What is the largest living structure on Earth? No, it’s not
the 2200 acre Armillaria ostoyae fungus in
Oregon that we talked about previously. That is the largest single organism,
but there is something bigger.
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The
Great Barrier Reef houses more species of
coral
than any other place on earth, more than 600
species
call the reef home. You see how many shapes
they
can take. Does this mean they are
asymmetric
animals?
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This can add up quickly, because the Great Barrier Reef is
more than 132,973 sq. miles (344,400 sq. km) in area. And if you still don’t
believe it is a living structure, get this; it’s moving south! Climate change
is warming the waters off the coast, and corals and tropical fish are moving south
with the warmer temperature. Meanwhile, the northern edge recedes as the waters
get too hot for corals.
Corals take all sorts of shapes (see picture above), and despite
what we talked about last week, they don’t seem to be either bilaterally or
radial symmetric. Sure, brain corals look radial, but most corals don’t have a
repetitive shape. Are these asymmetric animals?
Nope - remember that the coral you see is the exoskeleton of
the polyp, not the animal itself. Just because the apartment building isn’t
symmetric, it doesn’t mean the animal is as well. Coral polyps are definitely
radially symmetric, so our discussion of last week still holds.
Corals and other radial animals are exceptions, since 99% of
animals are actually bilaterally symmetric. But there are exceptions with the
bilateral animals as well. Some species that have been bilateral for millions
of years then evolved a tweak to the system. Some part them became asymmetric
in order to give them an advantage. Their stories are exceptional and we should
explore some of them.
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Coral
polyps are cnidarian animals. They live inside
the
calcium shells they produce, and it is the shells
that
seem asymmetric in many cases. But the polyps
show
radial symmetry. Each stalk and white “flower”
is
an individual polyp.
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Let’s start with the first animals that became bilaterally
symmetric– the platyhelminthes, or flatworms. Most flatworms are small, just
barely visible with the human eye, and most swim in the water. There are
free-living versions and parasitic species and it is in the parasites that we
find our first animal that has decided that completely bilateral isn’t
necessary.
Polyopsithocotylea
monogenea is a group of flatworms that live on and feed on fish gills. About
0.05-1 mm long, these platyhelminthes attach themselves to one of the gill
ridges and take up residence there for life. The attachment they use is called
a haptor, and they come in different
shapes and sizes.
The oncomiracidium stage of the worm is directly after the
egg stage and before it becomes an adult. This stage is completely bilaterally
symmetric. But when the adult stage is reached and it’s time to settle down and
starting feeding on some fish’s gills, they become asymmetric via their haptor
attachment.
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Opisthaptors,
or just haptors, are the attachment
organs
for many parasites, including parasitic
flatworms.
They can use suckers or clamps and hooks
in
order to anchor the worm in its preferred habitat.
You
can see that these haptors maintain animal
symmetry,
but not all do.
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The initial haptor is usually located on the posterior end,
on one side only. So much for bilateral symmetry. In some species this haptor
has suckers, in others it has hard clamps, and yet other species have both.
As the adult grows, more haptors may develop, just where the
animal touches the gill. Some may have 50 or more haptors arranged around their
posterior, becoming more and more asymmetric. But there is a plan, they only
grow where attachment is possible; some signal is generated by contact and this
stimulates growth of more attachments.
Here we have a family of parasitic
worms that aren’t symmetric living within a phylum of worms that were the first
to be bilaterally symmetric - exceptional. But, take one step up the chain and
you see the fish they live on. It just so happens that at least one group of
fish parasitized by P. monogean worms
are asymmetric themselves.
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These
are the profiles of some monogean flatworms. The
haptors
of these parasites grown in odd places and
destroy
the bilateral symmetry of the animal. But it is
necessary
for the worm to attach to the gills of their
prey
fish.
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The cichlids are successful because different species have
developed different feeding niches, and this is where we meet our asymmetric
cichlids – they eat the scales off other fish! One genus, Perissodus, has at least six species that eat scales, all endemic
to Lake Tangayiki (although there are also other scale eaters in other
locations).
Scale ripping and eating is called lepidophagy (lepido = scale, and phagy
= eat). Scales are an unexpectedly good source of nutrition. They chock full of
protein and calcium phosphate, and their attachments are both cartilaginous,
fatty, and come with some carbohydrate. Remember that the next time you order
fish in a fancy restaurant. Tell the chef not to scale it – you’ll be quite the
topic of conversation in the kitchen.
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Fish
scales have many uses. Some, especially from
herrings,
are used to make the pearlescent cosmetics
that
are sold today. Gals, your putting fish scales on your
eyes
and lips. Fish scales can also be turned into artificial
bone,
or they can be food for lepidophageous fish.
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But, you can imagine that the fish being unfrocked don’t
appreciate it very much and fight back or swim away quickly. That doesn’t even
take into account how hard it is to bite the scales off a swimming fish.
Therefore, lepidophages must evolve anatomies and behaviors that give them a
chance to succeed. Or perhaps it would be better to say, they acquired
characteristics that made being a lepidophage an advantage.
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Here
are two P. microlepis scale eating
fish. One is right
mouthed
and the other is left mouthed. You can see how
the
way their mouth develops breaks their bilateral
symmetry.
The right-mouthed version (on the left) will
only
eat scales from the left side of fish. How is it that both
versions
can be maintained in a population?
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A 2012 video study showed that right-mouthed individuals
almost always attack prey from the left, and their strikes are more powerful
and successful when coming from the preferred side. One could ask, why are
their both types? How did right- and left-mouthed individuals come to evolve
and why are there still both types?
A different 2012 study shows that juveniles prefer one side or the
other, even before their mouth bend has become pronounced, so it is a deeply
penetrating characteristic, both heritable and perhaps partially acquired.
There’s evolution and genetics at work here.
The prevailing model is that at any one time, right or
left-mouthed individuals will predominate in a population. Let’s say that right
now, right-mouthed feeders are the majority. The prey fish will learn to pay
more attention to their left side, as this side is more vulnerable.
This makes it harder for right-mouthed individuals to feed,
but easier for left-mouthed fish, because the prey fish ignore their right side
relative to the left. In time, the right-mouthed individuals breed less well
and the numbers in the population will shift. The left-mouthed feeders will become
the majority. This is an example of negative
frequency-dependent selection, where as a trait becomes more common it
becomes less advantageous, and there is a balancing selection.
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The
flu virus comes with one of many hemagglutin proteins
and
one of many neuraminidase proteins. If one genetic
version
is too successful, most people will develop
immunity
to it and it becomes less fit in the population of
hosts.
Its success is its downfall and a more rare version
will
rise up. This is negative frequency-dependent selection.
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However, a 2012 study throws this into question. They found
equal distributions of right-and left mouthed individuals in five populations
they studied. Also, left-mouthed and right-mouthed individuals mated with each
other just as often as they mate with same-sided individuals (called disassortative mating). Perhaps the
mouth bend is not a true dimorphism (di
= two, and morph = shape).
Or, as a 2008 study suggests, negative frequency-dependent
selection works best when there is disassortative mating. This may be necessary
since a 2007 study showed that lefty:lefty matings give 2:1 lefty offspring,
right:lefty mates give equal righty and lefty offspring, but righty:righty
pairs ONLY give righty kids. Figure out the genetics of that. The authors
proposed two possibilities – mendelian genetics with lefty being dominant and
dominant homozygous being lethal, or
cross-incompatibility that is predominant in lefty:lefty homozygotes, (meaning
lefty homozygotes can’t mate successfully mate with the other types).
Today we have seen a swimming flatworm that feeds on some
fish, and some fish that feed on the scales of other fish. And both of them
achieve this only because they have adapted their bilateral symmetry to become
just a bit asymmetric.
Next week, there are other animals that break symmetry to
survive. Flatfish lay on their sides at the bottom of lakes and oceans, yet
they still use binocular vision. How can that be?
For
more information or classroom activities, see:
Great
Barrier Reef –
Coral
polyps –
Cichlids
–
http://www.cichlid-forum.com/articles/lake_tanganyika_diverse.php
http://www.news.leiden.edu/news/super-fast-evolution.html
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