How Red Crabs Get In Shape Without Breaking A Sweat

Once a year, Christmas Island experiences a red tide completely unlike any other in the world. The roads are blockaded as a swarm of Christmas Island Red Crabs (Gecarcoidea natalis), one hundred million strong, makes its way from the forests to the sea. This massive migration is fueled by the strongest urge an animal has: to reproduce. They must travel from their forest homes to the sea to spawn. The drive is so strong that the crabs will crawl their way onto, into and over whatever they must to reach the water's edge. So many migrate that the horde of legs, claws and carapaces can be seen from the air, and there can be so many in one area that the crabs are literally shoveled off of the streets.

This strange phenomena has fascinated scientists ever since the crabs were discovered. It's strange enough to find a sea creature living so contentedly inland. The crabs have adapted well to forest life, scavenging fallen fruit, leaves, seedlings and carrion. They are a key species in the forests, recycling nutrients and aerating the soil with their burrows. Indeed, what really puzzled scientists wasn't how they live in the forest, it's how they make it to the sea. Crabs that spend their lives within a short scuttle of their burrows suddenly take it upon themselves to travel 3 to 4 miles to the ocean to spawn. When you think about it, their annual migration is akin to a couch potato suddenly deciding to compete in the Iron Man to get a girlfriend. What goes on in their bodies to drive such a dramatic shift? And how is it they can make it at all?

The answer, it turns out, comes down to their genes. Scientists have been researching the impressive migration of the red crabs for years. They know that the moon and rainfall helped trigger when the crabs decided to migrate. But what they couldn't figure out is how a relatively lazy forest dweller is so quickly able to change form and make the long journey to the shore. In many species, such a shift would be almost impossible - or at least take a lot of training.

Steve Morris, from the University of Bristol, knew there must be more to the story. To undergo a feat like that, the crabs must somehow make major changes to their muscle composition over a very short time. During the dry season, while the crabs are in the forest, they move only 10 minutes a day. But during their march to the sea they scurry for 12 hours or more.

In humans, that kind of change in exercise level is just about impossible to do quickly. The problem lies in our muscles. Muscles must have energy to move. Almost all living organisms use aerobic metabolism (a series of reactions which require oxygen) to create ATP, the cell's unit of energy. But when a tissue runs out of oxygen, it's not completely screwed. It can switch to a much more inefficient way of producing ATP called anerobic metabolism - but it can't last long this way. It must rest again soon or the buildup of toxic metabolic byproducts will damage it.

The oxidative capacity (or how long a muscle can go without switching to anerobic metabolism) and the fatigue resistance of muscle depends on a number of enzymes and the composition of its fiber proteins. There are multiple kinds of muscle fibers, and each has different levels of endurance. For us, those components are ruled by two major things: genes and training. The former we have no control over - no matter how hard some people try, their muscles are just not well built for marathons. But we can work with what we've got by training our muscles, which causes shifts in certain proteins. If I worked out really hard, I could improve the aerobic capacity and fatigue resistance of my muscles a lot, but it would take time.

The red crabs, however, don't train. They go from muscles that tire easily and switch to anaerobic metabolism in less than 30 minutes to those that stay aerobic for hours on end. Morris and his team figured that the answer therefore had to lie in the crab's genes.

So they took muscle samples from crabs during the wet season when they migrate and during the dry season when they don't and looked at mRNA levels, the precursors to proteins, to see what genes were being expressed differently between the active and lazy crabs. What they found that the gene expression profiles of non-migrating crabs were very different from their migrating counterparts. The traveling swarm had upregulated the endurance-version of genes like actin, troponin and tropomysin. The authors explain that the changes they saw in gene expression "would result in an increased representation of aerobic fibres which is also consistent with the concomitant improvement in aerobic exercise capacity of the red crabs at the onset of the annual migration."

Basically, the crabs have figured out a way to become athletes without all the training it takes us. Exactly when the genetic shift occurs, though, has yet to be determined. The team hopes to take what they've learned a step further, and follow the transcriptional changes in the crabs' muscles all year long, to see exactly when the crabs change their musculature in preparation for the great migration, and what might trigger the change in gene expression.

Sadly, it's unlikely that we'll ever be able to match the crab's feat of getting in shape without working for it. Crustacean muscles are very different from our own, and while we can learn from this kind of investigation, it's not going to lead to miracle muscle building in people any time soon.

Postel, U., Thompson, F., Barker, G., Viney, M., & Morris, S. (2010). Migration-related changes in gene expression in leg muscle of the Christmas Island red crab Gecarcoidea natalis: seasonal preparation for long-distance walking Journal of Experimental Biology, 213 (10), 1740-1750 DOI: 10.1242/jeb.033829