Predators need to catch their food, so they need depth
perception. For this you need two overlapping images. You don’t do the math
consciously, but your brain uses the differences in each image to tell you how
far away the target prey is. To get two images simultaneously, you need both
eyes to be on the front of your face.
Prey animals, usually herbivores, have to worry about
being chased down by a predator. Prey animals are usually quick, but they need
clues to get a good start before the predator gets too close. Their eyes are
designed to pick up motion; it doesn’t matter how far away it is. If something
moves in their line of sight, they assume it’s a predator and they bolt. By
having their eyes on the sides of their head, they have a maximal range of
vision which gives them the best chance to see that lioness coming.
Just to show you how important evolution thinks it is to
have at least two eyes, let’s discuss a group of animals that have found a way
to make two drastic changes work for them. Flatfish
have evolved to lie on their side, but they’re predators so they can’t afford
to have one eye constantly seeing nothing but the sandy ocean bottom.
Consequently, they have moved one eye to the other side of their head!
All the Pleuronectifromes
start out as fry that swim upright. Their top is at the top, and the bottom is
toward the bottom. They live nearer the top of the water than the bottom, have
pigment coloration on both sides of their body, and feed on phyto- and
zooplankton. But the surface isn’t the safest place to live; bigger fish are always
around to eat the small fry.
So, as they start to develop into adults, many changes take
place. They swim down to the floor of whatever body of water they call home.
One eye starts to move! It travels over the top of their head and onto the
other side, like the Mr. Potato Head of some deranged child.
As you can imagine, moving an eye isn’t an easy thing to do.
Their brain has to move, as do the cranial and facial bones. Their mouth has to
make room for the eye coming its way. All in all, it’s a tough piece of work.
Well, there are evolutionary intermediates – even some
living examples. A paper in 2008 introduced us to two extinct species of
flatfish. In each (Amphisitium and Heteronectes), one eye had migrated,
making the fish asymmetric, but it had not crossed the crown of the skull and
made it to the other side. These are definitely intermediate species, but we
can go one better.
The Psettodes
genus of flatfish (the turbots - yummy by the way) have one eye that is located
right at the crown of their skull. Perhaps technically you could say that is
has migrated to the other side, but just barely. And this brings us to another
question, which eye moves?
Some flatfish are right-eyed (dextral) that swim left side down.
Others are left-eyed (sinistral) and
swim with their right side down. Some species are strict and some are more
likely to have reversants (individuals that lay on their other side). A 2005
paper stated that only about 7 of the 700 species of flatfish show lateral polymorphism (lateral = side,
poly = more than one, and morph = shape), ie. some right-eyed and some
left-eyed individuals.
a 2009 paper describes
reversants for two right-eyed flounder species which are the first left-eyed
individuals ever seen in these two species (Microstomus
achne and Cleisthenes pinetorum).
It goes the other way too – a 2013 study describes the first right-eyed
individual ever seen in a megrim (sometimes called a whiff, Lepidorhombus whiffiagonis). But in the
left-eyed California halibut (Paralicythys
californicus), up to 40% of the individuals are right-eyed. Perhaps right-eyed
species are stricter than left-eyed species.
The exception to that rule is the starry flounder (Platichthys
stellatus). It’s a member of a right-eyed family of flounders, but in some
cases half of the individuals are left-eyed. In this case, there seems to be
more to the story - the lateral polymorphism occurs only in populations of
specific geographic areas.
A paper from 2007 looked into the mystery that 7 species
that show lateral polymorphism, but only two (starry flounder and P. fleusus) show a geographical
distribution in their polymorphs. Off the coasts of Japan and Russian, 100% of
the starry flounders are left-eyed, but near Alaska they are only 75% sinistral
and from Washington state to central California the populations are about 50/50.
The researchers looked into several questions. Was there
more to being left-eyed or right eyed than just which side the eye went to?
Does the side make a difference that could account for the geographic
distribution? They found out – yes, and yes.
They saw that the right and left-eyed individuals have more
asymmetries between them than just the side of the body that the eye migrates
to. They have differences in mouth size and angle, as well as tail size of all
things. Right-eyed individuals had significantly longer and wider tails than
did left-eyed individuals!
The research also shows that in areas where the two groups
compete the most, the differences between the dextral and sinistral individuals are the greatest. This suggested to them that
the differences allow them to compete for different prey – to fill different
ecological niches. The hypothesis is that the polymorphic asymmetries give them
different advantages which they then exploit and this is why there is a stable
geographic distribution in populations.
A 2009 paper started to investigate this.
During development of the embryo, certain internal asymmetries develop (will
talk more about this in a few weeks). In fish and other vertebrates, this is controlled
by expression of certain genes in the habenula
of the brain. One of the gene products (proteins) in the habenula is called pitx2.
There are actually two habenulae, one in each hemisphere of the brain,
as part of the thalamus. The researchers looked at the brains of right-eyed and
left-eyed flounder species and detected some things that were the same and some
things that were different.
After the pitx2 did its job in the embryo, it was turned
off. But right before metamorphosis – when the eye migrates and the fish lies
down on one side, pitx2 was turned back on – only in the left habenula, regardless
of which way the fish’s eye was going to migrate and which way it was going to
lie down.
Not only that, but the right habenula started to grow bigger
than the left habenula in both dextral and sinistral species. The only thing
that was different was the rotation of the brain, with the left habenula moving
forward in the right-eyed species and the opposite occurring in the left-eyed
species. The same changes occurred in fish no matter which eye migrated!
By the way – at the beginning of today’s
post I asked if there were any real animals with one eye. I made it sound like
there aren’t but in fact there are exceptions. In some small crustaceans
(arthropods) called copepods, the majority of species have a single eye right
in the middle of their head! What’s more, a 1994 study showed that the holdfast
of a particular copepod parasite is asymmetric (like we saw last week) and this
particular copepod is a parasite of only flatfishes! How symmetric this tale of asymmetry turned out to be!
Next week - can a single tooth render an animal asymmetric? Well, that depends on the tooth, doesn't it.
Next week - can a single tooth render an animal asymmetric? Well, that depends on the tooth, doesn't it.
For
more information or classroom activities, see:
Flatfish
–
Copepods
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Habenula
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