Protostome Animals

Chapter 32
Pages 724-748
Protostome Animals
Protostome Animals
One of two monophyletic groups of
bilaterally symmetric,
(mostly)coelomate animals that
share a pattern of embryological
development, including spiral
cleavage, formation of the mouth
earlier than the anus and includes
arthropods, mollusks, and annelids.
And that means????
• Monophyletic groups:
– refers to any group of organisms that includes the
most recent common ancestor of all those
organisms and all the descendants of that
common ancestor
Monophyletic, paraphyletic, & polyphyletic
Bilaterally Symmetric
• In bilateral symmetry (also called plane
symmetry), only one plane, called the sagittal
plane, will divide an organism into roughly mirror
image halves (with respect to external
appearance only).
• Often times leads to cephalization (head and
body)
• Bilateral symmetry permits streamlining, favoring
the formation of a central nerve center
• Bilateral symmetry is an aspect of both chordates
and vertebrates
Bilateral symmetry
sagittal plane
Coelomate Animals
• Possess a body cavity between the gut and the
body wall that is completely lined by
mesoderm tissue.
• The coelom serves several important
functions, such as:
– providing space for the development and
function of complex internal organs
– providing a hydrostatic skeleton to facilitate the
movements of soft-bodied animals.
• ***Not all protostomes are coelomates
Acoelomate and Pseudocoelomate
• There was a reversion of protostomes to
complete lack of a body cavity by flatworms
(Platyhelmenthes). This is called acoelomate.
– Flukes, tapeworms, and other types of flatworms are
all acoelomate animals but ARE protostomes.
• Some protostomes have a false body cavity that
forms between the mesoderm and the endoderm
layers called a Pseudocoelomate
– Because the cavity is not within the mesoderm and it
does not support organs it is not a real body cavity
– Protostome examples of pseudocoelomates are
roundworms (Nematoda) and rotifers (Rotifera)
What is an Animal?
• An animal:
– Eukaryotic
– Heterotrophic
– Multicellular
– Generally mobile at some stage
– Highly evolved
– complex
Spiral Cleavage
• Spiral cleavage is characteristic of
protostomes.
• Found in EARLY embryonic development of
organisms
• A developing embryo has spiral cleavage if:
– it undergoes cell division (cleavage)
– changes from a four-cell embryo to an eight-cell
embryo, with the cells divide at slight angles to
one another
– none of the four cells are in one plane
• Caused because mitotic spindles are at angles
Spiral cleavage
Spiral vs. Radial cleavage
Mouth First Development
• During the gastrulation of protostome organisms,
the initial invagination that forms within the
embryo becomes the mouth
• The coelom forms later in development when
openings form within the blocks of mesodermal
(middle) tissue
• This mouth first development leads into a head
and brain development
• This leaves little time for advanced development
of cerebral structures
– This is theorized to cause lower cerebral functioning
and smaller brains all together
The winning group???
• The question may have came to you, “Why study
protostomes?”
• Protostomes are widely the champions of the
theory of evolution
– Maybe not the champions, but they definitely took
the idea natural selection by adaptation to a new level
by rapid levels of speciation
• There is remarkable number of different
protostome species
– 925,000 different arthropod species have been
formally identified (80 percent of the world's species)
• It has been hypothesized that the actual # may be as high as
10 mil
WHY???
• This remarkable monophyletic group not only has
easily the most species rich clade, but its overall
numbers on this planet surpass even the imagination
– Within one single acre of rural land in England an
estimated 17, 825, 000 beetles will reside
– At any time, it is estimated that there are some 10
quintillion (10,000,000,000,000,000,000) individual insects
alive.
– Recent figures indicate that there are more than 200
million insects for each human on the planet!
– A recent article in The New York Times claimed that the
world holds 300 pounds of insects for every pound of
humans
• How is this even possible???????????????????????
One in a Quintillion
• Having the ability to not only have a
tremendous number of offspring
• But also to have those offspring reach
maturity rapidly
• Natural selection speeds up to a rapid pace
comparable only to prokaryotes and viruses
• The rate of possible mutation increases as well
allowing for overall speciation to increase
Tube Within a Tube
• Most members of the protostome super-phyla
have a wormlike bodies
– These bodies contain a basic tube within a tube
setup
– The outside is the skin which is derived from the
ectoderm
– The inside tube is the gut, derived from the
endoderm
– Muscles and organs derived from the mesoderm
located between the two tubes
– In some organisms this becomes the
hydroskeleton
Importance of Protostomes as Food?
• Shellfish such as clams, mussels, and oysters
are consumed throughout much of the world
• Crustaceans such as shrimp, lobsters, and
crabs make up some of the highest priced
meet in the world. (26% of all marine fishery
harvests)
• Snails are considered a delicacy in some
countries, where they appear on menus as
"escargot”
Protostomes in the Medical World
• Leeches have been used for a wide variety of medicinal
purposes as far back as 2,500 years ago
– Leeches secrete a substance that prevents blood from clotting,
and they can consume five times their own body weight in
blood
– Currently they are being used in plastic and reconstructive
surgery
• Studies of horseshoe crab eyes have led to treatments for
human eye disorders
• Chitin forms the chemical chitosan that removes metals
and toxins from water, and its fat-absorbing properties help
remove fat and cholesterol from human bodies
• Mollusks extract compounds have promise as anticancer
drugs.
• Compounds in the venom of cone shells are being
considered as potential drugs for treating neurological
disorders or acting as painkillers.
Medical leaches
Ecological Value of Protostomes
• Some species of protostomes, particularly those
that burrow and feed in soft sediments, exert a
major influence on the structure and functioning
of their habitats
• These organisms have remarkably individual
niches
• They make up the largest biomass on the planet
and exist as primary, secondary, and tertiary
consumers
• They make up the largest group of prey on the
planet
Model Organisms
• Two of the most famous protostomes in the
field of science are:
– Drosophilia melanogaster the fruit fly
– Caenorhabditis elegans the round worm
• Pronounced see-no-rab-DIE-tiss
• D. melanogaster and C. elegans are
exceptional model organisms for a science lab
because of slightly different reasons…
Drosophilia melanogaster
• Since the early 1900s D. melanogaster has been a
key experimental subject for genetics beacause:
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Its genome has been fully sequenced
It is easily reared in a laboratory
It is an inexpensive model organism
Mating can be arrange within a controlled setting
The life cycle is completed within two weeks
Females lay large numbers of eggs
Genetic defects can be seen in early embryonic
development
– Insight into eukaryotic gene development and
influence
Drosophilia melanogaster
Caenorhabditis elegans
• C. elegans emerged as a model organism in
developmental biology in the 1970s because of
Sydney Brenner and colleagues.
– Its cuticle is transparent, allowing for easy viewing of
individual cells
– Adults have exactly 959 nonreproductive cells and
– The fate of each of those cells can be predicted
• If researchers examined a 64 celled embryo of C. elegans,
they can predict the fate of EVERY one of those cells
» AND THERE IS MORE…
Caenorhabditis elegans and apoptosis
• Apoptosis is the term for programmed cell death
– We all have (nonfunctioning) gills and webbing on our
hands and feet during late embryonic development
within the womb
– The cells in this webbing and gill material go through
apoptosis before birth, and we are born without them
• C. elegans has 959 adult cells… but 1090
embryonic cells… the 131 cells that are missing
are programmed to die
• However… if a C. elegans is born with more than
959 cells it generally leads to cancer in
adulthood…
• So this means cancer now has two definitions…
Caenorhabditis elegans
Lophotrochozoa and Ecdysozoa
• Molecular biology suggests that a major
branching occurred within the protostome
lineage creating the groups:
– Lophotrochozoa
• Mollusks, annelids, rotifers, and flatworms
– All members grow by increment additions to their bodies
– Ecdysozoa
• Roundworms (Nematodes) and Arthropods
– All members grow by molting
Lophotrochozoans
• Mollusca:
– Bivalvia (2 shells)
• Clams, Mussels, Scallops, Oysters
– Gastropoda (belly-feet)
• Snails, slugs, nudibranchs
– Polyplacophora (many-plate-bearing)
• Chitons
– Cephalopoda (head-feet)
• Squid, Nautilus, Octopus
Edysozoans
• The lineage Edysozoa is named for the distinctive
molting done by all members of its group
– Ecdysis is the Greek root which means to escape or
slip out of
• During a molt, individuals split their exoskeleton
or cuticle and then slip out of it
• Their bodies are allowed to then expand, and a
new cuticle or exoskeleton forms
– Organisms are INCREDIBLY vulnerable to attack and
damage during a molt
Edysozoa
• Nematoda
– Roundworms
• Arthropoda
– Myriapods (millipedes and centipedes)
– Chelicerata (Spiders, ticks, mites, horseshoe crabs,
daddy longlegs, scorpions)
– Insecta (insects)
– Crustaceans (Shrimp, lobsters, crabs, barnacles,
isopods, copepods)