BIOL 201: Invertebrate Zoology

Chapter 4: Introduction toMetazoa

Rob SwatskiAsst. Prof. Biology

HACC-York1

Metazoans

Animals in Kingdom Animalia

Multicellular

1-30 million species in 29 major phyla

Colonial protozoan ancestors are

choanoflagellates (sister taxon) 2

Metazoan Characteristics

Eukaryotic heterotrophs

Larger bodies: costs/benefits?

Most motile (even sessile adults have

motile larvae)

Anterior-posterior polarity; oral-aboral

3

Anterior

Posterior4

Oral

5

Aboral

Metazoan Ground

Plan

Specialized cells organized as tissues

(division of labor)

Primary (original) tissues: epithelial &

connective

Epithelium: sheets of cells bound to each

other by cell-adhesion molecules

Cuticle: surface coat of glycoproteins or

secreted extracellular matrix (ECM) 6

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Connective Tissue

Widely separated, nonadjoining cells in ECM of proteoglycan

gel with collagenproteins

Skeleton: support, protection, muscle

attachment

Exoskeleton: thickened outer

cuticle with proteins or minerals

Endoskeleton: internal stiffening of ECM with

cross-linking (cartilage) or mineral

secretion (bone)8

Asexual (Clonal)

Reproduction

Budding

Fragmentation

Fission

Parthenogenesis: development of

individual from an unfertilized egg (virgin

birth)9

Fragmentation 10

Parthenogenesis 11

Sexual Reproduction

Diploid adults form haploid gametes(egg & sperm) via

meiosis

Fertilizationdiploid zygote

Zygote divides by mitosis into an

embryo (ontogeny)

Embryo divides by cleavage early

cells called blastomeres

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Embryo Development

Blastula: 1-cell layer thick hollow or solid ball

of cells

Gastrulation:invagination of one wall of blastula 2-layered

gastrula

Morphogenesis

Ectoderm & endoderm(primary germ layers)

form around gelatinous blastocoel

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Indirect Development

Life cycle includes a larval stage

Larva: independent stage with different

anatomy & niche

Biphasic life cycle:benthic adult &

planktonic larva value?

Larva settles in suitable habitat & undergoes

metamorphosis into an immature juvenile 15

Direct Development

Embryo develops directlyinto juveniles without a

larval stage

Juvenile resembles a miniature version of the

adult (grasshoppers)

Considered to be a derived trait

Indirect development with external fertilization & planktonic larva is the

ancestral pattern16

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 rather than self-fertilize

why?17

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Maternal Provisioning

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 into her body

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Oviparous aphids

Viviparous pea aphids

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Spider with her brood

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Body Size & Division of

Labor

Most metazoans are 0.5 mm – 1 m in size

Prokaryote (seed) Protozoan (grapefruit)

Animal (stadium)

Cell specialization improves efficiency

Requires functional compartmentalization &

cellular integration (started with protozoans)

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Hierarchy of Functional Compartments

OrganismOrgan system

Organ

Tissue

Cell

Organelle

Surface Area-to-Volume

Ratio

SA:V is significantly affected by increases

in body size

As a cell grows larger, its area is squared & its volume is cubed

Eventually, SA (supply) will not be able to

support cell volume (demand)

Limits exchange of gases, nutrients, &

wastes24

6:1 0.6:1 0.006:1

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Adaptations That Increase

SA:V

Microvilli, pseudopodia, &

internal membranes

Tissues arranged as thin 2-D sheets around

a metabolically inert ECM core

Body plans that are flat & thin or long &

slender

Fractal body plans: tubes in tubes

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Size & Transport

Rates of diffusion slow drastically over great

distances

Effective diffusion distance is roughly 0.5 mm for most animals

Body diameters larger than 1 mm may be diffusion-limited

Circulatory system needed for bodies larger than 1 mm in diameter

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Size & Metabolism

Metabolic rate increases with body size

However, 1g of shrew tissue consumes more

power than 1g of elephant tissue

Poikilotherms (cold-blooded animals)

consume 8 times more mass-specific energy

than protozoans

Homeotherms(mammals & birds)

consume 29 times more energy than a

poikilotherm of equal mass 32

Advantages of Large Body

Size

Mass-specific decrease 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

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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

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Pedomorphosis

A trait of a descendent species resembles an

ancestral larval or juvenile developmental

trait

Results in smaller & simpler descendents with short generation

times

Common in species living in unpredictable

or changing environments

Adapted to colonize entirely new habitats

(interstitial descendents of polychaete worms)

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Cockroach nymphs

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Peramorphosis

A trait of a descendent species that develops

beyond the ancestral trait

Results in larger & more complex descendents

with longer generation times

Favored in constant or predictable environments

(deep sea, coral reefs)

Larger body size is a major trend in metazoan

evolution39

Colossal Squid (Mesonychoteuthis)12-14 m long!

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Origins of Metazoa

Colonial theory: Metazoans are derived

from colonial flagellated protozoans

Most widely accepted theory, supported by

morphological & molecular data

Choanoflagellates & Metazoa are sister taxa

Spherical colony of flagellated cells divided by mitosis, but daughter

cells held within ECM41

Proterospongia sp.

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