Origins of Multicellular Animals Three Hypotheses Syncytial

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Origins of Multicellular Animals
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Three Hypotheses

• Syncytial ciliate hypothesis
• Ancestor is single celled ciliate with multiple
  nuclei
• Colonial flagellate hypothesis
• Ancestor is colonial flagellate like Volvox
• Polyphyletic hypothesis
• There may be multiple ancestors

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THE PHYLUM PORIFERA
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INTRODUCTION TO PORIFERA

• unusual animals
• originally thought they were plants
• Are primarily marine, mostly in shallower waters
• Are sessile and attached to substrate or objects-
  occasionally on other animals such as crabs
• Are the most primitive metazoans and have neither
  true tissues or organs

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Morphology of Sponges
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Sponge Cell Types

• Pinacocytes- outer cells equivalent of epiderm

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Sponge Anatomy
Porocyte
Amoebocyte
Pinacocyte
Choanocyte
Pechenik, 1996
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Sponge Cell Types

• Pinacocytes- outer cells covering sponge
  equivalent of epidermis
• Choanocytes-
• similar to choanoflagellates
• collared cells with flagella - create water
  current and collect food matter.

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Sponge Anatomy
Porocyte
Amoebocyte
Pinacocyte
Choanocyte
Pechenik, 1996
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Sponge Cell Types

• Pinacocytes- outer cells covering sponge
  equivalent of epiderm
• Choanocytes-
• similar to choanoflagellates
• collared cells with flagella - create water
  current and collect food matter
• Amoebocytes-
• amoeba-like cells
• store, digest and transport food, excrete wastes,
  secrete skeleton
• give rise to buds in asexual reproduction

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Mesophyl(Mesenchyma)

• Beneath the pinacocytes - a gelatinous protein
  layer
• it contains the skeletal material (ie. spongin
  and spicules) and amoebocytes

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Sponge Anatomy
Porocyte
Amoebocyte
Pinacocyte
Choanocyte
Pechenik, 1996
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Types of Spicules4 general types

• Monaxon- needle-like or rod-like straight or
  curved

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Types of Spicules4 general types

• Monaxon- needle-like or rod-like straight or
  curved
• Tetraxon- has 4 prongs

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Types of Spicules4 general types

• Monaxon- needle-like or rod-like straight or
  curved
• Tetraxon- has 4 prongs
• Triaxon or Hexaxon- 3 or 6 rayed
• Polyaxon- multiple short rods radiating from a
  common center burr shaped, star shaped or like a
  child's jack.
• Some species have a mixture of types

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SponginGive phylum its common name

• Some species have no spicules, but do have
  spongin
• spongin is a type of hardened secreted protein
• Some species have both spicules and spongin

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Three Basic Sponge Types

• Asconoid
• Syconoid
• Leuconoid

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Asconoid Sponges

• most primitive and simplistic in structure
• have radial symmetry
• are tube shaped

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Asconoid Spongetwo basic openings

• Ostia-
• incurrent pores that open into a central cavity
  called the spongocoel
• it is lined with choanocytes or collar cells
• Osculum
• the opening of the spongocoel to the outside
• water leaves the sponge

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Asconoid Sponge Design

• Imposes definite size limits to sponges due to
  the problem of water flow
• The spongocoel contains such a large volume of
  water that it is hard to push it out rapidly

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Syconoid Sponges

• next level of complexity
• walls are invaginated
• allowing for greater surface area over which
  water can pass
• typically vase shaped like the asconoid sponges
• radial symmetry

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Syconoid Structure

• helps to rectify some of the water movement
  problem
• increasing the surface area
• so there are more choanocytes to water volume
• decreasing the spongocoel volume
• these sponges able to get bigger than asconoid

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Leuconoid Sponges

• highest level of complexity in sponges
• lost radial symmetry and are very irregular in
  shape and may attain large sizes
• invaginated canals are even further invaginated
  and folded to from small flagellated chambers

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Leuconoid Sponge Design

• further increase in surface area makes these
  sponges highly efficient in moving and filtering
  water
• spongocoel is gone except for canals that lead to
  the osculum- or there may be a series of
  excurrent openings
• the largest sponges most hydrologically
  efficient

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Sponge Reproduction

• Sexual
• Asexual

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Sexual Reproduction in Sponges

• gametes formed by amoebocytes
• there are both hermaphroditic and dioecious
  species
• most hermaphroditic species produce eggs and
  sperm at different times so they do not self
  fertilize
• sperm is released into environment via osculum
  and is brought in by another sponge via ostia
• fertilization takes place in parent sponge
• zygote is expelled - it drops to bottom and
  begins to develop

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Asexual Reproduction in Sponges

• two types
• Budding- fragmentation of body wall, buds appear
  as outgrowth on sides of sponge
• when they reach a certain size they drop off and
  settle to bottom to form a new sponge
• Gemmules- occurs only in freshwater sponges
• gemmules are groups of food laden amoebocytes
  that deposit a hard covering of spicules around
  them
• formation is triggered by environmental
  conditions such as decreased temperatures
• they allow the sponge to pass the winter or
  periods of drought
• after which the outer covering breaks open and a
  new sponge develops

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Osmoregulation and Excretion in Sponges

• no special organs
• main waste is ammonia
• it is removed by water currents within the sponge

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HIGHER CLASSIFICATION OF SPONGES

• 4 classes of sponges

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Class Calcarea

• spicules composed of calcium carbonate
• spicules are monaxons or tri or quadraxons
• all three types of sponges exhibited
• All less than 10 cm high
• ex. Leucosolenia and Grantia
• found in shallow coastal waters
• all are marine

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Class Hexactinellida(glass sponges)

• Spicules of Silica and fused to form a lattice
  like skeleton
• cup or vase shaped with well developed spongocoel
• most beautiful example is Euplectella - venus
  flower basket
• chiefly live in 500-1000 meter depth
• are syconoid sponges
• all are marine
• may have commensal relationship with shrimp -
  where a male and female live inside the sponge
  get trapped inside when they out-grow the pores
  of sponge

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Class Demospongiae

• Largest class - 95 of sponges in this class
• spicules are silicious if present otherwise
  skeleton is made of spongin
• variously shaped some are huge
• all are leuconoid
• all but one family is marine- Spongillidae- is
  freshwater about 150 freshwater species
• this is the group from which we get our
  commercial sponges

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Class Sclerospongiae

• proposed in 1970 to include 6 species from
  Jamaica
• have silicious spicules and spongin
• also have an outer covering composed of calcium
  carbonate
• are leuconoid sponges