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Sponges And  Placozoa
 

Sponges And Placozoa

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    Sponges And  Placozoa Sponges And Placozoa Presentation Transcript

    • Sponges and Placozoans
    • 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 .
    • Dendrogram of Major Phyla Porifera Actinopoda Cnidaria Apicomplexa Ciliophora Mollusca Annelida Euglenozoa Bryozoa Platyhelminthes Rhizopoda Rotifera Granuloreticulosa Nematoda Priapulida Kinorhyncha Loricifera Phoronida Brachiopoda Echinodermata Hemichordata Chelicerata Uniramia Crustacea Protochordata Vertebrata
    • Choanoflagellates
      • Choanoflagellates are solitary or colonial protozoans with a flagellum surrounded by a collar of microvilli.
    • 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
    • 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.
    • 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
    • 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
    • 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.
    • 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.
    • Phylum Porifera
      • Sponges range in size and shape.
        • Up to 2 meters in diameter!
        • Encrusting, boring, finger, tube or vase shaped.
    • Neighbors
      • Many organisms, including crabs, nudibranchs, mites, bryozoans, and fish live as commensals or parasites in sponges.
    • 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
    • Suspension Feeders
      • Sponges are suspension feeders capturing food particles suspended in the water that passes through their body.
    • 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 .
    • 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.
    • 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.
    • 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.
    • 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.
    • 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.
    • Types of Cells
      • Choanocytes , flagellated collar cells, generate a water current through the sponge and ingest suspended food.
    • 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 ).
    • 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.
    • 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.
    • Reproduction
      • Most sponges are hermaphrodites meaning that each individual functions as both male and female.
        • Monoecious
        • Gametes are derived from choanocytes or sometimes archaeocytes.
    • 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.
    • 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.
    • Class Calcarea
      • Calcareous sponges ( Class Calcarea ) have spicules composed of calcium carbonate.
      • Small, usually vase shaped.
      • Asconoid, syconoid, or leuconoid in structure.
    • 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.
    • 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.
    • 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.
    • Cladogram of Sponge Classes
    • 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.
    • 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.
    • 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.