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Title: Smaller Ecdysozoans


1
Smaller Ecdysozoans
  • Chapter 12

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3
Diversity
  • Many protostomes possess a cuticle
  • Non-living outer layer secreted by epidermis
  • Cuticle restricts growth and must be molted via
    ecdysis
  • Members of Ecdysozoa molt cuticle as they grow
  • Regulation of molting achieved by the hormone
    ecdysone

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Phylum Nematoda Roundworms
  • 12,000 species have been named
  • As many as half a million may exist
  • Found in virtually all habitats in all biomes
  • Topsoil may contain billions per acre
  • Nematode parasites exist in nearly all animal and
    plant species

6
Phylum Nematoda Roundworms
  • Form and Function
  • Outer body covering is a thick, noncellular
    cuticle, secreted by the underlying epidermis-
    hypodermis
  • Cuticle serves to contain the hydrostatic
    pressure exerted by fluid in the pseudocoelom
  • Collagen is the primary protein in layers of the
    cuticle
  • Muscles
  • Longitudinal muscles lie beneath the cuticle
  • No circular muscles
  • Run in four bands, marked off by epidermal cords
    that project inward to pseudocoelom
  • Unlike other animals, the muscle extends to the
    nerve cords

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Phylum Nematoda Roundworms
  • Hydrostatic skeleton-muscle contractions press on
    fluid
  • No circular muscles to compliment longitudinal
    muscle in movement (antagonist) so the cuticle
    assists.
  • Fluid is force to opposite side when longitudinal
    muscles constrict forcing the cuticle to expand
  • Compression and stretching of the cuticle returns
    the body to resting position when muscles relax
  • Produces the thrashing movement of nematodes

9
Phylum Nematoda Roundworms
  • Digestion
  • Gut tube consists of mouth, pharynx, non-muscular
    intestine, short rectum and anus
  • Muscular pharynx sucks food in
  • Intestinal wall is one cell thick-no muscles
  • Food moves back as new food enters and the body
    moves
  • Defecation occurs from opening the anus and
    allowing pseudocoelomic pressure to expel waste
  • Some parasitic adults have an anaerobic
    metabolism aerobic metabolism are absent
  • Free-living nematodes and free-living stages of
    parasitic nematodes have both anaerobic and
    aerobic metabolism

10
Phylum Nematoda Roundworms
  • Ring of nerve tissue and ganglia around the
    pharynx give rise to small nerves to the anterior
    end and to two nerve cords, one dorsal and one
    ventral
  • Sensory organs at head and tail
  • Most are dioecious with males smaller than
    females
  • Male has copulatory spicules
  • Fertilization is internal
  • Nematode sperm has no flagella, in female
    reproductive tract sperm is ameboid and moves by
    pseudopods
  • Eggs are stored in uterus until deposited
  • Cuticle is shed between each of four juvenile
    stages

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Figure 12_03
12
Representative Nematode Parasites
  • Representative Nematode Parasites
  • Some are parasites of humans
  • Most are tropical
  • Ascaris lumbricoides
  • Occurs in up to 25 of people in some areas of
    the southeastern U.S.
  • More than 1.27 billion affected worldwide
  • A. suum is found in pig intestines
  • A female Ascaris may lay 200,000 eggs a day,
    which pass out in hosts feces

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Figure 12_04a
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Representative Nematode Parasites
  • Survive for long periods in soil
  • When humans eat undercooked vegetables
    contaminated with shelled juveniles, or when
    children put soiled fingers or toys in their
    mouths, consumed juveniles hatch, and burrow
    through intestinal wall
  • Carried through the heart to the lungs, they
    enter into alveoli causing pneumonia and are
    carried up to tracheae
  • Coughed up and swallowed, they mature in the
    intestine
  • They feed on intestinal contents and may block
    the intestines

15
Figure 12_04b
16
Representative Nematode Parasites
  • Hookworms
  • Anterior end of these small worms has a hook-like
    curve
  • Necator americanus, most common hookworm.
  • Sexes are separate
  • Large plates in mouth cut into intestinal mucosa
    and suck hosts blood
  • Pump through more blood than they digest
  • Heavy infections cause anemia
  • Eggs pass out in feces and juveniles hatch in
    soil
  • If human skin comes in contact with soil,
    infective juveniles burrow through skin to blood
  • Travel in blood to the lungs, are coughed up to
    be swallowed, and mature in the intestine

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Figure 12_05
18
Representative Nematode Parasites
  • Trichina Worm
  • Trichinella spiralis causes a potentially lethal
    trichinosis
  • Adult worms burrow into intestinal mucosa of the
    hosts small intestines and females directly
    produce juvenile worms
  • Juveniles penetrate blood vessels and circulate
    throughout the body to all tissues and spaces
  • Penetrate skeletal muscle cells, redirecting gene
    expression of the musculature
  • Cells lose striations and becomes a nurse cells
    to the parasite
  • When poorly cooked meat (pork or bear) containing
    encysted juveniles is swallowed, worms are
    liberated and mature in the intestine

19
Representative Nematode Parasites
  • Infect humans, pigs, rats, cats and dogs
  • Pigs can become infected eating uncooked scraps
    of infected meat or rats
  • Four other sibling species with variable
    distribution, freezing resistance, etc.
  • Heavy infections cause death
  • 25 cases of trichinosis are reported per year in
    the US

20
Figure 12_06
21
Representative Nematode Parasites
  • Pinworms
  • Most common worm parasite in the U.S.
  • Adults live in large intestine and cecum
  • Females, about 12 mm in length, migrate to anal
    region at night and lay eggs, causing itching
  • Scratching the anal region contaminates hands and
    bedclothes
  • Eggs develop rapidly and become infective within
    six hours at body temperature
  • When swallowed, hatch in duodenum and mature in
    large intestine
  • Members of this order have haploid males from
    unfertilized eggs
  • Females are diploid and come from fertilized eggs
    (haplodiploidy)

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Representative Nematode Parasites
  • Filarial Worms
  • Eight species of filarial nematodes infect humans
  • Some cause serious diseases
  • Wucheria bancrofti and Brugia malayi live in
    lymphatic system
  • Cause inflammation and obstruction of the
    lymphatics vessels
  • Females release live young, tiny microfilariae,
    into blood and lymph
  • Mosquitoes ingest microfilariae when they feed
  • Worms develop to infective stage and move into
    the mosquito bite wound when it bites a human
  • Elephantiasis is caused by repeated exposure
  • Swelling and growth of connective tissue causes
    enormous swelling of body parts-scrotum, legs,
    arms

24
Representative Nematode Parasites
  • River blindness or onchocerciasis is carried by
    black flies and infects 37 million people in
    tropics
  • Dog heartworm, Dirofilaria immitis, is carried by
    mosquitoes and is the most common U.S. filarial
    worm
  • Heartworm pills

25
Figure 12_08
26
Figure 12_09
27
Representative Nematode Parasites
  • Dracunculus medinensis, the guinea worm
  • Larvae live within planktonic copepods (water
    fleas)
  • Upon ingestion of contaminated water, water fleas
    are digested by human host, but larvae survive
    and penetrate the stomach/intestinal wall
  • Worms live and mate within body cavity
  • Gravid females migrate through tissues to the
    lower extremities and produce an open ulcer
  • Causing burning pain, the human host immerses leg
    into water and female releases thousands of eggs
    into water
  • Eggs are eaten by copepods and the cycle
    continues

28
Representative Nematode Parasites
  • Only known treatment is to carefully remove
    ulcerated females carefully with a stick treated
    this way for thousands of years

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30
Phylum Nematomorpha
  • Diversity
  • Horsehair worms resemble coarse hairs
  • Adult structures resemble those seen in
    nematodes cuticle, epidermal cords, only
    longitudinal muscles, and a similar nervous
    system pattern
  • Currently placed as the sister taxon to nematodes
  • About 320 species are known
  • Occur worldwide
  • Adults are free-living in moist habitats
  • Juveniles are parasites of arthropods

31
Phylum Nematomorpha
  • Larvae encyst within host and do not emerge from
    an aethropod host unless water is nearby
  • Juveniles of freshwater forms use terrestrial
    insects as hosts
  • Marine nematomorphs infect certain crabs
  • Digestive system is vestigial
  • Larvae absorb food from arthropod hosts
  • Adults can absorb organic molecules through
    vestigial gut and body wall

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Figure 12_10
33
Phylum Tardigrada
  • Known as water bears
  • Very small, less than 1 mm long
  • About 900 live in a water film around mosses and
    lichens
  • Some live in freshwater and some are marine
  • Most are terrestrial that occupy a film of water
    surrounding mosses and lichens
  • Trunk bears eight short unjointed legs, each with
    claws
  • A pair of stylets and sucking pharynx protrude to
    pierce nematodes or plant cells
  • Body covered by non-chitinous cuticle that is
    molted four or more times during lifetime
  • Most of the body cavity a hemocoel
  • No circulatory or respiratory systems

34
  • Cryptobiosis
  • Terrestrial tardigrades can suspend metabolism to
    survive harsh conditions
  • Tardigrades can dehydrate from 85 water to only
    3 water
  • In this state they can resist extreme
    temperatures, ionizing radiation, oxygen
    deficiency, etc. for years
  • When water is available, they become
    metabolically active again

35
  • Reproduction
  • Sexes are separate
  • In parthenogenetic freshwater and moss-dwelling
    species, males are unknown

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Figure 12_15
37
Phylogeny and Adaptive Diversification
  • Phylogeny
  • Evolutionary relationships among ecdysozoans are
    not well-understood
  • Members of this clade do not share a common
    cleavage pattern
  • Nematodes and nematomorphs
  • Cleavage is unique, not spiral or radial
  • Cleavage in tardigrades
  • Has yet to be studied

38
Phylogeny and Adaptive Diversification
  • Recent phylogenies place Nematoda and
    Nematomorpha as sister taxa since they share a
    collagenous cuticle

39
Phylogeny and Adaptive Diversification
  • Tardigrades have some similarities to rotifers,
    particularly in their reproduction and
    cryptobiotic tendencies
  • Tardigrades and arthropods also share
    arthropod-type setae and muscles inserted on the
    cuticle

40
Phylogeny and Adaptive Diversification
  • Adaptive Diversification
  • Nematodes show the most impressive adaptation
  • Found in almost every habitat available to
    animals
  • Body structure is plastic enough to allow
    adaptation
  • Life cycle ranges from simple to complex
  • Have been known to survive in suboptimal
    conditions
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