Lecture 4 Bryozoans

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Lecture 4 Bryozoans Sponges
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Bryozoans
Bryozoans (Phylum Bryozoa) are marine or
freshwater colonial invertebrates. Like some
sponges and corals, they form colonies of various
shapes and sizes, whose skeleton is usually
calcite, and so have a high fossilisation
potential. The individual bryozoan animals
typically measure 1 mm or less in diameter and
are called zooids. Most are adapted for feeding
and are called autozooids, but some, called
heterozooids, have other functions in the colony.
Marine bryozoans usually inhabit shallow
environments from the poles to the equator.
Reptadonella insidiosa from the Coralline Crag,
Suffolk.
Fenestella from the Lower Carboniferous of County
Fermanagh
Bryozoa first appeared in the fossil record
during the Ordovician approximately 20,000
fossil species and 5,000 living species are now
known.
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Colony forms
Bryozoan colonies come in many shapes and sizes.
Typically colonies reach 100 mm in diameter. In
the geological past many particular morphologies
have evolved repeatedly. Most bryozoans are
sessile, permanently cemented to a firm
substrate one living genus moves by coordinated
action of its vibracula which act like walking
legs. Some ctenostomes have a boring habit.
Encrusting colonies may be simple, with a
runner-like morphology, or be composed of single
or multilayered sheets of zooids. Commonly
bryozoan colonies are erect, which allows them to
feed from cleaner and faster flowing water above
the sediment of the sea floor. They may be
simple expansions of dividing branches or complex
and tree-like with many branches. Branches may
be cylindrical - thick (ramose) or thin
(dendroid), jointed or unjointed. They may be
flattened with zooids budded either side of a
median lamina (bilaminate) or with zooids on one
surface only (unilaminate). Many unilaminate
forms form a reticulate pattern of branches
joined by crossbars. Some bryozoans form dense
small domed colonies, while others take on the
shape of the shell or other substrate encrusted.
Usually erect colonies become broken into smaller
branch fragments after death and are rarely found
preserved intact in the fossil record.
Various different bryozoan colony forms 1.
Dendroid - slender circular articulated branches
2. Foliaceous
3. Reticulate meshwork
of branches.
4. Ramose - thick circular
branches. 5. Short dichotomously dividing
branches.
6. Adnate - encrusting sheet.
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Flattened bilaminar branches. Scale 20 cm

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The bryozoan animal
The bryozoan animal The autozooid comprises a
polypide (the feeding unit) and epithelial
tissue which secretes the skeleton of a tubular
or box-like shape. The polypide consists of a
lophophore (tentacle crown), a U-shaped gut and
reproductive organs.
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Zooids with the lophophore retracted into the
zooecium
A freshwater bryozoan with the lophophores
everted (extended) into the surrounding water for
feeding.
A freshwater bryozoan with the lophophores
everted (extended) into the surrounding water for
feeding.
A freshwater bryozoan with the lophophores
everted (extended) into the surrounding water for
feeding.
A freshwater bryozoan with the lophophores
everted (extended) into the surrounding water for
feeding.
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The bryozoan skeleton
Individual animals live in calcified chambers
that make up the zoarium of the colony.
Basically there are two forms (A) the
stenolaemate pattern where the living chambers
are generally simple and cylindrical in shape
and (B) the gymnolaemate pattern where chambers
are box-like.
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Feeding
Lophophores composed of up to 30 tentacles are
protruded (everted) from apertures for feeding,
or retracted by means of a retractor muscle while
at rest. Beating of cilia on the tentacles
creates inhalent water currents which carry food
particles to the mouth, and exhalent currents
which carry waste away. Some bryozoans can
generate incoming and outgoing water currents
that carry food to, and waste products away from
the colony (see practical).
Drawing showing the arrangement of everted
lophophores in Fenestella.
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Classification of bryozoans
Bryozoans are divided into three classes Class
Phylactolaemata Freshwater bryozoans with no
zooid polymorphism, no calcification form
resistant overwintering structures known as
statoblasts. No fossil record except for a few
statoblasts from Quaternary sediments. Class
Gymnolaemata Mostly marine bryozoans with
cylindrical or flattened zooids. Lophophore is
protruded by action of muscles pulling on frontal
wall. Includes the majority of living bryozoan
species, traditionally classified into two
sub-groups 1. Order Ctenostomata Lower
Ordovician - Recent Skeleton gelatinous or
membranous. Some species have a boring habit,
which with bioimmuration, is responsible for
their fossil record. Buskia nigribovis
Jurassic Normandy, France 2. Order
Cheilostomata Upper Jurassic - Recent Colonies
erect or encrusting, composed of box-like
zooecia. Avicularia common. Aperture generally
closed by an operculum. 2a. Suborder
Anasca Frontal wall membranous or chitinous,
uncalcified. Wilbertopora woodwardi (Brydone)
Upper Cretaceous Hampshire. 2b. Sub-order
Ascophora Calcified frontal wall, perforate or
imperforate with flexible sac or ascus beneath.
Aspidelectra melolantha Recent Sheppey, Kent.
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1
2a
1
2b
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Class Stenolaemata Marine bryozoans with
tubular zooids with calcified walls. Lophophore
is protruded by action of annular muscles.
Includes five orders 3. Order Cystoporata
Lower Ordovician - Triassic Erect or encrusting
colonies, of long, simple zooecial chambers with
basal diaphragms. Vesicular tissue and/or
hood-like lunaria on apertures diagnostic.
Fistulipora incrustans Lower Carboniferous Co.
Fermanagh 4. Order Cryptostomata Lower
Ordovician - Permian Cylindrical or bifoliate
erect colonies. Autozooecia budded from axis or
median lamina. Zooecial chambers short, some with
internal partial partitions called hemisepta.
Apertures regularly disposed. Rhabdomeson
progracile Upper Carboniferous Yorkshire. 5.
Order Fenestrata Lower Ordovician -
Permian Colonies erect, formed of thin dividing
branches or reticulate meshwork arranged in fans
or cones (Fenestella s.l.) or coiled around an
axis (Archimedes). Autozooecia open onto one face
only. Fenestella ivanovi Lower Carboniferous
Co. Fermanagh. 6. Order Trepostomata Lower
Ordovician - Upper Triassic Colonies encrusting
or erect branches up to 5 mm thick. Apertures
circular to irregular. Zooecial chambers long,
many diaphragms walls thin in centre (endozone)
thicken towards surface (exozone).
Parvohallopora ramosa Ordovician Cincinnati,
U.S.A. 7. Order Cyclostomata Lower Ordovician
- Recent Colonies encrusting or erect.
Autozooecial chambers long. Communication pores
between zooids and brooding gonozooids in most
genera. Actinopora disticha Cretaceous
Northfleet, Kent.
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Sponges
Sponges (Phylum Porifera) are the most simple of
the multicellular (many-celled) animals. Their
cells are not organised into tissues and
therefore they are not regarded as true
metazoans. Sponges are halfway between the
protozoans and the metazoans and are sometimes
referred to as parazoans. They are all aquatic
and are mostly marine, sessile, benthic animals,
and are all filter feeders they pump water
through their porous bodies to filter out
suspended nutrients.
Morphology
A typical sponge has an upright bag-shaped body
with a central cavity the paragaster, which
opens at the top via the osculum. Water enters
the sponge through minute pores on the outer
surface, known as ostia, which connect to
ramifying incurrent canals serving the sponges
soft tissue. Water flow is maintained by
countless flagellae arising from cells known as
choanocytes. These cells also collect nutrients
from the passing water whch is then ejected
through the osculum.
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Given their small number of cells types, sponges
have evolved a remarkable array of body plans,
from the typical large cup-shapes to thin
encrusting and even boring forms. Their cells
secrete various skeletal products, including
minute siliceous or calcareous bodies known as
spicules. Spicules have many shapes and sizes
from needle-like to many-rayed forms, and these
serve to classify the sponges.
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Classification
Living sponges are divided into 4
classes Hexactinellida the glass sponges
have siliceous spicules with 3 to 6 rayed
symmetry, fused into a rigid framework present
day hexactinellids are mainly deep-water
types. Desmosongiae the horny sponges - also
possess siliceous spicules, usually 1 to 4 rayed,
but often lose their spicules are have a flexible
skeleton of spongin, a proteinaceous fibre.
Constitute 95 of all modern sponges. Calcarea
the calcisponges - 3 to 4 rayed spicules
composed of calcium carbonate. Sclerospongiae
the coralline sponges possess a solid
calcareous skeleton, like the corals - not just
isolated spicules. Discovered in the 1960s
living in cryptic habitats now clear that many
enigmatic fossil groups are actually coralline
sponges Archaeocyathids cone-shaped skeleton
- common group in Cambrian previously thought
to be an extinct phylum. Stromatoporoids
Dome-shaped or encrusting forms previously
thought to be hydrozoans. Chaetetids similar
colonies to stromatoporoids, but previously
thought to be tabulate corals.
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Raphidonema - Cretaceous
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Archaeocyathid - Cambrian
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Chaetetid - Carboniferous