place
  • Provide support for sponges without any spicules
• Body with ostia and oscula, canals and central cavity form the water canal system

  • Ostia are more abundant
  • Ostia is for water inflow, oscula is for water outflow
  • Flow of water is unidirectional

  The coordinated beating of the flagella creates the flow of water
• Flow of water is important to

  o   Bring in materials for filter feeding

  o   Brings out metabolic and excretory waste

  o   Brings in oxygen and gives out carbon dioxide

  o   Reproduction purposes (mentioned down)
• Sponges are monoecious/hermaphrodites – do not need a partner to mate and reproduce

  Spawning – process of releasing sperm cells into the water to be carried by currents to another sponge (since they are sessile)

  • Choanocytes differentiate to form sperm cells
  • Sperm pushed out by water flow via the oscula into the big wide ocean
• Fertilisation

  o   Amoebocytes differentiate to form ova in the mesohyl

  o   Water current brings in sperm for fertilisation

  o   Fertilisation occurs in mesohyl

  o   Water brings out zygote and larvae for dispersal
• Sponges use filter feeding to eat
• Flagella of choanocytes

  • Have microvilli
• Microvilli

  Surface has mucus at the top to trap food particles
• How food is trapped

  1. Gaps in the microvilli allow water to flow through

  2. Causes tiny food particles to get trapped in the mucus
• Movement of food

  1. Mucus moves down the flagella nearer to the choanocyte cell body

  2. Through the movement of flagella and water
• No coordination of movements, purely by chance
• Engulfing of food

  1. Cell body of choanocytes engulf the food via phagocytosis or endocytosis

  2. To form food vacuole
• Phagocytosis


  Cytoplasm extends to engulf the food
• Endocytosis


  Cytoplasm folds in to take in the food
• Digestion of food

  1. Lysosome fuses with food vacuole

  2. To digest it via intracellular, enzymatic, chemical digestion (a primitive form of digestion)
• Amoebocyte associates with choanocyte


  To transport digested food to other cells of the sponge since amoebocyte is motile but choanocytes are not
• Exocytosis and endocytosis

  1. Exocytosis of food vacuoles from choanocyte

  2. Endocytosis of amoebocytes
• Challenge of using filter feeding is that sponges can only eat small food that can enter via the pores – limited nutrition for sponges
• Syconoid and Leuconoid structures of sponges increase surface area to volume ratio to allow more choanocytes to be present on the inner layer of cells – more efficient feeding