Origins of Multicellular Animals. Three Hypotheses Syncytial ciliate hypothesis –Ancestor is...
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Transcript of Origins of Multicellular Animals. Three Hypotheses Syncytial ciliate hypothesis –Ancestor is...
Origins of Multicellular Animals
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
THE PHYLUM PORIFERA
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
Morphology of Sponges
Sponge Cell Types
• Pinacocytes- outer cells; equivalent of epiderm
Sponge Anatomy
Pechenik, 1996
Pinacocyte
Choanocyte
AmoebocytePorocyte
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.
Sponge Anatomy
Pechenik, 1996
Pinacocyte
Choanocyte
AmoebocytePorocyte
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
Mesophyl(=Mesenchyma)
• Beneath the pinacocytes - a gelatinous protein layer
• it contains the skeletal material (ie. spongin and spicules) and amoebocytes
Sponge Anatomy
Pechenik, 1996
Pinacocyte
Choanocyte
AmoebocytePorocyte
Types of Spicules4 general types
• Monaxon- needle-like or rod-like; straight or curved
Types of Spicules4 general types
• Monaxon- needle-like or rod-like; straight or curved
• Tetraxon- has 4 prongs
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
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
Three Basic Sponge Types
• Asconoid
• Syconoid
• Leuconoid
Asconoid Sponges
• most primitive and simplistic in structure
• have radial symmetry
• are tube shaped
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
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
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
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
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
• 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
Leuconoid Sponge Design
Sponge Reproduction
• Sexual
• Asexual
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
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
Osmoregulation and Excretion in Sponges
• no special organs
• main waste is ammonia – it is removed by water currents within the
sponge
HIGHER CLASSIFICATION OF SPONGES
4 classes of sponges
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
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
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
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