It takes a lot of effort to grow a new coat, and no one can afford to be profligate! Yet every time a crab feels the squeeze in its shell, the metabolic cost of upsizing is the last thing on its mind. So how can a moulting crab make good economy when upgrading to a bigger shell?
Studies on male crabs indicate that their stores of essential trace metals vary throughout their moult cycles. In addition, uptake of water around the time of moulting causes an increase in haemolymph (blood) volume and an associated gain in mass. Recent experiments by Ulrik Nørum and his colleagues at the University of Southern Denmark take these findings a little further. Nørum's team wondered if it was possible to metabolically cost the crab's moult cycle and was intrigued to know whether males and females tackled moving up a size in the same way.
The Danish researchers grouped nearly 200 male and female shore crabs(Carcinus maenas) into six major moult stages. They then measured blood volume at each moult stage by first injecting crabs with a known amount of the radiolabelled [14C]inulin. Two hours after injection, they drew blood samples and measured [14C]inulin levels by placing the samples in a radiation detector. Their results reveal an impressive threefold increase in blood volume immediately before moulting. And there was a difference between the sexes; female crabs retain less water than males.
In order to discover where, and when, trace metals were lurking, the team also measured copper and zinc concentrations in midgut gland tissue, gill tissue, muscle tissue and exoskeleton at each moult stage. By pooling these results, the scientists were able to estimate whole-body copper and zinc content.
Nørum's team found that shore crabs were able to maintain stable body levels of precious trace metals during most of their moult cycle. Amazingly, this stability persisted despite major changes in the concentrations of copper and zinc within different tissues. Newly formed exoskeletons, for instance, contain 74% less copper than premoult exoskeletons.
The Danish team reckons that, against this backdrop of cyclic ion fluctuations, premoult dilution of the blood counteracts changes in trace metal content. Put simply, by diluting their blood and altering where and when they store trace metals, crabs maintain whole-body stability in metal content throughout their upsizing activities.
The research also suggests that male and female crabs employ different strategies for keeping hold of their valuable trace metals. For example,female crabs store more copper and zinc in their blood and midgut, and more zinc in their gills, than do males. Although cyclical changes in trace metal levels are likely to reflect muscle breakdown, starvation, exoskeleton resorption and moulting in both sexes, the reasons behind these different storage strategies pose another interesting puzzle.
The Danish research clearly reveals that crabs are armed with an array of sophisticated strategies to minimise the metabolic costs of upsizing. The study also underlines the need to measure both tissue concentrations and whole-body contents if we are to determine exactly how crabs keep their metal balance.