Biology concepts – bilateral asymmetry, directional
asymmetry, antiasymmetry, crabs, evolution, mate choice, sexual selection,
sexual dimorphism
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Hermit crabs aren’t true
crabs since they don’t
have ten legs (two go to form
claws in true crabs)
and their antennae and eyes
are different from
true crabs. Many don’t even
live in the water, but
that is true for true crabs
as well. King crabs,
horseshoe crabs, coconut
crabs, they all have
crab in their name, but they
aren’t true crabs.
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Most snails live in shells of their own making. As they grow
larger, they add to the aperture (opening) of the shell to make it bigger –
they remodel instead of relocating. As they add on to their shell, most take on
a curve, so that over time, they are spiraled.
The vast majority of snail shells spiral asymmetrically to
the right. They are dextral,
although the occasional sinistral
(left-handed) shell is found, and some species are equally left and right
shelled. Because the asymmetry of the shell is almost always to the right, this
is called a directional asymmetry (see last week’s post).
The right handedness of the spiral isn’t a distinction without a
difference – it matters. Since the snails reproduce with organs that are
located in their heads, they have to be able to line up correctly for mating;
this means they need right handed spirals that fit together well when they face
each other.
How does this affect the hermit crab and its survival?
Because hermit crabs use gastropod shells as protection for their soft
abdomens, that's how. Hermit crabs have hard exoskeletons on the front of their body, but
their abdomen is covered in a very soft shell that is easy prey for a predator.
Their abdomen is reduced
as well. The legs are absent or very short, and this leaves a tapered back end
that fits well into an abandoned gastropod shell. The furthest south point on
the hermit crab abdomen has a specialized function for gripping the inside of
the shell. So, he/she carries the shell around for protection – until he/she outgrows
it.
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Shells are at such a premium
that even before
one is abandoned, there are
others lining up to
use it. Notice how they line
up from large to
small. When largest finds a
new shell, each of the
others will be able to move
up to new digs as well.
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Shell number is the main stressor in a hermit crab’s life. The
availability of appropriate shells is the major limiting factor in keeping
hermit crabs safe. If there are too few, hermits have to find alternate housing
(maybe the neck of a coke bottle or some such thing). Snail shell numbers can and do limit the size of hermit crab
populations.
We stated earlier that the vast majority of snail shells are
dextrally asymmetric, so it isn’t surprising that hermit crabs greatly prefer
dextral shells. This is a case of an animal born symmetric but grows asymmetric. The dextral spiral
of the shell molds the abdomen as the crab grows, so that it becomes asymmetric.
When a hermit crab finds an appropriate new shell, it pulls
its abdomen out and quickly enters the new shell. This is the time when the
crab is most vulnerable to predators. When it comes out of the shell, you can
see that the abdomen is soft and reduced, but you also see that it is straight.
The asymmetry is an adaptation see when they coil, but under muscular control it will be straight.
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You can see the asymmetric
twist of the hermit
crab’s abdomen to the right
side, but it isn’t
born that way. This happens
after growing up
with a dextral shell for a
home. They are born
with straight abdomens and
can easily
straighten them out if they
so choose, since
the shell is so soft on the
abdomen.
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But with left spiraling shells, they only had a 50/50 chance
of turning them correctly, based on which direction they were facing. The crabs
don’t instinctively know how to make use of a sinsitral shell. Good thing their
abdominal asymmetry is labile; they can use a sinistral shell if that’s all
they have to choose from, or even an old pop can if need be.
The hermit crab’s story is one of directional asymmetry and
acquired directional asymmetry, but another crab gives us a story of
directional symmetry and antisymmetry. This crab is the arm wrestling champion
of the crab world.
There are 92 species of fiddler crab (genus Uca). The males of this genus have one
oversized cheliped, called the major
claw. That makes this a sexually dimorphic trait (see this post). They wave the major claw
(more on this below) and long ago it looked to someone as if they were playing a fiddle –
so they're called fiddler crabs. One guy and his observation; he must have been
some kind of big wig, because the name stuck.
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This shows the major claw of
one of the 92
species of fiddler crab.
These are in fact true
crabs, as opposed to hermit
crabs above. Not
that the burrow is out of the
water, but will be
inundated by a high tide.
That’s important
for mating.
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A 2007 study showed that the tides are very important for
reproduction by washing the larvae into the water. So, if the females are
mating five days before the highest tides, they choose the burrows of the
largest males to slow down the maturation of the eggs. If they mate late, they
choose the smaller burrows of smaller-clawed males, so the temperature will be
higher and the eggs will hatch faster and be ready for the coming high tide.
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The male waves his major claw
to attract
females, so it is a dimorphic
sexual ornament.
But it also functional, as
the length of the claw
determines the width of the
burrow and is
used for fighting off other
males. Some males
don’t wave, they just snatch
females into their
burrow when they come close.
How rude.
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In 90 of the 92 fiddler crab species, the major claw is
antisymmetric. This means that roughly half of the individuals will have the
major claw on their left side and the other half will be right-clawed. This
suggests that the exaggerated growth of the major claw is not controlled
genetically. The pressure on claw direction is either negative-frequency based
(see this post), or there is no pressure to choose a side.
However, there are exceptions. In two species, the major
claw is directionally asymmetric; 99% of the males will have their major claw
on their right side. This does suggest a genetic component to the side on which
the major claw occurs. It’s interesting that these two very different
mechanisms occur in species that are so closely related. Did antisymmetry
evolve from directional asymmetry or did only a couple of antisymmetric species
become directionally asymmetric?
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Here you see the exceptional
fight; one right-
clawed and one-left clawed
individual. If they
were both right-clawed, they
could clench claws
sort of like a hand shake.
This is how they were
designed to be used and work
best for
crushing force.
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A left-clawed male in a right-clawed species is usually at a
disadvantage because of the mechanics described above. A 2007 study showed that
left-clawed males lost their burrows to right-clawed males more often and kept
their burrows for shorter times than right-clawed males. The right-clawed males
are also more likely to pick a fight and try to take another crab’s burrow.
Therefore, these researchers hypothesize that left clawed
individuals in a right-clawed species are not advantaged in a
negative-frequency dependent manner – in fact, they are significantly
disadvantaged, and this may keep the percentage of left-clawed males low.
The idea of whether the antisymmetric claws are not
heritable traits and the directional ones are is an interesting topic. And it gets more interesting when you start
talking about regeneration – fiddler crabs can regrow the claws or limbs they
lose.
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Every once in a while you may
see a fiddler crab
with two major claws. This
isn’t such a good
thing. It costs a lot of
energy to build and maintain
one, let alone two, so they
will be at a survival
disadvantage. More
importantly, it’s hard to pick
things up and eat them using
the major claw. The
small claw is important for
feeding.
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But in other species, handedness is set
and can’t be broken – if the major claw is lost, the regenerating claw will be
the one of the same size no matter what. Dr. Brook Swanson of Gonzaga
University has observed these regenerations in a right-clawed species. He
stated that the major claw will regenerate almost completely in one molting,
but to accomplish this the overall growth and size of the crab will be reduced.
They have to shrink to get the claw to grow fast. Remember, this claw is basic
to their competition and mating.
This leads to an amazing feature of fiddlers who regenerate a
major claw. The new one is often not as strong as the old one, but they aren’t
going to tell their opponent that. The crab will behave as though it's as
strong as ever; he will feign wanting to fight even though he is aware that he is not
as strong. They fake it – like that bully in middle school that ran away if someone stood up to him. This is called a dishonest signal, as opposed to the fluctuating asymmetry and honest signal that we talked about last week.
Next week, let’s switch it up and talk about asymmetries
that occur on the inside of the
animal body.
For more information or
classroom activities, see:
True and false crabs –
