Smaller Ecdysozoans

1
Smaller Ecdysozoans

• Chapter 12

2
(No Transcript)
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

4
(No Transcript)
5
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

7
(No Transcript)
8
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

11
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

13
Figure 12_04a
14
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

17
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)

22
(No Transcript)
23
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

29
(No Transcript)
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

32
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

36
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