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Sponges and Placozoans

Sponges And Placozoa

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Page 1: Sponges And  Placozoa

Sponges and Placozoans

Page 2: Sponges And  Placozoa

Origin of Metazoa

Evolution of the eukaryotic cell was followed by diversification into many lineages including:Modern protozoansPlantsFungiAnimals

Multicellular animals are called metazoans.

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PoriferaPorifera

ActinopodaActinopoda

CnidariaCnidaria

ApicomplexaApicomplexa

CiliophoraCiliophora

MolluscaMolluscaAnnelidaAnnelida

EuglenozoaEuglenozoa

BryozoaBryozoa

PlatyhelminthesPlatyhelminthes

RhizopodaRhizopoda

RotiferaRotifera

GranuloreticulosaGranuloreticulosa

NematodaNematodaPriapulidaPriapulidaKinorhynchaKinorhynchaLoriciferaLoricifera

PhoronidaPhoronidaBrachiopodaBrachiopoda

EchinodermataEchinodermata

HemichordataHemichordata

ChelicerataChelicerata

UniramiaUniramia CrustaceaCrustacea

ProtochordataProtochordata

VertebrataVertebrata

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.

Resulted to two origins of multicellularity: Syncitial ciliated and Colonial flagellated

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Syncitial Ciliate Hypothesis

Syncitial ciliate hypothesis – metazoans arose from an ancestor shared 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 like

cellularization occurs.Does not explain flagellated sperm in

metazoans. Implies that radial symmetry is derived.histoincompatibility

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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

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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.MonoeciousGametes 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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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 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.