12. sponges
TRANSCRIPT
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Sponges and Placozoans
Chapter 12
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Origin of Metazoa
Evolution of the eukaryotic cell wasfollowed by diversification into manylineages including:
Modern protozoans Plants
Fungi
Animals Multicellular animals are called
metazoans.
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Porifera
Actinopoda
Cnidaria
Apicomplexa
Ciliophora
Mollusca
Annelida
Euglenozoa
Bryozoa
Platyhelminthes
Rhizopoda
Rotifera
Granuloreticulosa
NematodaPriapulida
Kinorhyncha
Loricifera
Phoronida
Brachiopoda
Echinodermata
Hemichordata
Chelicerata
Uniramia Crustacea
Protochordata
Vertebrata
Dendrogram of Major Phyla
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Choanoflagellates
Choanoflagellatesare solitary orcolonial protozoans
with a flagellumsurrounded by acollar of microvilli.
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Choanoflagellates
Choanoflagellatesresemble sponge feedingcells (choanocytes).
Scientists are studyingcolony formation and cell-to-cell communication inchoanoflagellates in search
of clues to the evolution ofmulticellularity.
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Syncitial Ciliate Hypothesis
Syncitial ciliate hypothesismetazoans arose from an ancestorshared with single celled ciliates.
Recall multiple nuclei in reproducing ciliates.
Later, each nucleus becomes partitioned.
Trend toward bilateral symmetry as in
flatworms.
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Syncitial Ciliate Hypothesis
Problems:
In flatworm embryology nothing likecellularization occurs.
Does not explain flagellated sperm inmetazoans.
Implies that radial symmetry is derived.
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Colonial Flagellate Hypothesis
Colonial Flagellate Hypothesismetazoans descended from ancestorscharacterized by a hollow, spherical
colony of flagellated cells.
Individual cells became specialized fordifferent functions.
Radially symmetrical, similar to a blastula. First proposed by Haeckel in 1874
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Phylum Porifera
Sponges, Phylum Porifera, are multicellularheterotrophs.
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 sessileanimals that have aporous body andchoanocytes.
Supported by askeleton of tinyneedlelike spiculesand protein.
They live in bothfresh and marinewaters.
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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 fishlive as commensalsor parasites insponges.
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Skeletal Framework
The skeletal framework of a sponge may be fibrousor rigid.
The fibrous part comes from collagen fibrils in theintercellular matrix.
Spongin Rigid skeletons consist of needlelike spicules.
Calcareous
Siliceous
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Suspension Feeders
Sponges are suspension feederscapturing food particles suspended in thewater that passes through their body.
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Suspension Feeders
Water flows inthrough incurrentpores called dermal
ostia. It flows past the
choanocytes wherefood particles are
collected on thechoanocyte collar.
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Suspension Feeders
Choanocytes take in small particles byphagocytosis. Protein molecules aretaken in by pinocytosis.
Sponges can also absorb nutrientsdissolved in the water.
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Canal Systems
Asconoid thesimplest canalsystem.
Choanocytes line the
spongocoel. Water enters through
the ostia and exitthrough the large
osculum. Usually tube shaped.
Found only in theClass Calcarea.
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Canal Systems
Syconoid tubularbody and singularosculum likeasconoids.
The walls of thesponge are folded toform choanocytelined canals.
Increased area forfeeding.
Class Calcarea.
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Canal Systems
Leuconoids mostcomplex, permits anincrease in spongesize.
Choanocytes line thewalls of smallchambers wherethey can filter all the
water that flowsthrough.
Most sponges.
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Types of Cells
Absence of tissues & organs means thatfundamental processes occur on thecellular level.
Respiration and excretion occur bydiffusion in each cell.
Mesohyl is the gelatinous matrixcontaining skeletal elements & amoeboidcells.
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Types of Cells
Choanocytes, flagellated collar cells,generate a water current through thesponge and ingest suspended food.
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Types of Cells
The choanocytes passfood particles toarchaeocyte cells fordigestion.
Digestion occurs entirelywithin cells, there is nogut.
Other cell types secrete
spicules (sclerocytes),spongin (spongocytes),& collegen(collenocytes).
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Types of Cells
Pinacocytes arethin, flat,epithelial-type
cells that coverthe exterior andsome interiorsurfaces of the
sponge. Almost a true
tissue.
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Reproduction
Sponges have remarkableregeneration capabilities.
Regeneration followingfragmentation is a form ofasexual reproduction.
External buds can breakoff to form new sponges.
Internal buds (gemmules)in freshwater sponges canremain dormant in times ofdrought.
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Reproduction
Most sponges are hermaphroditesmeaning that each individual functionsas both male and female.
Monoecious
Gametes are derived from choanocytes orsometimes archaeocytes.
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Reproduction
Most sponges are viviparous.
After fertilization, the zygote is retainedand is nourished by the parent. Ciliatedlarvae are later released.
Some are oviparous releasing gametesinto the water.
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Reproduction
Sponges in the class Calcarea and a fewDemospongiae have an unusual developmentalpattern 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) havespicules 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 mostlydeep sea forms. Spicules are six-rayed and made of silica.
Hexactinellids lack a pinacoderm or gelatinousmesohyll.
Chambers appear to correspond to both syconoid andleuconoid types.
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Class Hexactinellida
Some advocate placinghexactinellids in a subphylumseparate from other sponges.
Trabecular reticulum made of afusion of archaeocytepseudopodia - forms the
chambers opening to spongocoel. Trabecular reticulum is largest
continuous syncytial tissueknown in Metazoa.
Choanoblasts are associated with
flagellated chambers. Collar bodies do not participate in
phagocytosis this is the functionof the primary and secondaryreticula.
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Class Demospongiae
Class Demospongiaecontains most of thesponge species.
Spicules are siliceous,
but not six-rayed.
Spicules may be boundtogether by spongin, orabsent.
All leuconoid, mostlymarine.
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Cladogram of Sponge Classes
Ph l d Ad ti
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Phylogeny and AdaptiveDiversification
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 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 adhaerensis the sole species ofphylum Placozoa(marine).
No symmetry No muscular or
nervous organs
Placozoans glide overfood, secrete digestiveenzymes, and absorbnutrients.
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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 representsendoderm.