Sponges and Placozoans

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Sponges and Placozoans

• Chapter 12

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Origin of Metazoa

• Evolution of the eukaryotic cell was followed by
  diversification into many lineages including
• Modern protozoans
• Plants
• Fungi
• Animals
• Multicellular animals are called metazoans.

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Dendrogram of Major Phyla
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Choanoflagellates

• Choanoflagellates are solitary or colonial
  protozoans with a flagellum surrounded by a
  collar of microvilli.

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Choanoflagellates

• Choanoflagellates resemble sponge feeding cells
  (choanocytes).
• Scientists are studying colony formation and
  cell-to-cell communication in choanoflagellates
  in search of clues to the evolution of
  multicellularity.

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Colonial Flagellate Hypothesis

• Colonial Flagellate Hypothesis metazoans
  descended from ancestors characterized by a
  hollow, spherical colony of flagellated cells.
• Individual cells became specialized for different
  functions.
• Radially symmetrical, similar to a blastula.
• First proposed by Haeckel in 1874

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Phylum Porifera

• Sponges, Phylum Porifera, are multicellular
  heterotrophs.
• They are asymmetrical.
• They lack true tissues and organs.
• Molecular evidence suggests they do share a
  common ancestor with other animals.
• Kingdom Animalia is monophyletic.

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Phylum Porifera

• Sponges are sessile animals that have a porous
  body and choanocytes.
• Supported by a skeleton of tiny needlelike
  spicules and protein.
• They live in both fresh and marine waters.

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Phylum Porifera

• Sponges range in size and shape.
• Up to 2 meters in diameter!
• Encrusting, boring, finger, tube or vase shaped.

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Neighbors

• Many organisms, including crabs, nudibranchs,
  mites, bryozoans, and fish live as commensals or
  parasites in sponges.

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Skeletal Framework

• The skeletal framework of a sponge may be fibrous
  or rigid.
• The fibrous part comes from collagen fibrils in
  the intercellular matrix.
• Spongin

• Rigid skeletons consist of needlelike spicules.
• Calcareous
• Siliceous
• Composition and shape the spicules forms the
  basis of sponge classification.

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Suspension Feeders

• Sponges are suspension feeders capturing food
  particles suspended in the water that passes
  through their body.

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Suspension Feeders

• Water flows in through incurrent pores called
  dermal ostia.
• It flows past the choanocytes where food
  particles are collected on the choanocyte collar.

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Suspension Feeders

• Choanocytes take in small particles by
  phagocytosis. Protein molecules are taken in by
  pinocytosis.
• Sponges can also absorb nutrients dissolved in
  the water.

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Canal Systems

• Asconoid the simplest canal system.
• Choanocytes line the spongocoel.
• Water enters through the ostia and exit through
  the large osculum.
• Usually tube shaped.
• Found only in the Class Calcarea.

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Canal Systems

• Syconoid tubular body and singular osculum like
  asconoids.
• The walls of the sponge are folded to form
  choanocyte lined canals.
• Increased area for feeding.
• Class Calcarea.

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Canal Systems

• Leuconoids most complex, permits an increase in
  sponge size.
• Choanocytes line the walls of small chambers
  where they can filter all the water that flows
  through.
• Most sponges.

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Types of Cells

• Absence of tissues organs means that
  fundamental processes occur on the cellular
  level.
• Respiration and excretion occur by diffusion in
  each cell.
• Mesohyl is the gelatinous matrix containing
  skeletal elements amoeboid cells.

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Types of Cells

• Choanocytes, flagellated collar cells, generate a
  water current through the sponge and ingest
  suspended food.

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Types of Cells

• The choanocytes pass food particles to
  archaeocyte cells for digestion.
• Digestion occurs entirely within cells, there is
  no gut.
• Other cell types secrete spicules (sclerocytes),
  spongin (spongocytes), collegen (collenocytes).

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Types of Cells

• Pinacocytes are thin, flat, epithelial-type cells
  that cover the exterior and some interior
  surfaces of the sponge.
• Almost a true tissue.

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Reproduction

• Sponges have remarkable regeneration
  capabilities.
• Regeneration following fragmentation is a form of
  asexual reproduction.
• External buds can break off to form new sponges.
• Internal buds (gemmules) in freshwater sponges
  can remain dormant in times of drought.

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Reproduction

• Most sponges are hermaphrodites meaning that each
  individual functions as both male and female.
• Monoecious
• Gametes are derived from choanocytes or sometimes
  archaeocytes.

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Reproduction

• Most sponges are viviparous.
• After fertilization, the zygote is retained and
  is nourished by the parent. Ciliated larvae are
  later released.
• Some are oviparous releasing gametes into the
  water.

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Reproduction

• Sponges in the class Calcarea and a few
  Demospongiae have an unusual developmental
  pattern where the embryo turns inside out.
• Flagellated cells become choanocytes
  archaeocytes.
• Larger cells become pinacocytes.

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Class Calcarea

• Calcareous sponges (Class Calcarea) have spicules
  composed of calcium carbonate.
• Small, usually vase shaped.
• Asconoid, syconoid, or leuconoid in structure.

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Class Hexactinellida

• Glass sponges (Class Hexactinellida) are mostly
  deep sea forms.
• Spicules are six-rayed and made of silica.
• Hexactinellids lack a pinacoderm or gelatinous
  mesohyll.
• Chambers appear to correspond to both syconoid
  and leuconoid types.

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Class Hexactinellida

• Some advocate placing hexactinellids in a
  subphylum separate from other sponges.
• Trabecular reticulum made of a fusion of
  archaeocyte pseudopodia - forms the chambers
  opening to spongocoel.
• Trabecular reticulum is largest continuous
  syncytial tissue known in Metazoa.

• Choanoblasts are associated with flagellated
  chambers.
• Collar bodies do not participate in phagocytosis
  this is the function of the primary and
  secondary reticula.

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Class Demospongiae

• Class Demospongiae contains most of the sponge
  species.
• Spicules are siliceous, but not six-rayed.
• Spicules may be bound together by spongin, or
  absent.
• All leuconoid, mostly marine.

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Class Homoscleromorpha

• A fourth class, Homoscleromorpha, was formed to
  contain sponges without a skeleton or with
  siliceous spicules without an axial filament.

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Cladogram of Sponge Classes
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Phylogeny and Adaptive Diversification

• Sponges appeared before the Cambrian.
• Glass sponges expanded in the Devonian.
• One theory - sponges arose from
  choanoflagellates.
• However, some corals and echinoderms also have
  collar cells, and sponges acquire them late in
  development.

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Phylogeny and Adaptive Diversification

• Molecular rRNA evidence suggests a common
  ancestor for choanoflagellates and metazoans.
• Sponges and Eumetazoa are sister groups with
  Porifera splitting off before radiates and
  placozoans.

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Phylum Placozoa

• Trichoplax adhaerens is the sole species of
  phylum Placozoa (marine).
• No symmetry
• No muscular or nervous organs
• Placozoans glide over food, secrete digestive
  enzymes, and absorb nutrients.

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Phylum Placozoa

• Cell layers
• Dorsal epithelium
• Thick ventral epithelium of monociliated cells
  and nonciliated gland cells.
• Space between the epithelia contain fibrous
  cells within a contractile syncytium.
• Grell considers it diploblastic.
• Dorsal epithelium represents ectoderm and ventral
  epithelium represents endoderm.