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Introduction to Metazoa
Metazoans
Multicellular organisms
Animals in kingdom Animalia
Believe to have evolved from protozoans; specially choanoflagellates
Metazoan Chracteristics
• Eukaryotic Heterotrophs
• Polarized along an anterior-posterior locomotory axis
• Most motile
Ground Plan
• Specialized cells organized as tissues
• Primary Tissues: epithelial & connective
• Epithelium: sheets of cells bound to each other by cell-adhesion molecules.
Ontogeny
• “Origin of being”
• Development of an organism
• Zygote formed by the union of egg and sperm nuclei
• Multicellular embryo is formed in process known as cleavage (division)
Embryo Development
• Blastula: 1 cell layer thick hollow ball or solid ball of cells• Gastrulation: invagination of one wall of blastula -> 2 layered
gastrula• Morphogenesis• Ectoderm & endoderm form around gelatinous blastocoel.
Cells, Tissues, Skeletons
• Protozoans posses very little specialization
• Most protozoans rely on organelles to carry out all functions.
• Different tissue types allow for a partioning of labor
• Damage cells can be regenerated; however if a paramecium is damaged the whole oraganism dies.
Indirect Development
• Life cycle includes a larval stage• Larva: independent stage with different anatomy & niche.• Biphasic life cycle: benthic adult & planktonic larva.• Larva settles and undergoes metamorphosis into an immature
jevenile.
Direct Development
• Embryo develops directly into jeveniles without a larval stage
• Considered to be a derived trait• Indirect development with external
fertilization & planktonic larva is the ancestral pattern.
Reproductive Adaptations
• Improve chances of fertilization & embryo survival: increase synchrony & proximity
• Hermaphroditism: common in species with small population densities & sessile lifestyles.
• Any nearby individual is a potential mate• Most hermaphrodites cross-fertilize than self
fertilize.
Maternal Provisioning
Oviparous
Viviparous
• Adaptations providing physical protection & nutrients to offspring are very valuable.
• Oviparous: eggs spawned before or just after fertilization
• Viviparous: internal fertilization, embryos, gestate in maternal body & release larva or juveniles.
• Brooding: eggs released from mother, but are retained on or taken back to her body
Functional Consequences of Body Size
• Most metazoans are 0.5 mm – 1m in size
• Prokaryote (seed) Protozoan (grapefruit) Animal (stadium)
• Cell specialization improves efficiency• Requires functional compartmentalization &
cellular integration.
Size, Surface Area and Volume
• SA:V is significantly affected by increases in body size.
• As a cell grows larger, its area is squared & its volume is cubed.
• SA (supply) will not be able to support cell volume (demand)
• Limits exchange of gases, nutrients and wastes.
DIFFUSION
Size and Transport
• Rates of diffusion slow drastically over great distances.
• Effective diffusion distance is roughly 0.5mm for most animals.
• Body diameters larger than 1mm may be diffusion-limited.
• Circulatory system needed for bodies larger than 1mm in diameter.
Size and Metabolism
Metabolic rate increases with body size.Poikilotherms (cold-blooded animals) consume 8
times more mass-specific energy than protozoans.
Homeotherms (mammals & birds) cosume 29 times more energy than a poikilotherm of equal term.
Advantages of a Large Body Size
• Mass specific decreases in metabolic rate• Reduced risk of predation by protozoa• Larger metazoans can prey upon protozoans• Motile metazoans move faster than protozoans• Multicellularity allows ability to regenerate.
Ontogeny & Phylogeny
• Metazoan ontology includes developmental stages subject to natural selection
• Heterochrony: changes in the timing of developmental events- allows potential for evolutionary change.
• Two types of heterochrony• Pedomorphosis & Peramorphosis
Pedomorphosis
• A trait of descendent species resembles an ancestral larval or jevenile developmental trait
• Results in smaller and simpler descendents with short generation times.
• Common in species living in unpredictable or changing enviroments
• Adapted to colonize entirely new habitats.
Peramorphosis
• A trait of descendant species that develops beyond the ancestral trait
• Results in larger & more complex descendants with longer generation times.
• Favored in constant or predicatable enviroments.
• Larger body size is a major trend in metazoan evolution.
Origins of Metazoa
• Colonial Theory: Metazoans are derived from colonial flagellated protozoans.
• Choanoflagellates & metazoa are sister taxa • Spherical colony of flagellated cells divided by
mitosis, but daughter cells held within ECM.
Sources• http://www.google.com.ph/imgres?q=metazoa&hl=fil&biw=1366&bih=585&gbv=2&tbm=isch&tbnid=ITnAcb6yU3INiM:&imgrefurl=http://
www.ucmp.berkeley.edu/phyla/phyla.html&docid=nVJrWUumXbrhFM&imgurl=http://www.ucmp.berkeley.edu/phyla/animcoll.jpg&w=405&h=342&ei=feKwT_6kOOXjmAXEjsmlCQ&zoom=1&iact=rc&dur=169&sig=114980428662261257468&page=1&tbnh=122&tbnw=144&start=0&ndsp=23&ved=1t:429,r:0,s:0,i:67&tx=100&ty=36
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