Everyone knows that all crabs moult. Its no secret. When you wear you skeleton on the outside, it makes it difficult to grow. So every now and then crabs and other arthropods, nematodes, tardigrades and some other "minor" phyla shed their exoskeleton or cuticle resorbing the Calcium held in it. They secrete a new one thats a little bigger, reusing the calcium from the old exoskeleton, letting them grow a little more from the inside out.
The exoskeleton of crabs is mad of a long-chain polymer called chitin (to the right, from here). It is a very sturdy material and an excellent defense against many would-be predators. But it might not be only for growth that crabs moult. There have been studies demonstrating the crabs have excess heavy metals in their exoskeleton and it has been hypothesized that moulting may be a mechanism to rid the crab of toxic metals. Metals could be bound and woven into the chitin matrix, then removed with the crab's moult.
An intriguing new study by Bergey & Weis, currently In Press at Marine Environmental Research, studied the distribution of Copper, Lead and Zinc during postmoult and intermoult times. These metals are common the Atlantic estuaries where Uca pugnax is found. Obviously all creatures need some amount of trace metals to various functions, such as enzyme cofactors, so why are some heavy metals harmful to crabs? In addition to sluggishness, lack of responsiveness, premature egg dropping, larval swimming behavior altered (Mercury), larval development and metabolic rate altered (Cadmium)
"Metals can cause disruption of molting, limb regeneration, alteration of blood glucose levels, color changes, and impairment of reproduction in crustaceans."-Bergey & Weis 2007Yikes, lets get rid of this stuff! Now, what could a crab do with excess metals to get it outside of its body? One way is to excrete it out with the rest of the waste products, such as ammonia, across epithelia or gill (see Weihrauch et al. 2004). But then this would be processed through several organ systems and tissues which could potentially be harmed by the metals. The other way is to hide it away somewhere until you can get rid of a large portion all at once. Sort of like bulk trash pickup day when the undergrads leave for the summer.
Bergey & Weis studied two populations of fiddler crabs from New Jersey. One population was inside a large, protected salt marsh (the clean population). The other population was collected outside of a sewage outfall (the polluted population):
"The site is surrounded by industrial sites, a sewage treatment plant, and a major highway. Oil spills... and possible leachate from landfills have also been a source of organic contaminants"-Bergey & Weis 2007I found the result to be very interesting. Namely, that the fiddler crabs from the polluted site eliminated 12% Copper, 76% Lead, and 22% Zinc. Notice the interesting skew? Thats a lot lead shed off with the moult! Conversely, fiddler crabs from the clean site eliminated a significantly lower amount compared to ones from the polluted site (3% Cu, 56% Pb, 8% Zn). So not only are fiddler crabs in lead-polluted sites mobilizing excess Lead to the exoskeleton, they are preferentially doing so potentially as a depurification mechanism. Quite brilliant actually, you already have the genetic machinery to moult. It seems to me an easy evolutionary story to visualize. Crabs that were able to make physiological adaptations to concentrate damaging heavy metals in their cuticles or exoskeletons had more viable offspring than those who sequestered the metals and let it damage their tissues and reproductive system. Of course zinc and copper are enzymatically important metals and are in much higher demand, which is why you don't see much elimination of these two elements. Lead has no known vital function in fiddler crabs
One thing I kept thinking about when reading this was whether thee crabs ate their moult. When a crustacean moults, it is unarmored and vulnerable. In fact, many crabs don't even bother feeding or coming out from a good hiding spot for a month while they wait for their new exoskeleton to harden (helped out by the recycled Calcium by the way). The benefit is lost if crabs eat their moult. This is common in insects as the moult may provide a first meal post-moult before retreating to safety. Some hermit crabs and the coconut crab are known to eat their moults. It would be interesting to see a comparative study of several different types of crab from polluted vs. clean sites that are known to either eat or not eat their moults. Also, perhaps doing a similar study on crabs that are known to ingest moults and have a treatment allowing them eat and treatment removing moults (not allowing them to eat) to see if metals are being reingested from the moult.
See also News@Nature.com story.