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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.