Download Protostomes

Protostome Animals
Protostomes
• Biologists estimate that there are > 10 million arthropod
species
• A hectare of pasture in England may contain 45 million
beetles
• In tropical rainforests, 40% of total mass of organisms are
beetles and other insects
• Ants: estimates are 1 million billion individuals
• Mollusks: > 93,000 species
Protostomes
include some of the
most familiar and
abundant animals
on Earth
Protostomes: consumers, predators, and scavengers in many marine ecosystems
The Relative Abundance of Animal Lineages
Major protostome phyla
Using the Fossil Record
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Major non-protostome phyla
Arthropoda
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Insects
Nematoda
all of the protostome phyla appear very early in the history of animal evolution
two major events occurred in the fossil record of protostomes:
• extinction of the trilobites (250 mya)
• appearance of insects
Mollusca
Platyhelminthes
Annelida
Ectoprocta
Chordata
Echinodermata
Cnidaria
Porifera
Other phyla
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Overview of Protostome Evolution
1.
-
An Overview of Protostome Evolution
undergo spiral cleavage after
fertilization
during gastrulation, the initial pore
becomes the mouth
-
coelom arises when openings form
within blocks of mesodermal tissue
Protostomes
Protostome development
Coelom
-
have 3 embryonic tissues
have a coelom: exceptions -
3.
some exhibit segmentation; Annelids,
Arthropods
4.
monophyletic group
1.
Lophotrochozoans
•
•
grow by adding to body elements
flatworms, rotifers, annelids, molluscs
2.
Ecdysozoans
•
•
grow by molting
nematodes & arthropod, etc.
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Ecdysozoa
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Lophotrochozoa
are bilaterally symmetrical
Protostomes: 2 main lineages:
Protostomes: A Monophyletic Group Comprising Two Major
Lineages
There are 22 phyla of protostomes,
but the eight major phyla shown
account for about 99.5% of the
known species
2.
Once the protosome pattern
of development had evolved,
protostomes split into the
lophotrochozoans and
ecdysozoans
Triploblasty
Bilateral symmetry
2 major subgroups exist within the protostomes
What Is a Lophotrochozoan?
14 phyla of lophotrochozoans include: molluscs, annelids,
flatworms
Lophotrochozoan:
• presence of a feeding structure: lophophore
• a type of larva: trochophore (common to many phyla)
• Lophophore: specialized structure where the mouth is in the
center of a ring of tentacles: functions in suspension feeding
Lophotrochozoans Have Distinctive Traits
Lophophores function in suspension feeding in adults.
Food particles
Water
current
Anus
Mouth
Tentacles have ciliated cells along surface
Gut
What Is a Lophotrochozoan?
Lophotrochozoans Have Distinctive Traits
Trochophore larvae swim and feed.
14 phyla of lophotrochozoans include: molluscs,
annelids, flatworms
Lophotrochozoan:
• Trocophores: type of larva common to several phyla of
lophotrochozoa
• common in marine mollusks and annelids
• may represent a feeding and a dispersal stage in the life cycle
ring of cilia along the middle
Mouth
Cilia used in
locomotion
and feeding
Anus
What Is an Ecdysozoan?
• The primary contrast between lophotrochozoans
and ecdysozoans involves their methods of growth
• Ecdysozoans: grow by molting—shedding of
cuticle or exoskeleton
• most prominent of the 7ecdysozoan phyla are
roundworms (Nematoda) and the arthropods
(Arthropoda)
Ecdysozoans
Exoskeleton:
• is a firm, nonliving covering that is difficult to penetrate
• exoskeletons pose a problem: they cannot grow as the
animal body inside them grows
solution evolved in the ancestors of the ecdysozoans
• they shed outgrown exoskeleton
• the presence of the exoskeleton required the evolution of
new forms of movement and respiration
Ecdysozoans
• some have wormlike bodies covered by flexible
exoskeletons called cuticles
• a thin cuticle allows the exchange of gases,
minerals, and water across the body surface, but
restricts the animal to moist habitats
• bodies are supported primarily by hydrostatic
skeletons
Ecdysozoan
• Explaining the evolution of body coverings:
hypothesis:
support of hypothesis:
-
How Do Body Plans Vary among Phyla?
Themes in the Diversification of Protostomes
all protostomes
• protostomes have diverged into 22 different phyla that are
recognized by distinctive body plans or specialized
mouthparts
- are bilaterally symmetrical and triploblastic
• Result: millions of different species
- undergo embryonic development in a similar
way
• What triggered this diversification?
consider evolutionary innovations
- most protostome phyla have wormlike bodies
How Do Body Plans Vary among Phyla?
The Nature of the Coelom Varies among Protostomes
No
coelom
Pseudocoelom
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• acoelomate body plan – reversion to lack of body cavity (for example,
platyhelminthes (flatworms) are acoelomate)
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Lophotrochozoa
Reduced
coelom Pseudocoelom
• pseudocoelom, which forms from an opening that originates between
the ectoderm and mesoderm layers, arose independently in rotifers and
nematodes
Reduced coelom
Coelom
Pseudocoelom
No coelom
Pseudocoelom
• radical changes occurred in coelom formation
How Do Body Plans Vary among Phyla?
• A fully functioning coelom has two roles:
(1) providing space for fluids to circulate among
organs
(2) providing a hydrostatic skeleton for
movement
• In arthropods and molluscs, the coelom is
drastically reduced…….Why???
Evolution of Arthropods
• some wormlike ecdysozoans developed a layer of
protective material, chitin, a strong, flexible,
waterproof polysaccharide
• a rigid body does not allow wormlike movement;
such animals require appendages that can be
manipulated by muscles
Arthropod Body Plan
Arthropods Have Specialized Body Plans
Arthropod body plan (external view)
Arthropods
• have segmented bodies that are organized into prominent regions:
head, thorax, and abdomen
Tagma
Head
Thorax
Abdomen
• distinguished by jointed limbs and exoskeleton made of chitin
• locomotion is based on muscles that apply force against the
exoskeleton to move legs or wings (hydrostatic skeleton is
unnecessary)
• have a hemocoel that provides space for internal organs and
circulation of fluids
Success of Arthropods
• this exoskeleton made aquatic arthropods excellent
candidates for invading terrestrial environments
• arthropods are the dominant animals on Earth,
approaching 2 million described species
Arthropods and Their Relatives
Onychophorans
- have a flexible cuticle that contains chitin, and
reduced coelom
- have soft, fleshy, unjointed legs that form from
outgrowths of the body
- recent molecular evidence links the
onychophorans (phylum Onychophora) to
arthropod lineages
Exoskeleton
(covers body)
Jointed limbs
Segmented body
Fossil Arthropods
trilobites (phylum Trilobita)
• were once the dominant line of arthropods before
becoming extinct by the Paleozoic era
• heavily armored, had jointed appendages
Arthropods and Their Relatives
Water bears:
• (phylum Tardigrada) also have fleshy, unjointed
legs and reduced coelom
• are extremely small, and lack circulatory systems
and gas exchange organs
• when dry conditions occur, the animal shrinks to a
small barrel-shaped object that can survive for a
least a decade
Molluscan Body Plan
Molluscs Have Specialized Body Plans
Mollusc body plan (internal view)
• The molluscan body plan is based on three major components:
(1) foot: a large muscle located at the base of the animal and usually
used in movement
(2) visceral mass: the region containing the internal organs and
external gill
(3) mantle: a tissue layer that covers the visceral mass and that
secretes a shell in some species
• In molluscs, the coelom's functions are replaced by the visceral mass
and the muscular foot
Variation Among Body Plans of Wormlike
Phyla
• The wormlike protostomes are characterized by similar body plans but
specialized mouthparts or feeding structures
Gill
Mantle
(secretes shell)
Visceral mass
(internal organs
and external gill)
Muscular “foot”
Some Protostome Phyla with Wormlike Bodies Have Specialized Mouthparts
Echiurans (“spoon worms”)
Mouth
Proboscis
• Echiurans: spoon worms – suspension feeders
• Priapulids: penis worms – sit and wait predators
• Nemerteans: ribbon worms – active predators
Some Protostome Phyla with Wormlike Bodies Have Specialized Mouthparts
Priapulids (“penis worms”)
Everted throat
Mouth
The toothed throat
everts (turns insideout), grabs the prey,
and then retracts
Food particles
are trapped in
mucus in the
“gutter” of the
proboscis and
then moved to
the mouth by
cilia
Some Protostome Phyla with Wormlike Bodies Have Specialized Mouthparts
Nemerteans (“ribbon worms”)
Mouth Proboscis
The barb-tipped proboscis
everts (from a different
opening than the mouth)
and spears or entangles
the prey, and then retracts
it toward the mouth
Evolution of Protostomes
• most protostome evolution took place in oceans
• many different lineages of animals evolved
feeding structures designed to extract small prey
from the water, most likely because this was the
only type of food available during much of animal
evolution
Evolution of Protostomes
• external body coverings probably evolved as a
result of the selective pressures resulting from
predation
• body coverings evolved independently in
lophotrochozoan and ecdysozoan lineages
• although greater complexity has evolved in many
animal lineages, several lineages that have
remained simple have been very successful
Evolution of Protostomes
• flowing water brings food with it: thus the sessile
lifestyle evolved repeatedly during
lophotrochozoan and ecdysozoan evolution
• competition for space is a consequence of the
sessile lifestyle