Lecture 12 Chapter 12 – Parasitism
Some parasites can alter the behavior of their host in order to complete their life cycles.
o Ex: figure 13.1 the wood cricket’s behavior was manipulated by the hair worm emerging
from its body, by causing the cricket to jump into the water to die, so that the hairworm is
able to continue its life cycle.
o Alters its perception to thirst.
Not all enslaver parasites appear to act by manipulating the host’s chemistry, but the mechanisms
are still unknown.
Many other parasites “enslave” their hosts.
o Term enslave for several fungal species that alter the perching behavior of their fly hosts
in such a way that fungal spores can be dispersed more easily after the fly dies.
o Figure 13.2: Flies infected by the fungus perch in an atypical position, from which fungal
spores can easily spread to the healthy flies. Healthy flies typically perch on the upper
surfaces of lowlying vegetation where they are exposed to fungal spores.
Shortly before they die from the infection, yellow dung flies infected by the
fungus move to the downwind side of a relatively tall plant and perch on the
underside of one of its leaves. This position increases the chance that fungal
spores released will land on healthy yellow dung flies.
Vertebrates can be enslaved by parasites too.
o Rats infected with the protozoan parasite Toxoplasma gondii do not avoid cats, and in
some cases are actually attracted to cats.
o This increases the chance that the parasite will be transmitted to the next host in its
complex life cycle – a cat.
The parasitoid wasp manipulates its host, the orbweaving spider to spin a special cocoon web.
o Parasitized spider makes normal webs up to the night when the wasp induces it to make a
cocoon web. Suggested that the wasp might inject the spider with a chemical that alters
Wasp’s removal sometimes resulted in the construction of a web that was very
similar to a cocoon web, but more often resulted from a web that is different
from a cocoon web and a normal web. However, days after the removal things
went back to normal.
Chemical appears to act in a dosedependent manner; otherwise, we would
expect the spiders exposed to the chemical to build cocoon webs only.
Symbionts are organisms that live in or on other organisms
o More than half of the millions of species that live on Earth are symbionts.
o Our own bodies can be a home to many other species.
o Some symbionts are mutualists, but the majority are parasites
Parasites consume the tissues or body fluids of the organism on which it lives, its host.
Pathogens are parasites that cause disease.
Figure 13.3 – the human body. Different parts of our bodies provide suitable habitat for a wide
range of symbionts, many of which are parasites. Ex: our faces are home to mites that feed on
exudates from the pores of our skin and on secretions at the base of our eyelashes.
Parasites differ from predators.
o Unlike predators, parasites have a higher reproductive rate than their hosts, and typically
harm but do not immediately kill the organisms they eat.
Parasites Natural History: Parasites, which constitute roughly 50% of the species on Earth, typically feed on only one or a
few host species.
Macroparasites are large, such as arthropods and worms
Microparasites are microscopic, such as bacteria.
Most parasites feed on only one or a few individual host organisms throughout their lifetime.
Defined broadly, parasites include herbivores such as aphids or nematodes that feed on one or a
few host plants. Parasitoids, whose larvae feed on a single host, almost always kill it.
Most species are attacked by more than one parasite, and even parasites can have parasites.
Parasites tend to have a close relationship to the organisms they eat, and many parsites are closely
adapted to particular host species, and many attack only one or few host species.
Figure 13.4 – in a case study conducted in Britain, most host species were found to harbor more
than one parasite species. The number of parasite species shown here for fishes, birds, and
mammals includes only helminth worm parasites and hence is likely to underestimate the actual
number of parasite species found in these vertebrates.
They live on the outer body surface of their host.
Examples are the plant parasite such as the Dodder or the mistletoe that grow on, and obtain water
and food from another plant. The Dodder obtains water and food from the host plant from
specialized roots called haustoria.
Mistletoes are hemiparastic they extract water and mineral nutrients from their hosts, but since
they have green leaves and can photosynthesize, they don’t rely exclusively on their host for carbs.
Many fungal and animal parasites are ectoparasites. More than 5,000 species of fungi attack
important crop and horticultural plants, resulting in billions of dollar damage. Some fungi attack
plants, including mildews, rust and smuts, grow on the surfaces of the host plant and extend their
hyphae (fungal filaments) within the plant to extract nutrients from its tissue.
Plants are also attacked by animal ectoparasites including aphids, whiteflies, scale insects,
nematodes, beetles and juvenile cicadas.
o These animals can be thought of as both herbivores and parasites (especially if they
remain on one plant their entire life).
Picture of cicada in lecture – dunno why doe
Animals also have many ectoparasites. Ex: athlete’s foot
fungus, fleas, mites, live, and ticks. Some of these parasites
also transmit disease organisms, including fleas that spread the
plaque and ticks that spread Lyme disease.
Live within the host, in the alimentary canal, or within cells or
tissues. The alimentary canal is excelled as a habitat for many parasites.
Many do not eat host tissue, but rob the host of their nutrients.
Many disease organisms are endoparasites.
Figure 13.6 Tapeworm uses its suckers and hooks to attach to the intestinal wall of its
mammalian host, often a rodent, rabbit, or cat. Tuberculosis
causes lung disease tuberculosis, which kills 12m people. Soft
rot potato tuber shows the destruction wrought, that causes soft rot. Affected areas
become soft with decay and develop a distinctive odor.
Table 13.1 advantages and disadvantages to living in or on a host.
Defense and Counter defenses:
Hosts have adaptations for defending themselves against
parasites, and parasites have adaptations for overcoming host defenses
Host organisms have many kinds of defense mechanisms. Protective outer coverings include skin and exoskeletons. Many parasites that do gain entry are
killed by the host’s immune system.
Plants have many chemical weapons called secondary compounds. Some animals eat specific
plants to treat or prevent parasite infections. Ex: Wooly bear caterpillars switch from their usual
food plants to poison hemlock when parasitic flies lay eggs on their bodies.
Chimpanzees infected with nematodes specifically seek out and eat a bitter
plant that contains compounds that kill or paralyze the nematodes and can
also deter many other parasites. Figure 13.8 Chimpanzee chews on Vernonia