Biology concepts – bilateral asymmetry, directional asymmetry, antiasymmetry, crabs, evolution, mate choice, sexual selection, sexual dimorphism
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.
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.
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.
A 1994 experiment showed that when presented with dextral shells, the hermits turned them correctly to pour out any sand inside.
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.
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
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.
A study in 2012 showed that in crowds, the males wave their claw faster. Waving for a long period is energetically costly, so they wave fast when competition is highest and slow down when fewer males are present.
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?
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
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.
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 –