Introduction to animal diversity

1. An Introduction to
Animal Diversity
Chapter 33

2. 2
 Characteristics of Animals
 Animal Classification
 Molecular Views of Animal Diversity
Chapter 33 Outline

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Cellular Features
 Animals are multicellular
 No cell walls – increases flexibility
 Extracellular matrix forms strong fibres outside the cell
 Cell junctions play an important role in holding animal cells
in place and allowing communication between cells
33.1 Characteristics of Animals

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 Animals are heterotrophs
 Different modes of feeding:
 Suspension feeding – filtering particles from surrounding water
 Bulk feeding – eating large food pieces
 Fluid feeding – sucking sap or animal body fluids
 Rely on internal absorptive nutrition
Modes of Nutrition

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 Most have muscle cells and nerve cells organized into tissues
 Most are capable of some kind of locomotion
 Important for food acquisition and escape from predators
 Coordinated by sensory structures, muscular-skeletal system,
and nervous system
 Sessile species, such as barnacles, have moving appendages or
a swimming larval stage
Movement

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 Nearly all animals reproduce sexually
 Small, mobile sperm unites with large egg to produce a zygote
 Internal fertilization common in terrestrial species
 External fertilization common in aquatic species
 Certain insects, fish, and lizard species can reproduce asexually
 Metamorphosis occurs in some species
 Developmental phenomenon in which animal changes from a juvenile to an
adult form
 Reduces competition for food between juveniles and adults, and facilitates
dispersal
Reproduction and Development

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 Multicellular animals emerged at the end
of the Proterozoic eon
 >590 mya
 First animals were invertebrates
 A sudden increase in animal diversity
occurred during the Cambrian explosion
 Favorable environment: warm
temperatures, increases in atmospheric
and aquatic oxygen, development of ozone
layer
 Evolution of the Hox gene complex
 An evolutionary “arms race”
History of Animal Life

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 First vertebrates were fishes ~520 mya
 Plants colonized land around the same time
 Provided a food source for animals on land, but also presented new
challenges
 Colonization of land led to new adaptations
 e.g. Lungs, tetrapod locomotion, internal fertilization, amniotic egg
 Reptiles dominated the Earth for millions of years
 Dinosaurs died out ~65 mya, giving way to an explosion in the number and
diversity of mammals
Vertebrate Evolution

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 Most biologists agree animal kingdom is monophyletic
 All taxa evolved from a single common ancestor
 About 35 recognized animal phyla
 We will focus on the 12 that have the greatest number of species
Animal Classification

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 Closest living relative of animals
are choanoflagellates
 Single-celled protists with a single
flagellum surrounded by a collar
of cytoplasmic tentacles
 Some species are colonial
 Bear a striking similarity to
sponge choanocytes
Animal Evolution

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 Morphological and developmental features traditionally used to
classify animals:
1. Presence or absence of different tissue types
 Parazoa vs. Eumetazoa
2. Type of body symmetry
 Radial vs. Bilateral
3. Specific features of embryonic development
 Protostomes vs. Deuterostomes
 Molecular analysis is used to further classify protostomes as:
4. Ecdysozoa or lophotrochozoa
Animal Classification

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1
2
3
Tissues
Symmetry
Embryonic development
4
Molecular

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 Collectively, animals are known as metazoa
 Divided into two subgroups based on whether they have
specialized tissues
 Parazoa: Without specialized tissues or organs
 May have distinct cell types
 Consists of a single phylum, Porifera (sponges)
 Eumetazoa: Have more than one type of tissue, and (usually)
different types of organs
 All other animals
Tissues
1

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 Eumetazoa are divided by their type of symmetry
 Radiata: Radially symmetric animals
 Can be divided equally by any longitudinal plane through the central axis
 Often circular or tubular in shape, with a mouth at one end
 Consists of a single phylum, Cnidaria
 Bilateria: Bilaterally symmetric animals
 Can be divided along a vertical plane at the midline to create two halves (left
side and right side)
 Have cephalization, dorsal/ventral (upper/lower) sides, and
anterior/posterior (head/tail) ends
Symmetry
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Early Divisions in Animal Phylogeny

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 Radiata and Bilateria also differ in number of embryonic cell
layers (germ layers)
 Radiata have two layers (diploblastic)
 Bilateria have three layers (triploblastic)
 Cell layers develop during gastrulation
 Inner layer – endoderm
 Outer layer – ectoderm
 Mesoderm – 3rd layer in bilateral animals
 Forms muscles and most other organs
Germ layers

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Formation of Germ Layers

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 In gastrulation, the endoderm forms an indentation called the blastopore
 Opening of the archenteron to the outside
 In protostomes, the blastopore becomes the mouth
 Cleavage is determinate: Fate of embryonic cells is determined early
 Often exhibit spiral cleavage
 In deuterostomes, the blastopore becomes the anus
 Cleavage is indeterminate: Each cell produced by early cleavage can develop
into a complete embryo
 Radial cleavage
 Consists of two phyla: Echinodermata and Chordata
Embryonic Development
3

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 In the past, presence or absence of a coelom, and the presence
of body segmentation was used in construction of
phylogenies
 Molecular data suggest these features are unreliable in terms
of understanding evolutionary history
 However, they continue to be useful in describing differences
in animal structure
Other Morphological Characteristics
used in Classification

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 Coelom: a fluid-filled body cavity
 Coelomates have a coelom
completely lined with mesoderm
 = True coelom
 Pseudocoelomates have a coelom
only partially lined with mesoderm
 Acoelomates lack a body cavity and
instead have mesenchyme
Body Cavity

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 Cushions internal organs
 Enables movement and growth of internal organs independent of the
body wall
 In some soft-bodied invertebrates, functions as a hydrostatic skeleton
 Fluid-filled body cavity surrounded by muscles that gives support and
shape to the body
 Muscle contractions push fluid from one part of the body to another
 Fluid can act as a simple circulatory system
Functions of the Coelom

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True Coelom
Pseudocoelom
Absent

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 Division of the body into a series of repetitive segments
 Allows specialization of body regions
 Presence/absence of segmentation previously used in
construction of animal phylogeny, but likely it has arisen
multiple times
Segmentation

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 Changes in specialization of body segments traced to changes in Hox
genes
 Involved in pattern formation
 In vertebrates, Hox gene expression along the anteroposterior axis
controls whether cervical (neck) vertebra or thoracic (chest) vertebra
develop
 Expression of a gene called HoxC-6 determines the cervicothoracic
boundary
 Shifts in where HoxC-6 is expressed affects neck length
Hox Genes & Segment Specialization

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Molecular Views of Animal Diversity
 Scientists now use molecular systematics to classify animals
and clarify questions posed by earlier data
 Compares similarity in DNA, RNA, and amino acid sequences
 Closely related organisms have fewer differences than those
more distantly related
 Advantage over morphological data in that genetic sequences
are easier to quantify and compare
 DNA has only A, T, C, G
 Morphological data are more subjective
33.3 Molecular Views of Animal Diversity

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Comparison of SSU rRNA gene
sequence

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Feature Investigation
Molecular Analysis of Arthropod SSU rRNA

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Important ramifications:
 Drosophila melanogaster and Caenorhabditis elegans are more closely
related than previously thought
 Arthropods and annelids are not as closely related as thought based on
segmentation
Findings

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 Genetic data provided evidence for the division of the
protostomes into two separate clades
 Ecdysozoa: Consists of nematodes, arthropods, and a few
other minor phyla
 Named for ecdysis (molting)
 Members secrete an exoskeleton that must be shed and
regrown as the animal increases in size
 Lophotrochozoa: Consists of mollusks, annelids, and
several other phyla
 Named for the lophophore (feeding tentacles) and
trochophore larva
 Some members have neither of these features (e.g.
platyhelminthes), so classified strictly based on molecular
data
Division of Protostomes
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 Jump to long image description

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Deuterostomia
 The deuterostomes includes two phyla:
Echinodermata and Chordata
 Share similar pattern of development
 Relationship supported by molecular
evidence
 Echinoderms includes sea stars, sea urchins
and sea cucumbers
 Chordates includes all vertebrates and some
invertebrates
A tiny bit of 34.6

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 Free-swimming larvae have bilateral symmetry, while adults
have a modified radial symmetry
 Five parts pointing out from the centre
 No brain, simple nervous system
 Endoskeleton consisting of calcareous plates overlaid by a thin
skin
 Water vascular system with tube feet functions in movement,
gas exchange and feeding
 Reproduce sexually with separate sexes and external
fertilization
Phylum Echinodermata

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Brittle star larva & adult

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Four critical innovations in body plan exhibited at some point
during development:
 Notochord: Single flexible rod that lies between the digestive
tract and the nerve cord
 Provides skeletal support for early diverging chordates
 In vertebrates, mostly replaced by complex jointed backbone
 Dorsal hollow nerve cord: Hollow tube that develops dorsal
to the digestive tract
 In vertebrates, develops into brain and spinal cord
Phylum Chordata

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 Pharyngeal slits: Slits in pharyngeal region, close to mouth, that open to the outside
 Permits water to enter through mouth and exit via slits without entering digestive tract
 In early-diverging chordates functions as filter-feeding device
 In later-diverging chordates develops into gills for gas exchange
 In terrestrial chordates slits do not fully form, and are modified for other purposes
 Postanal tail: Tail of variable length that extends posterior to the anal opening
 Used in locomotion (e.g. swimming) or for other purposes
Phylum Chordata

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Molecular Analysis of Humans &
Various Invertebrates

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Invertebrate Chordates
Subphylum Cephalochordata
(The Lancelets)
Subphylum Urochordata
(The Tunicates)

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Retain all chordate characteristics, plus have:
 A vertebral column: Notochord is replaced by a bony or cartilaginous
column of interlocking vertebrae
 Provides support & protects nerve cord
 Cranium: Bony or cartilaginous housing that protects a more developed
brain
 Endoskeleton of cartilage or bone
 Very strong yet flexible
 Also have a greater diversity and complexity of internal organs
Vertebrates

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Major Clades
of
Vertebrates

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