CHAPTER 12
Sponges and
Placozoans
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Origins of Multicellularity
Advantages
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Nature’s experiments with larger organisms
without cellular differentiation are limited
Increasing the size of a cell causes problems
of exchange
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Cell assemblages in sponges are distinct from
other metazoans
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Multicellularity avoids surface-to-mass problems
Molecular evidence demonstrates that sponges are
phylogenetically grouped
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Origin of Metazoa
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Evolution of the Metazoa
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Evolution of eukaryotic cell followed by
diversification
Modern descendants
 Protozoa, plus multicellular animals
Multicellular animals
 Referred to collectively as metazoans
Metazoans placed in Opisthokont clade
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Origin of Metazoa
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Choanoflagellates
 Solitary or colonial aquatic eukaryotes
Each cell (choanocyte) has a flagellum
surrounded by collar of microvilli
 Beating the flagellum draws water into collar
 Microvilli collect mostly bacteria
 Most are sessile
 One species attaches to floating diatom
colonies
 Strongly resemble sponge feeding cells
 Much debate whether sponge choancytes are
ancestral to choanoflagellates
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 One
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approach to metazoan origins
suggests transitional forms between
protozoan ancestors and simple
metazoans
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Origin of Metazoa
Theories of Unicellular Origin of Metazoans
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Some propose
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Metazoans arose from syncytial (multinucleate) ciliated
forms
Cell boundaries evolved later
Body form resembled modern ciliates with tendency
toward bilateral symmetry
Would resemble flatworms, but their embryology fails
to show cellularization, and flatworms have
flagellated sperm
This would mean that radial cnidarians had a bilateral
ancestor
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Origin of Metazoa
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Others suggest
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Metazoans arose from a colonial flagellated form
Cells gradually became specialized
First proposed by Haeckel (1874)
As cells in a colony became more specialized
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Colonial ancestral form at first
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Radially symmetrical
Reminiscent of a blastula stage of development
Hypothetical ancestor was called a blastea
Some believe ancestral forms similar to a gastrula
existed
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Colony became dependent on them
Gastraea
Bilateral symmetry evolved when the planula larvae
adapted to crawling
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Origin of Metazoa
Molecular Evidence
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By comparing the genomes or proteomes of
simple metazoans like sponges with more
complex taxa, scientists can discover what
cell transmitters or morphogens the first
metazoans possessed
Recent research indicates
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Sponge genome contains elements that code for
regulatory pathways of more complex metazoans
Includes proteins involved in spatial patterning
Some hypothesize modern sponges are less
morphologically complex than their ancestors
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Phylum Porifera
General Features
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Sessile sponges are filter feeders
Porifera means “pore-bearing”
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Sac-like bodies perforated by many pores
Use flagellated “collar cells”, or
choanocytes, to move water
Body is efficient aquatic filter
Approximately 15,000 species of sponges
 Most are marine
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Few live in brackish water, 150 in fresh water
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Phylum Porifera
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Marine sponges found in all seas at all
depths and vary greatly in size
Many species are brightly colored because of
pigments in dermal cells
Embryos are free-swimming, adult sponges
always attached
Some appear radially symmetrical but many
are irregular in shape
Some stand erect, some are branched, and
some are encrusting
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Phylum Porifera
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Growth patterns often depend on
characteristics of the environment
Many live as commensals or parasites in or
on sponges
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Also grow on a variety of other living organisms
Few predators
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Sponges may have an elaborate skeletal structure
and often have a noxious odor
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Phylum Porifera
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Skeletal structure of a sponge can be fibrous
and/or rigid
 If present, rigid skeleton consists of
calcareous or siliceous spicules
 Fibrous portion
Collagen fibrils in intercellular matrix
 Several types of one form of collagen, spongin,
exists
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Composition and shape the spicules
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Forms the basis of sponge classification
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Phylum Porifera
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Fossil record of sponges dates back to the early
Cambrian
Living sponges traditionally assigned to 3 classes:
Calcarea, Hexactinellida, and Demospongiae
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Members of Calcarea typically have calcium carbonate
spicules with one, three, or four rays
Hexactinellids are glass sponges with six-rayed siliceous
spicules
Members of Demospongiae have siliceous spicules,
spongin, or both
A fourth class, Sclerospongiae, was formed to
contain sponges with massive calcareous skeletons
and siliceous spicules
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Phylum Porifera
Form and Function
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Body openings consist of small incurrent
pores or dermal ostia
Incurrent pores: Average diameter of 50 μm
Inside the body
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Water is directed past the choanocytes where
food particles are collected
Choanocytes (flagellated collar cells) line some of
the canals
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Keep the current flowing by beating of flagella
Trap and phagocytize food particles passing by
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Phylum Porifera
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Sponges non-selectively consume food
particles sized between 0.1 μm and 50 μm
 The smallest particles are taken into
choanocytes by phagocytosis
 Protein molecules may be taken in by
pinocytosis
 Two other cell types, pinacocytes and
archaeocytes, play a role in sponge
feeding
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Phylum Porifera
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Types of Canal Systems
 Asconoids: Flagellated Spongocoels
 Simplest
body form
 Small and tube-shaped
 Water enters a large cavity, the
spongocoel
Lined with choanocytes
 Choanocyte flagella pull water through
 All Calcarea are asconoids
 Leucosolenia and Clathrina are examples.
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Phylum Porifera
 Syconoids:
Flagellated Canals
 Resemble
asconoids but larger with a
thicker body wall
 Wall contains choanocyte-lined radial
canals that empty into spongocoel
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Water enters radial canals through tiny
openings called prosopyles
 Spongocoel
is lined with epithelial cells
rather than choanocytes
 Food is digested by choanocytes
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Phylum Porifera
 Flagella
draw water through internal pores
called apopyles into the spongocoel and
out the osculum
 Sycons pass through an asconoid stage in
development but do not form highly
branched colonies
 Flagellated canals form by evagination the
body wall
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Developmental evidence of being derived
from asconoid ancestors
 Classes
Calcarea and Hexactinellida have
syconoid species (ex: Sycon)
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Phylum Porifera
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Leuconoids: Flagellated Chambers
Most complex and are larger with many
oscula
 Clusters of flagellated chambers are filled
from incurrent canals, and discharge to
excurrent canals
 Most sponges are leuconoid
 The leuconoid system
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Evolved independently many times in sponges
System increases flagellated surfaces
compared to volume
More collar cells can meet food demands
 Large sponges filter 1500 liters of water per day
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Phylum Porifera
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Types of Cells
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Sponge cells are arranged in a gelatinous matrix,
mesohyl
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Only visible activities of sponges are
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Connective “tissue” of sponges
Absence of true tissues or organs requires that all
fundamental processes occur at the level of individual cells
Slight alterations in shape, local contraction, propagating
contractions, and closing and opening of incurrent and
excurrent pores
Movements occur very slowly
Still interesting in that they suggest a whole body response
in organisms lacking organization above the cellular level
Apparently excitation spreads from cell to cell by an
unknown mechanism
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Phylum Porifera
 Choanocytes
 Oval
cells with one end embedded in
mesohyl
 Exposed end has one flagellum surrounded
by a collar
 Collar consists of adjacent microvilli
Forms a fine filtering device to strain food
 Particles too large to enter collar are trapped in
mucous
 Moved to the choanocyte and phagocytized
 Food engulfed by choanocytes is passed to
archaeocytes for digestion
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Phylum Porifera
 Archaeocytes
 Move
about in the mesohyl
 Phagocytize particles in the pinacoderm
 Can differentiate into any other type of
cell
Sclerocytes secrete spicules
 Spongocytes secrete spongin
 Collencytes secrete fibrillar collagen
 Lophocytes secrete collagen
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Phylum Porifera
 Pinacocytes
 Form
pinacoderm
 Flat epithelial-like cells
 Somewhat contractile
 Some are myocytes that help regulate
flow of water
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Phylum Porifera
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Cell Independence: Regeneration and
Somatic Embryogenesis
Sponges have a great ability to regenerate
lost parts and repair injuries
 Complete reorganization of the structure
and function of participating cells or bits
of tissue occurs in somatic embryogenesis
 Process of reorganization differs in
sponges of differing complexity
 Regeneration following fragmentation is
one means of asexual reproduction
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Phylum Porifera
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Asexual reproduction can occur by bud
formation
 External buds
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Small individuals that break off after
attaining a certain size
 Internal
buds or gemmules
Formed by archaeocytes that collect in
mesohyl
 Coated with tough spongin and spicules
 Survive harsh environmental conditions
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Phylum Porifera
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Sexual Reproduction
Most are monoecious
 Sperm sometimes arise from transformed
choanocytes
 In some Demospongiae and Calcarea
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Gametes develop from choanocytes
 Some derive gametes from archaeocytes
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Sponges provide nourishment to zygote
until it is released as a ciliated larva
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Phylum Porifera
In some, one sponge releases sperm
which enter the pores of another sponge
 Choanocytes phagocytize the sperm
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Transfer sperm to carrier cells
 Transport sperm through mesohyl to oocytes
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Some sponges release both sperm and
oocytes into water
 The free-swimming larva of sponges is a
solid parenchymula
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Phylum Porifera
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Calcarea and some Demospongiae
Hollow stomoblastula develops with flagellated
cells oriented toward the interior
 Blastula then turns inside out (inversion)
 Flagellated cells or micromeres of larva located
at anterior end
 Larger non-flagellated macromeres located at
posterior end
 Macromeres overgrow the micromeres during
metamorphosis during settlement
 Flagellated micromeres become choanocytes,
archaeocytes and collencytes
 Nonflagellated cells give rise to pinacoderm and
sclerocytes
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Phylum Porifera
Class Calcarea (Calcispongiae)
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Calcareous sponges with spicules of calcium
carbonate
Spicules are straight or have three or four
rays
Most are small with tubular or vase shapes
Many are drab in color, but some are bright
yellow, green, red, or lavender
Leucosolenia and Sycon are marine shallowwater
Asconoid, syconoid and leuconoid body
forms
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Phylum Porifera
Class Hexactinellida (Hyalospongiae)
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Glass sponges with six-rayed spicules of silica
Nearly all are deep-sea forms
Most are radially symmetrical
Stalks of root spicules attach them to
substrate
Spicular system forms a network
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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
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Phylum Porifera
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Choanoblasts are associated with flagellated
chambers
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Choanoblasts are located on the primary
reticulum
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Layers of the trabecular reticulum separate into a
primary reticulum and a secondary reticulum
Each has one or more process extending to collar
bodies.
Hexactinellids
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Lack a pinacoderm or gelatinous mesohyll
Myocytes are absent
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Phylum Porifera
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Chambers appear to correspond to both
syconoid and leuconoid types
Adapted to a deep-water habitat with a large
and easy flow of water
Some advocate placing hexactinellids in a
subphylum separate from other sponges
Collar bodies do not participate in
phagocytosis
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Function of the primary and secondary reticula
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Phylum Porifera
Class Demospongiae
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Contains 95% of living sponge species
Spicules are siliceous but not six rayed
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Leuconoid body form
All marine except for Spongillidae, the
freshwater sponges
Freshwater sponges
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Absent or bound together by spongin
Widely distributed in well-oxygenated ponds and
springs
Flourish in summer and die in late autumn
Leave behind gemmules
Reproduce sexually, but existing genotypes may
also reappear annually from gemmules
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Phylum Porifera
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Marine demosponges
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Highly varied in color and shape
Bath sponges
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Lacks siliceous spicules
Have spongin skeletons
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Phylum Porifera
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Phylogeny and Adaptive Diversification
Sponges appeared before the Cambrian
 Glass sponges expanded in the Devonian
 One theory
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Sponges arose from choanoflagellates
 However, some corals and echinoderms also have
collar cells, and sponges acquire them late in
development
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Molecular rRNA evidence suggests
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 Porifera
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Classes of sponges
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Phylogenetic studies indicate
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Distinguished on basis of spicule form and
chemical composition
Sponges with calcareous spicules in class
Calcarea belong in a separate clade than those
with spicules made of silica
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Phylum Porifera
Adaptive Diversification
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Poriferans are a highly successful group
Diversification centers on their unique watercurrent system and its degree of complexity
New feeding mode has evolved for a family of
sponges found in deepwater caves
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Tiny hook-like spicules cover body
Spicule layer entangles crustaceans
Filaments of the sponge body grow over prey
Carnivores, not suspension feeders
Contain siliceous spicules, but lack choanocytes
and internal canals
Illustrates the non-directional nature of evolution
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Phylum Porifera
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Classification
Class Calcarea
 Class Hexactinellida
 Class Demospongiae
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Phylum Placozoa
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Trichoplax adhaerens
Sole species of phylum Placozoa (marine)
K. G. Grell (1971) described the phylum
No symmetry
No muscular or nervous organs
Cell layers
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Dorsal epithelium
Thick ventral epithelium of monociliated cells and
nonciliated gland cells
Space between the epithelia contain fibrous “cells”
within a contractile syncytium
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Phylum Placozoa
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Glides over food, secretes digestive
enzymes, and absorb nutrients
Grell considers it diploblastic
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Dorsal epithelium represents ectoderm and
ventral epithelium represents endoderm
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