Oct 24 2013
More than half of the species living on earth are symbionts. Symbionts are organisms that live in
or on other organisms.
Our own bodies can be a home to many other species (exterior and interior)
A parasite consumes the tissues or body fluids of the organism on which it lives (the host).
Pathogens are parasites that cause diseases (an abnormal condition affecting the body of an
Parasites usually have a higher reproductive rate than their hosts.
Parasites typically harm, but don’t immediately kill, the organisms they eat (unlike predators).
The degree of harm varies widely. We can compare one parasite that causes mild symptoms
with one that has lethal effects. (fungus that causes athlete’s foot (nuisance), and Yersinia
pestis, the bacterium that causes the plague (FATAL)).
Parasite Natural History
Parasites typically feed on only one or a few host individuals over the course of their lives
Parasites can be defined broadly to include herbivores such as aphids or nematodes that feed
on one or a few host plants and Parasitoids which are insects whose larvae feed on a single host
and almost always kill it.
Parasites vary in size from large species to species too small to be seen with the naked eye
Macroparasites: Large species such as arthropods and worms.
Microparasites: Microscopic, such as bacteria protists and unicellular fungi.
Most species are attacked by more than one kind of parasite; even parasites have parasites.
Many parasites are closely adapted to particular host species (close relationship with the host)
and many attack only one or a few host species. This specialization helps explain why there are
so many species of parasites (many host species have at least one parasite that eats only them)
Rough estimate suggests that 50% of the species on earth are parasites. Ectoparasites live on the outer body surface of the host.
Endoparasites live inside their hosts, within cells or tissues, or in the alimentary canal.
Many fungi are ectoparasites. These live on the outer body surface of the host.
Ectoparasites include plants such as Dodder and mistletoe that grow on, and obtain water and
food from another plant.
Such parasitic plants use modified roots called Haustoria to penetrate the tissues of their hosts.
Dodder cannot photosynthesize and hence depends on its host for both mineral nutrients and
carbohydrates. Mistletoe however, is hemiparasitic. They extract water and mineral nutrients
from their hosts, but since they have green leaves and can photosynthesize, they do not rely
exclusively on their host for carbs. There are also many fungal and animal parasites that live on
the surfaces of plants and animals, feeding on their hosts tissues or body fluids.
More than 5,000 species of fungi attack crop plants causing billions of dollars in damage each
Mildews, rusts, and smuts grow on the surface and extend their hyphae (fungal filaments) into
the plant to extract nutrients from its tissues.
Plants are also attacked by numerous animal ectoparasites
Aphids, whiteflies, scale insects, nematodes, beetles, and juvenile cicadas.
They can be thought of as both herbivores and parasites.
Animals also have many ectoparasites
Athlete’s foot fungus, fleas, mites, lice, and ticks.
Some of these parasites also transmit disease organisms.
Many disease organisms are endoparasites. These are parasites living inside of their hosts.
Parasites are commonly found within the alimentary canal. People/animals are
constructed the same. Their bodies consist of tissues that surround an open tube called
the alimentary canal which extends from the mouth to the anus. Most parasites within
the alimentary canal do not eat host tissue, but rob the host of nutrients. For example, a
tapeworm attaches to the host’s intestinal wall and absorbs digested food.
Many endoparasites live in the host’s tissues/cells. They cause a wide variety of
symptoms as they reproduce or consume host tissues. Examples include Yersinia pestis,
the bacterium that causes the plague and Mycobacterium tuberculosis, the bacterium
that causes tuberculosis.
Plants also have endoparasites
Bacterial pathogens cause soft rot. Fungi can rot various plant parts from the inside out.
Some bacteria invade vascular tissues, disrupting the flow of water and nutrients,
causing wilting and often death.
Advantages & Disadvantages of Ectoparasitism and Endoparasitism
There are advantages and disadvantages to living in or on a host. Because ectoparasites live on the surface of their host, they can disperse more easily than
endoparasites (move from one organism to another). Endoparasites have evolved various
mechanisms for dispersal, including complex life cycles (stages specialized for dispersing from
one host species to another) and enslaver parasites (alter physiology or behaviour of host in
ways that facilitate their dispersal).
Some parasites of the alimentary canal are dispersed in feces.
Ectoparasites are more exposed to natural predators, parasites (mites that suck fluids from their
bodies), and parasitoids because they live on the surface of the host.
Aphids are eaten by many birds and insects, and attacked by parasites and parasitoids.
In contrast, endoparasites are protected from the external environment, and have easy
access to food (unlike an ectoparasite, an endoparasite does not have to pierce the
hosts protective outer surfaces to feed.
Endoparasites are at risk from attack by the hosts immune system. Some parasites have evolved
ways to tolerate or overcome immune system defenses.
Defenses and Counterdefenses
Hosts have evolved ways to protect themselves against parasites and parasites have evolved
counter measures to circumvent host defenses
Parasites exert strong selection pressure on their host organisms, and vice-versa.
Host organisms have many kinds of defense mechanisms.
Protective outer coverings include skin and exoskeletons. These can prevent
endoparasites from entering and keep ectoparasites from piercing its body.
Many parasites that do gain entry are killed by the host’s immune system.
Vertebrate immune systems have “memory cells” that can recognize microparasites
from previous exposures. Some are so efficient that the host can have lifelong immunity
against future attack by the same microparasite.
Other immune system cells engulf and destroy parasites or mark them with chemicals
that target them for later destruction.
Hosts can regulate biochemistry to deter parasites.
Bacterial and fungal endoparasites require iron to
grow. Vertebrate hosts have a protein called
transferrin that removes iron from blood serum
(where parasites have access) and stores it so
bacterial and fungal endoparasites can’t get it.
But some parasites can steal iron from the
transferrin to support their own growth. This allows
them to overcome the biochemical defense of the
Plants also have defense systems.
Resistance genes. These are different alleles which provide protection against microparasites that have particular genotypes
Nonspecific immune responses such as antimicrobial and antifungal compounds. (some
that attack the cell walls of bacteria and others that are toxic to fungal parasites)
Chemicals that stimulate deposition of lignin, which makes a barrier to stop an invader’s
Chemical signals that “warn” nearby cells of imminent attack.
Plants can also regulate their biochemistry to deter parasites
Plants have many chemical weapons called secondary compounds.
Some animals eat specific plants to treat or prevent parasite infections.
Example: Woolly bear caterpillars switch from their usual food plants to poison
hemlock when parasitic flies lay eggs on their bodies.
Chimpanzees infected with nematodes seek out and eat a bitter plant that
contains chemicals that kill or paralyze the nematodes (Huffman 1997).
Some hosts can encapsulate endoparasites, or their eggs, to make them harmless. Encapsulation
is a process in which some hosts can cover parasites or parasite eggs with capsules that kill them
or render them harmless. It is used by some insects to defend themselves against
macroparasites. Insect blood cells can engulf small invaders (bact