Symbiosis Reading worksheet
The word symbiosis literally means "together life". It refers to organisms that live in close
approximation; often one cannot live without the other. In extreme cases, one organism actually lives
inside the other organism.
There are 3 types of symbiosis:
1. Parasitism: parasite benefits, host is hurt.
Parasitism is a relationship in which one organism - the host - is the source of food and/or shelter for
another organism, the parasite. In this relationship, all of the benefits go to the parasite; the host is
harmed by the relationship. An example is a human and a tapeworm living in the intestines. The
tapeworm derives food (and shelter) from the human host; the human is denied the nutrition that is
consumed by the tapeworm.
2. Commensalism: one species benefits, the other is neither hurt nor helped.
In commensalism, one organism benefits from the relationship while the other is neither helped nor
hurt. Example: a few orchids growing epiphytically on a tree. If there are a lot of commensals on a
single "host" then it stands to reason that the host will be hurt and the relationship will slide towards
the parasitic (Note: some ecologists define parasitism functionally, that is the parasite must be adapted
to feed on the host; other ecologists define the terms logically, that is if the host is hurt and the parasite
benefits then the relationship is parasitic, even if feeding does not take place. This latter definition
makes many commensal relationships appear parasitic. )
3. Mutualism: both species benefit
Finally we come to mutualistic relationships where both parties benefit. In fact, many people use the
term symbiosis a bit too casually, using the term symbiosis to refer to mutualistic relationships (they
should really call a relationship where both species benefit mutualistic instead of symbiotic, as the latter
term leaves open the possibility that the relationship could be mutualistic, parasitic or commensal in
nature). Example: corals and zooxanthellae (zooxanthellae are algae that take up residence in a coral
animal. The photosynthetic zooxanthellae provide the coral with sugars in return for nitrogen and other
nutrients from the coral).
Tightness and Looseness: All relationships between organisms range over a continuum from obligate
(where one or both organisms would die without the other) to facultative (where the presence/absence
of the other isn't really necessary). Obligate relationships - such as a human tapeworm in our gut - are
considered "tight", while facultative ones - a squirrel living in a tree - are considered "loose". Some
ecologists place the 3 types of relationships first, that is there are parasitic, commensalistic, and
mutualistic relationships, and only the obligate ones in any of these 3 categories are called symbioses.
Parasitism tight and loose: The Catalpa Worm (above) is being parasitized by tiny wasp larvae. The
adult wasps (right) sting the caterpillar, injecting their eggs. The eggs hatch and devour the caterpillar
from the inside, being careful not to disrupt any vital functions. Eventually they emerge and spin
cocoons of silk in which they transition from larvae to adult. Technically, these insects are parasitoids,
since, unlike true parasites, they kill their hosts. This is a failry tight relationship; the wasps could not
survive without caterpillars in the same family (although the caterpillar would do just fine without the
wasps). Much looser parasitism is shown by ectoparasites, which feed from the outside. Mosquitoes of
course suck blood (only the females at that; they need the protein to make eggs). Oak Treehoppers
(left) suck sugar-rich juices from the phloem of trees).
Often these relationships are not species-specific; the
mosquitoes would probably go after any other warmblooded prey and the oak treehoppers pictured were in fact
on a sycamore tree
The squawroot (left) may look like a fungus, but it is
actually a flowering plant. It is parasitic on trees,
usually oaks, and gets its energy by tapping into the
oak's roots. It betrays its true nature when it comes
time to reproduce, however. I'm not sure about
squawroot in particular, but other related plants are
often self-pollinating and thus don't even need showy
flowers to attract pollinators. The squawroot is a
distant relative of the magnolia.
Commensalism? Many would argue that a flicker making its home in a cactus (below left) is a good
example of commensalism. In a forest, such a relationship usually is commensalistic; the flicker below
has excavated its nest in the dead wood of a living sycamore tree. To my eye, the desert bird has gone
through some living tissue to make its nest. Still, the overall damage to the cactus is small.
The white-winged dove (left) has a mutualistic relationship with the
Saguaro Cactus. The cactus provides food for the bird in the form
of a large fruit. The bird consumes the fruit, also ingesting the
cactus' seeds. The bird then flies off, and later deposits the seeds in
a new location (with a nice dose of fertilizer to boot!). In this way,
the cactus gets its seeds transported away from the parent plant,
allowing it to potentially colonize new places. This type of
mutualism is known as a dispersive mutualism.
The Cattle Egret (right) is often seen in the
company of grazing animals. The grazers
stir up insects, which the egret then eats.
This is probably a loose sort of
commensalism; there is no apparent
benefit to the cattle. The commensalism is
loose because the egrets will follow any
cattle; in Florida, in fact, I have seen them
following mowers.
Symbiosis in the seas: Some of the best examples of symbiosis are found in the oceans - not surprising
since life has had longer to evolve and form close associations in the oceans. Below, the corals are
perhaps the best example of a mutualistic symbiosis. Tiny coral animals (which individually resemble
this freshwater hydra) form huge colonies, with each hydroid encased in stone secreted by the animals.
Collectively, these colonies can grow very large. Brain coral (above right) typically forms huge colonies;
the dark "boulder" to the left of the picture immediately right is actually a colony of brain coral that may
be thousands of years old (the fish is 5 feet long). Each hydroid in turn may harbor cells of
photosynthetic algae (usually dinoflagellates); these algal endosymbionts are called zooxanthellae and
give the coral its brown or green appearance. As mentioned above, the zooxanthellae "trade" sugars for
nutrients; it's convenient that the wastes of the coral (CO2, ammonia, etc.) are the very things needed
by the algae for photosynthesis. Interestingly, both the corals and the zooxanthellae can survive without
the other (at least for a while); under conditions of stress the corals are known to expel the
endosymbionts in a phenomenon known as coral bleaching. Under happier times, the corals direct their
growth to maximize sun exposure for their algal guests; you can see this clearly in the photo of the
Elkhorn Coral below.
Finally, everyone who has seen "Finding Nemo"
knows about the association between Clownfish and
Anemones. By working its way carefully into the
anemone, the clownfish gradually accustoms the
anemone to the chemical makeup of the fish's skin;
this gradual acclimatization prevents the anemone
from stinging the clownfish (while fish with a
different "taste" will be stung and eaten). The fish
gets a safe house and some tidbits; the anemone
gets cleaned and has the clownfish working as lures to bring in potential prey, or chasing away fish that
would harm the anemone. Some scientists do not see any benefit for the anemone and classify this as a
commensalism.
The Sea Lamprey, left, is a sort of temporary parasite. It latches onto
a fish and uses the teeth to hold on and rasp away the skin, leaving an
open wound for the lamprey to feed on. It drops off, usually without
killing the "host". Sea Lampreys are not specific on any species of
fish; they will latch onto any living thing and try to feed.
The wasp to the right has stung and paralyzed a spider. It will take
the spider to a nest and lay an egg on it. The larvae will consume
the still-living spider; often from the inside. This is usually
considered to be a parasitoid relationship.
Two more relationships from the Costa Rican
forests. The Tree Sloth (left) has algae growing in
its fur. These algae help to camouflage the sloth
against the lichen-covered tree (note the brown
fur of the baby, not yet covered with algae). This
is a mutualistic relationship. There is even a moth
that lives only in the sloth's fur and consumes the
algae; this is a commensal relationship between
the moth and the sloth.