Module 1 - Phylum Porifera

• They have a porous body with numerous small openings called ostia that allow water to enter.
• Porifera are mostly marine animals consisting of loosely organized cells without tissues or organs
• There are three cell types in Porifera: pinacocytes, choanocytes, and amoebocytes
• The central cavity or a series of branching chambers in Porifera is used for water circulation during filter feeding
• Microvillar collar cells or choanocytes in Porifera surround flagella, with units arising from either single cells or syncytia
• Syncytial cells in Porifera are not separated by distinct membranes
• A syncytial layer in Porifera has multiple nuclei contained within a single, large plasma membrane for efficient exchange of materials and signals between cells
• Muscle cells in Porifera are syncytial cells, while liver cells are not
• General morphological features of Poriferans include being the simplest multicellular animal, having spongin or spicules, a body with pores and a canal system
• Porifera have a mesohyl, a jellylike middle layer that acts like an endoskeleton and produces spicules
• Archaeocytes/Amoebocytes in Porifera are amoeboid cells in the mesohyl that reproduce, secrete skeletal elements, transport food, store food, and form contractile rings around the opening
• Archaeocytes/Amoebocytes in Porifera also store and digest food, give rise to gametes, are involved in non-self recognition, eliminate wastes, and are totipotent (able to give rise to new cells/differentiation)
• Pinacocytes:

  • Thin, flat, epithelial cells covering the exterior and interior surfaces of a sponge
  • Derived from the epithelial layer during sponge development
  • Some pinacocytes are modified into myocytes or "muscle cells"
  • Functions:
  • Line the outer surface of a sponge
  • May be mildly contractile and contraction may change the shape of the animal
• Porocytes:

  • Specialized cells found in the walls of the sponge's body
  • Responsible for creating and maintaining the openings (ostia) through which water enters the spongeFunctions:
  • Regulate the flow of water into the sponge's internal channels for filter feeding and gas exchange
  • Cylindrical cells that create a tube-like structure within the sponge's body wall
  • Can contract to close the pores during unfavorable environmental conditions
• Choanocytes (collar cells):

  • Ovoid cells with one end embedded in mesohyl and the exposed end bears a flagellum surrounded by a collar
  • Functions:
  • Collar made up of microvilli connected to each other for filter feeding
  • Line the flagellated canals and channels to generate currents that help maintain circulation of seawater within and through the sponge
  • Capture small food particles and incoming sperm for fertilization
  • Believed to have arisen from choanoflagellates or choanoflagellate-like ancestor, which are the oldest living protozoan ancestors of animals
• Reproductive cells:

  • Oocytes: egg cells
  • Spermatocytes: sperm cells
• Rhabidiferous cells:

  • Secrete polysaccharides
  • Form part of the mesohyl
• Grey cells:

  • Function for immunity in sponges
• Poriferans are the simplest multicellular animals, an aggregation of cells embedded in mesohyl
• Pinacocytes are thin, flat, epithelial cells that cover the exterior and interior surfaces of a sponge
• Choanocytes (collar cells) are ovoid cells that line the flagellated canals and channels, generating currents for filter feeding
• Reproductive cells include oocytes (egg cells), spermatocytes (sperm cells), and rhabidiferous cells that secrete polysaccharides
• Monaxon: needle-like or rod-like spicule; straight or curved
• Tetraxon: has 4 prongs (spicule)
• Triaxon or Hexaxon: 3 or 6 rayed (spicule)
• Polyaxon: multiple short rods radiating from a common center; burr shaped, star shaped or like a child's jack
• Asconoid: tube-shaped body with 1 osculum, choanocytes line the spongocoel, imposes size limits due to water flow
• Syconoid: tubular body with invaginated walls, choanocytes in radial canals, can grow larger than asconoid
• Leuconoid: most complex, invagination of canals to form small flagellated chambers, loss of spongocoel, may attain large sizes
• Sponges are planktonic or motile at the larval stage and sessile at the adult stage
• Sponges can creep across the seabed at 1-4 mm per day due to amoeba-like movement of pinacocytes
• Sponges have no nervous tissues and exhibit coordination of movement through contractions of pinacocytes and myocytes
• Respiration and excretion in sponges are supported by a water flow system, absorbing oxygen and excreting waste products through diffusion
• Respiration and Excretion:

  • Supported by water flow system
  • Absorbs oxygen and excretes waste products (e.g., ammonia) through diffusion
  • Archeocytes/amoebocytes remove mineral particles that block the ostia by transporting and dumping into the outgoing water current
• Feeding:

  • Filter feeding: generally feed on suspended particles in the water pumped through the canal system
  • Filtered food by choanocytes and passed on to archeocytes
  • Pinacocytes may phagocytosize food particles at the surface followed by intracellular digestion
• Cell body:

  • Contains microvilli, microfibrils, and a collar
• Carnivorous sponge:

  • Mostly from the family Cladorhizidae
  • Techniques include using sticky threads or hooked spicules to entangle prey with fine threads
  • Possible use of venom
  • Inflate balloon-like structures for prey capture
  • Most lost the water flow system and choanocytes
  • Food-capturing mostly crustaceans and other small animals
• Host for endosymbionts:

  • Mostly freshwater sponges
  • Most common in water with relatively poor supply of food particles
  • Have leafy shapes that maximize the amount of light they collect
  • Host green algae within archeocytes and other cells
  • The Calcibacteria harboring Sponge, Hemimycale columella, from the Atlantic and Mediterranean
• "Immune" System:

  • Do not have a complex immune system
  • Can reject grafts from other species but accept them from other members of their species
  • Gray/grey cells reject foreign material
  • Produce a chemical that stops the movement of other cells in affected areas
  • Concentrate in the areas and release toxins that kill all cells in the area
• • Free-swimming larva is expelled via the osculum, settles at the bottom, and begins external development