Species Interactions II Parasitism

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Species Interactions IIParasitism
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Parasites

• organisms that live in or on another species
  (the host) and have a negative effect on survival
  /or reproduction of the host
• Differ from true predators in that they may not
  kill their host and if they do, it may take
  several generations of the parasite.

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Coevolution

• Parasites and hosts act as selective influences
  on each other
• Hosts benefit if they evolve resistance or
  tolerance to their parasites
• Parasites benefit if they penetrate the defenses
  of their hosts
• Parasites have generally evolved toward lower
  virulence
• All species of plants and animals are resistant
  to all but a few species of parasites ie
  parasites have also evolved toward
  specialization.

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• More than half the worlds species live on or in
  the bodies of other organisms
• Almost every organism is the habitat of others

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Parasites found on wood mice near Oxford,
England. From Elton C. (1940) The Ecology of
Animals, 2nd Ed, Methuen, London
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Conservation implications

• Eg. Passenger pigeon went extinct in 1914
• Host for two species of chewing lice, Columbica
  extinctus Campanulotus defectus, went extinct
  at the same time

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Why be a parasite?

• Benefits
• Many parasites live in a benign physical
  environment, regulated by their hosts
• When compared to free-living relatives,
  parasites can produce offspring at rates of
  several orders of magnitude higher because they
  can allocate more energy to reproduction

Parasitic isopod on Caribbean soldierfish
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Why be a parasite? (cont)

• Costs
• Although rates of egg production are high, rates
  of transmission between hosts are usually low
• Finding a host can be hard, parasites have to
  disperse through a hostile environment this has
  led to the evolution of complex life cycles in
  some cases
• Hosts may be able to recognize and mount a
  defense against the parasite, To counter this,
  some parasites produce chemicals that suppress
  the immune systems of the host

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Classification of parasites

• Microparasites reproduce directly within their
  hosts, have generation times shorter than their
  host, induce acquired immunity in recovered
  hosts.
• Macroparasites are usually larger, reproduce
  outside their hosts, have generation times
  similar to their hosts and generally induce, at
  best, short term immunity.

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Classification of parasites (cont)

• Endoparasites live inside the tissues of their
  host at some stage in their life cycle

Ectoparasites live on the outside of their hosts
or on the epithelial lining of body cavities
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Classification of parasites (cont)

• Parasites can be obligate (must live in or on a
  host) or facultative

Some facultative parasites kill their host then
continue to live on it eg eggs of blowfly Lucilia
are laid on a mammal such as a sheep, larvae feed
on dung or on a wound and then spread into living
tissue. May either kill the sheep or make it more
susceptible to dying from other causes. They then
continue to feed, pupate, and hatch, further
generations continue to live on the dead host
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Parasite life cycles

• Direct life cycle parasite lives and reproduces
  in one host
• Indirect life cycle parasite needs one or more
  intermediate hosts
• Host in which sexual reproduction takes place are
  the primary hosts, others are termed intermediate
  or secondary hosts

Many parasites are spread from one host to
another by a vector. A vector only disperses the
parasite the parasite does not draw resources
from the vector. Flying insects such as flies and
mosquitoes are important vectors plus ticks.
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Dog tick life cycle - direct
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Schistosoma life cycle - indirect
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Taxonomic representation main groups

• Platyhelminthes Trematodes, Cestodes
• Nemertea (ribbon worms) - few species
• Nematomorpha (hairworms), parasitic in
  arthropods as larvae
• Acanthocephala (spiny-headed worms)
• Nematoda (roundworms)
• Annelida Hirudinea (leeches)
• Crustacea Copepoda, Cirripedia, Isopoda,
  Decapoda
• Insecta Diptera , Hymenoptera (parasitoids)

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Parasitic helminths

• Diverse group which cause a ubiquitous and
  constant drain on the energetic resources of most
  free-living organisms.
• Class Trematoda flukes, 3 subclasses
• Monogenea simple life history ie direct life
  cycles, mostly ectoparasites on the skin and
  gills of fish, some have become internal
  parasites of the mouths and urinary bladders of
  amphibians and turtles

Monogenea parasite on fish gill
Monogenea parasites on copepod
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• Aspidobothridea distinguished by huge sucker,
  often as big as the body, mainly endoparasites of
  molluscs, living in the pericardial or renal
  cavities, some found in the guts of fish and
  turtles
• Digenea more pathogenic than the other 2 groups,
  endoparasites, more complex life cycle, living in
  the gut, bloodstream or tissues. Typical life
  cycle will have three kinds of hosts

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• Schistosoma spp.
• Lives in human blood vessels, cercariae, with
  forked tail, bore directly into the primary
  hosts skin when in water.
• Once adult, males and females stay incopula for
  the rest of their lives within the larger vessels
  of the bloodstream then migrate to the rectum or
  bladder, cause local haemorrhage as they lay
  their spined eggs. Intermediate host is snail
• Tropical distribution, causes schistosomiasis
  (bilharzia), extremely virulent,
• most important parasitic disease of humans after
  malaria (affects 200 million people)
• First species enters host, provokes an immune
  response (production of antibodies)
• Defends itself by coating itself with proteins
• Schistosomes that subsequently enter host face
  barrage of antibodies stimulated by first
  parasite
• If host has already been infected with
  schistosomes from livestock or wild animals,
  effect of virulent species is moderated cross
  resistance

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Class Cestoda

• Endoparasitic tapeworms
• Most cestodes are in the subclass Eucestoda
  (true tapeworms), 2 host life cycle.
• Adult found usually in vertebrates, eggs give
  rise to bladder worms in the tissues of either
  the vertebrate host or a secondary, intermediate
  host.

• Secondary host has to be eaten by the primary
  host to give rise to adult tapeworms again.

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Cestodes (cont)

• Most cestodes have little or no pathogenic effect
  on their vertebrate hosts
• Humans are host to a few tapeworm species and
  secondary hosts to some others eg Taenia solium
  (pig tapeworm) and T. saginata (beef tapeworm),
  can grow to 5 m (one recorded at 21 m),
• Cestodes attach by scolex, reproduce via eggs in
  proglottid segments which are shed from the end
  of the body
• Lack any digestive organs, absorb food directly
  across the body wall from the intestinal fluid of
  the host.
• Enormous reproductive potential - single T.
  saginata can shed 10 proglottids/day, each
  containing 80,000 eggs

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Acanthocephala

• Spiny-headed worms
• Endoparasites, 300 spp,
• No gut, have both vertebrate and invertebrate
  hosts

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Nematoda

• Some free-living, some parasites of animals or
  plants
• Huge diversity, range from microscopic to over 2
  m long
• Structurally very alike so present problems for
  taxonomists
• About 50 species known to occur in humans, most
  not pathogenic

• All nematodes have high reproductive potential,
  most efficient species are not lethal, have
  capacity for sustained diapause as eggs, encysted
  larvae, or encysted adults

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• Wucheria bancrofti larvae move to peripheral
  blood vessels at night, transmitted by
  mosquitoes, heavy infections can cause
  elephantiasis
• Cyclops, a copepod is the intermediate host,
• Filariasis is the 3rd most important
• tropical parasitic disease in humans

• Nematodes can migrate around the body in
  response to chemical cues such as pH eg
  Nippostrongylus brasiliensis in rats,
  experimentally transplanted into the small
  intestine, migrate back to their original habitat
  in the large intestine

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Hirudinea

• Leeches hermaphrodites, marine, freshwater
  and terrestrial, not considered true parasites
  because they only remain attached to their host
  for a short period

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Copepoda

• Abundant and diverse marine parasites, range
  from commensal scavengers that can be occasional
  parasites to ecto- and endoparasites that are
  highly modified
• Some live in the intestines of fish, marine
  mammals, large decapods, most are host-specific,
  although they usually only feed on faeces and
  rarely on the intestinal wall
• Modifications for a parasitic lifestyle include
  sucking mouthparts and elongated piercing
  stylets.

• Attachment to host may be permanent, adult
  becomes reduces except for gonads, males may be
  very shrunken and be permanently attached to
  females.

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• Lernaea
• In juvenile stage are temporary ectoparasites on
  fish gills, leave first host after moulting,
  mate, then males die
• Females become permanently attached to gadid
  fish, usually in the gill tissue.
• Body becomes vermiform with an enormously
  enlarged genital segment, anterior of head
  becomes modified into a series of branching roots
  that grow deep into the musculature of the fish
• Female survives as a bag of tissue (which is
  partly host tissue produced by reaction)
  enclosing the mature female gonads that undergo
  continued egg production.

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Cirripedia

• Parasitic barnacles, retain a typical barnacle
  life history.
• Some species, least modified, live as
  ectoparasites on whales, turtles and sharks,
  derive food through a root system in the host
  tissue.
• Others live in gill cavities of lobsters and
  crabs or bore through the shells of molluscs
• Order Rhizocephala - most modified of all
  parasites, adult is merely a branching structure,
  like a fungus, growing though the host tissues,
  no alimentary canal. Externally, has
  typical-looking nauplii and cyprid larvae but
  they dont have a gut either - just
  undifferentiated cells.

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• Thompsonia spp.
• Adult ramifies through a host like a root system
• Breaks through the joints in the exoskeleton to
  produce many brownish-yellow egg sacs
• Egg sacs consist of host tissue
• On bursting, the egg sacs release free-swimming
  barnacle larvae

Flower crab Portunis pelagicus infested with
Thompsonia sp.
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Insect parasites Parasitoids

• Probably comprise more than 10 of metazoan
  animals
• Parasitoids resemble both predators and
  parasites
• Obligate carnivores in the larval phase.

• Adult female lays her eggs in or close to the
  body of another arthropod (usually another
  insect)
• Host is eventually killed by the feeding of the
  parasitoid larva.

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Parasitoids (cont)

• All the energy and nutrients necessary for
  development are obtained from a single host.
• Parasitoid body size is always smaller than that
  of the host
• Sometimes eggs are attacked, sometimes larvae.

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Parasitoids (cont)

• Mobile hosts usually stung and paralysed by the
  female parasitoid prior to oviposition.
• In some cases paralysis is temporary and the
  host recovers and continues feeding, even though
  it now contains an immature parasitoid.
• Advantage of this strategy (called koinobiont)
  is that female can attack hosts that are too
  small to support the developing parasitoid.
• Parasitoid remains dormant in the host until the
  host has grown to a sufficient size to allow its
  development.
• Idiobiont development occurs when no host growth
  occurs after parasitism.

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Parasitoids (cont)

• Many parasitoids lay their eggs on the outside
  of the host (ectoparasitoids) and the larvae
  develop by feeding through small punctures in the
  hosts cuticle.
• Endoparasitoids inject eggs into the body cavity
  of the host and larvae feed internally.

• Parasitoids may be highly developed to withstand
  attack by the hosts immune system.
• Some species inject a symbiotic virus into the
  host which destroys the ability of the host to
  encapsulate the parasitoids eggs.

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Parasitoids (cont)

• Either a single or multiple larvae may develop
  in a single host.
• In some species a single egg is laid which then
  divides asexually to give rise to a number of
  genetically identical larvae (polyembryony).
• Superparasitism where a previously parasitised
  host is discovered by a second female of the same
  species and this female deposits an egg/s
• Many parasitoids can detect the presence of
  another parasitoid in a host and avoid that host
  reduces subsequent competition
• Multiparasitism where second female is another
  species.
• Larvae of solitary species may have mandibles
  that they use to destroy other larvae developing
  in the same host.

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Parasitoids (cont)

• Hyperparasitoids specialised to develop on other
  parasitoids
• Many species in the family Aphelinae have females
  that develop as normal endoparasitoids of scale
  insects, while the males develop as
  hyperparasitoids on the females.

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Parasitoids (cont)

• Host detection Chemical cues (kairomones) most
  common but sight, sound, heat and vibrations.
• Kairomones may be volatile substances that
  operate as attractants over long distances or
  short-range arrestants that have low volatility
  and inform the parasitoid of the immediate
  presence of a host.

• Parasitoids may also operate on cues from the
  hosts environment eg Drosophila parasitoids are
  attracted to odours of rotting fruit
• Trichogramma spp. Detect arrestant chemicals
  released my moth scales dislodged during
  oviposition

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• Parasitoid life style has evolved many times in
  insects
• Most parasitoid species belong to either
• (i) the Diptera in two families, the Bombylidae
  and the Tachinidae
• (ii) the Hymenoptera (sawflies, wasps, ants and
  bees)

Hymenoptera
Symphyta
(sawflies) primarily herbivorous
Aculeata
Parasitica
Apocrita (ants, bees, solitary wasps,
social wasps)
Mostly parasitoids 45 families includes
ichneumons, braconids, chalcids
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Impacts of parasites on hosts

• Parasites can affect
• Individuals (morphology, behaviour,
  survivorship, growth, reproduction)
• Populations
• Communities
• Majority of studies indicate that effects are
  correlated with parasite load

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Effects on host morphology

• Example 1 Frog limb malformations associated
  with infection by the trematode Ribeiroia ondatrae

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• Example 2 Sacculina
• Rhizocephalan parasite of crustacea has a
  nauplius larva, then settles as a cyprid on a
  host
• Adults become internal cuticular tumours
  (interna)
• Tumours ramify through host, external egg sac
  develops
• When crabs are parasitized by Sacculina, males
  develop a female appearance due to
• interference in the function
• of the androgenic gland
• by the parasite
• Parasite also prevents
• crab from moulting

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Effects on host behaviour

• Parasites often affect host behaviour in ways
  that increases probability of transmission
  between hosts /or reproduction
• Example 1 Human threadworm Enterobius
  vermicularis lives mainly in the caecum but at
  night the female migrates to the anus and lays
  eggs on the surrounding skin. This causes the
  host to itch and scratch thus spreading the eggs
  to the hands and then to reinfect via the mouth.

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Effects on host behaviour

• Example 2 Acanthocephalans (spiny-headed worms)
  amphipods
• Uninfected amphipods are negatively phototactic
  - stay on lake bottom, away from surface-feeding
  vertebrates
• Infected amphipods are positively phototactic,
  swim to the surface where they are preyed upon
• Amphipod behaviour remains unaltered until the
  acanthocephalan has reached a life stage called a
  cystacanth that
• is capable of infecting the vertebrate host.
• If eaten earlier the acanthocephalan
• would die without completing its life
• cycle.

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Effects on host behaviour (cont)

• Example 3 Fungal parasites of ants
• Infection by fungus alters ant behaviour such
  that the host climbs up on to plant stalks or
  leaves where it dies
• Fungal hyphae grow out of the brain and feet,
  produce fruiting bodies then disperse spores

Camponotus sp. infected by Cordyceps lloydii
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Parasites and Host Populations

• Balance between parasite and host populations
  influenced by the virulence of the parasite and
  the defenses of the host immunity
• Parasites can influence population cycles of
  their hosts
• Example 1 red grouse in Scotland populations
  cycle with a period of 4-8 yrs
• Population crashes associated with high levels
  of Trichostrongylus tenuis (nematode) infections
• Experimental removal of parasite load
  significantly reduced population fluctuations

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Effects on host populations (cont)

• Example 2 Red foxes and mange mites
• Sarcoptes scabei first found infecting red foxes
  in central Sweden in 1975.
• Causes hair loss, skin deterioration and death,
  mange spread over whole of Sweden
• Within 10 yrs, fox populations reduced by 70.
• Following this, the number of mountain hares
  (one of foxs main prey species) increased x 4

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Conclusions

• Parasitic way of life has been adopted by a
  diverse group of invertebrates
• Parasites are capable of enormous reproductive
  rates compared to their free-living counterparts
  (benign nature of environment, lack of allocation
  to other functions etc)
• Parasites show a wide range of adaptations to
  their way of life including morphology, behaviour
  and life cycles
• Parasites are ubiquitous and can have significant
  impacts on the morphology, growth and behaviour
  of their hosts, as well as affecting populations
  via their influence on survivorship and
  reproduction
• These population-level impacts can have flow-on
  effects to ecological communities