Transcript Biology 320 Invertebrate Zoology Fall 2005

Biology 320
Invertebrate Zoology
Fall 2005
Highlights from Chapter 4 –
Introduction to Metazoa
Metazoans
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Multicellular organisms
Really we are referring to
members of Kingdom
Animalia, or animals
Believed to have evolved
from protozoans;
specifically
choanoflagellates
Basic Properties
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Eukaryotic
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Typically large
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Motile
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Many costs / benefits associated with increased size
Usually even to some degree in sessile animals
Many have dual life stages – motile larvae and
sessile adults
Polarity
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Anterior – posterior
Oral - aboral
Ontogeny
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Means “origin of being”
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Fancy way of saying development
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Zygote formed by union of egg and sperm nuclei
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Soon after multicellular embryo is formed in process
known as cleavage (division)
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One important, early stage is known as a blastula
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Hollow ball of cells
Cleavage in a Sea Urchin Embryo
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Soon after, one wall of the
blastula invaginates
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In most individuals, primary
germ layers are established by
grastrulation
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Process known as gastrulation
Altered embryo is referred to as a
gastrula
Ectoderm – outer layer
Endoderm – inner layer
Mesoderm – middle layer
Organisms possessing ectoderm
and endoderm are said to be
diploblastic
Organisms possessing all three
germ layers are triploblastic
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If there is indirect
development, a larva
forms
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Producing larvae that
occupy a different niche
allows for resource
partitioning
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Lives independently of
adult
Very different from adult
Occupies different
ecological niche
Example: caterpillar to
butterfly
Eventually undergoes
metamorphosis to
become a juvenile
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Some undergo direct
development
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Juvenile looks just like
a miniature adult
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No larval phase
Example:
Grasshoppers
Believed to be a
derived trait rather
than primitive
Cells, Tissues, and Skeletons
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Remember, protozoans posses very little cellular
specialization
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Most protozoans rely on organelles to carry out all
cellular functions
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Different tissue types allow for a partitioning of labor
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Damaged or destroyed cells can be regenerated,
however if a Paramecium is badly damaged, the whole
organism dies
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Some metazoans lack true tissues
Main tissue types present in early metazoans are derived
from ectoderm and endoderm
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Epithelial tissue – line and cover body surfaces and organs
Connective tissue – most common and widely distributed
primary tissue type. Consists of cells in an extracellular matrix
(almost always containing collagen fibers)
Functional Consequences of Body Size
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On average, metazoans range in size from 0.5mm to
approximately one meter
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Surface area to volume ratio (SA:vol) is drastically
affected by an increase in body size
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Area is squared
Volume is cubed
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Body size is negatively correlated with SA:vol
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This effects the exchange of substance such as gases,
nutrients and wastes
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However, some
metazoans have
structural adaptations to
increase SA:vol
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Microvilli
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Pseudopodia
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Tissues arranged in sheets
with metabolically inert
ECM between
Vermiform or flat body plan
Body plans with fractal
relationships – example:
tubes in sponges
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Rates of diffusion slow
drastically over great distances
Effective diffusion distance is
approximately 0.5mm for most
animals
Body diameters of greater
than 1.0mm may be diffusionlimited
Some animals such as
flatworms carry out all gas
exchange via simple diffusion,
but others require a circulatory
system
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Generally, metabolic rates are positively correlated with
body size
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However, 1g of shrew tissue consumes more power than
1g of elephant tissue
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Poikilotherms (cold blooded animals) consume 8 times
more mass-specific energy than protozoans
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Homeotherms (mammals and birds) consume 29 times
the power of a poikilotherm of equal mass
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Possessing a circulatory system, and maintaining a
constant body temperature are both metabolically
expensive
Possible Advantages of Larger Body Size
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Mass-specific decrease in
metabolic rate
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Less danger of predation by
protozoa
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Predation upon protozoans by
metazoans
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Motile metazoans move at
greater speeds that protozoans
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Multicellularity provides the
ability to regenerate
Origins of Metazoans
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Colonial theory
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Metazoa is derived from colonial flagellated protozoans
Supported by morphological and molecular data
Accepted by the authors of your text
rRNA data indicated that Volvox evolved 50 to 75mya
and is not an ancestor to metazoans (believed to have
evolved 600mya)
Choanoflagellates are believed to be the sister taxon