Crabs use their shells for garbage disposal
Fiddler crabs rid excess lead from their system by moulting.
Shedding an old shell could be about more than just getting too big for it. Crustaceans living in polluted waters may dispose of toxic metals by dumping their old casings, say researchers.
Crustaceans need to moult their exoskeletons throughout their lives so they can grow. As the time for a moult nears, a new exoskeleton starts to form beneath the old one. To soften the old exoskeleton and make the shedding process easier, calcium is reabsorbed in the days before the moult. Once the new exoskeleton is in place and the old one gone, calcium that was absorbed from the old exoskeleton is shifted to the new one to harden it.
Marine biologists have long understood this process of calcium control. But they have only just started to realize how moulting might affect the distribution of other elements, such as trace metals. These can be toxic in excess and cause numerous health problems, such as disrupting reproduction, slowing limb regeneration and changing body colour. Copper, zinc and lead have all been found in the exoskeleton, leading some researchers to speculate that crustaceans shed their shells to rid their bodies of too much metal.
Full metal jacket
To check, Lauren Bergey and Judith Weis at Rutgers University in Newark, New Jersey, obtained fiddler crabs (Minuca pugnax) from two very different populations. One was from Linden, New Jersey, in a site adjacent to a sewage treatment plant and a highway and surrounded by industrial facilities. The other lived in the New Jersey Jacques Cousteau National Estuarine Research Reserve.
The team found that fiddler crabs could indeed manage their metals. Both populations shifted copper and zinc from the exoskeleton back into soft tissues just before the moult, but in the polluted population the moult served to rid the crabs of excess lead. "What I found really impressive was how much lead shedding we were seeing in polluted waters," says Bergey. Some crabs released 76% of their body's lead content in a single moult. The results will be published in an upcoming issue of the journal of Marine Environmental Research1.
Shifting around this much metal is probably bad for the crab. The long-term energy costs are unclear, but they are almost certainly less than the penalties of metal poisoning.
"We need to consider looking at moulting in a new light," says Bergey. Some researchers studying crustaceans response to metals have watched the creatures over only a short period of time. But this work on moulting shows that exposure to metals should be studied over the long-term, to capture the effect of shell shedding on dealing with pollution.
- Bergey, L. & Weis, J. Mar. Environ. Res. doi:10.1016/j.marenvres.2007.04.009 (2007).