MetazoansMulticellular organismsAnimals in kingdom AnimaliaBelieve to have evolvedfrom protozoans; speciallychoanoflagellates
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.
• 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.
• 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.
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 MetabolismMetabolic 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.
• 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.
• 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.