Wednesday, May 27, 2015

Hermit Houses And Fiddler Claws

Biology concepts – bilateral asymmetry, directional asymmetry, antiasymmetry, crabs, evolution, mate choice, sexual selection, sexual dimorphism

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.
Our first exception today concerns the hermit crab and the animal that determines whether hermit crabs live or die. Hermits don’t eat snails, but snails are the most important things in the life of a hermit crab. And it all relates to our ongoing to tale of bilateral asymmetry.

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.
When the crab molts a few times and becomes larger, he has to find a new home. Therein lies the rub, because snails keep one shell their entire life. Consequently, a shell only becomes available when a snail dies. And even then, they have to die in a way that doesn’t destroy the shell.

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.

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.
So what happens if the hermit is presented with a sinistral shell? What if it's the only shell available? Well, they may use it, but they don’t necessarily like it. 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
for mating.
The major claw has a few functions. One of the primary functions is as a sexual ornament for attracting females. But it’s an ornament with a reason for its size. The males prepare a burrow for the female to lay her eggs in, and the width of the burrow matches the width of the major claw.  A wider burrow leads to a lower temperature for egg development, so it takes longer for the eggs to mature.

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.

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.
So how does a male fiddler promote his burrow size? He waves his claw in front of the females. This draws attention to his claw and therefore his burrow size. Interestingly, they play faster when in an orchestra – well sort of. 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
crushing force.
The side the major claw is on has relevance for male fighting competitions. The fights between males are very different when they have claws on the opposite side as compared to when they are on the same side. The fight starts with the males facing each other and pushing against each other claw to claw. The fight might end right there if one is demonstrably stronger, but if not, they will interlock claws and try to move the other out of the way. Interlocking occurs one way if they are opposite and another if they are same handed. This could affect the outcome.

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.

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.
Sometimes the major claw is lost, but it can come back. In some species, the small claw will grow into a major claw, and the regenerated claw will become the small claw. This makes sense – regrowth is kind of slow because the growth is limited by the exoskeleton until there is a molt – or several. By making the small claw into the big one, a couple of molts are saved.

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.

Backwell, P., Matsumasa, M., Double, M., Roberts, A., Murai, M., Keogh, J., & Jennions, M. (2007). What are the consequences of being left-clawed in a predominantly right-clawed fiddler crab? Proceedings of the Royal Society B: Biological Sciences, 274 (1626), 2723-2729 DOI: 10.1098/rspb.2007.0666

Milner, R., Jennions, M., & Backwell, P. (2011). Keeping up appearances: male fiddler crabs wave faster in a crowd Biology Letters, 8 (2), 176-178 DOI: 10.1098/rsbl.2011.0926

Reaney, L., & Backwell, P. (2007). Temporal constraints and female preference for burrow width in the fiddler crab, Uca mjoebergi Behavioral Ecology and Sociobiology, 61 (10), 1515-1521 DOI: 10.1007/s00265-007-0383-5

Imafuku, M. (1994). Response of hermit crabs to sinistral shells Journal of Ethology, 12 (2), 107-114 DOI: 10.1007/BF02350055

For more information or classroom activities, see:

True and false crabs –


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