Beach-walkers
are starting to see the seasonal influx of dead Dungeness Crabs that
occurs at this time of year after storm waves deposit their load of
seaweed and other litter on the shore. Included may be tens or
sometimes hundreds of apparently dead crabs (only one is visible in the
accompanying photograph). However, if you look more carefully you will
see that rather than being a portent of doom, they are a signal that
all is well and good with the offshore Dungeness-crab population. Use
your finger to flip up the carapace from the back, as shown in the
inset photo, and you will see that what you have is a cast-off moult.
Crabs and other crustaceans live in what are essentially rigid boxes,
or exoskeletons ("outside skeletons"), that they seasonally or
yearly outgrow. At this time, usually in spring when temperatures and
light levels are increasing, the crabs grow a new skin, partly from
energy material stored in their soft tissues and partly from residues
resorbed from the old skeleton. The old exoskeleton soon becomes
brittle and, by swelling up the body with water and after much
wriggling and pulling, the crab breaks free of it and escapes out the
back. All body parts are pulled free. The large claws, now soft, are
extracted through the small joints by much stretching. Even the
eyestalks are extracted, and one clue that what you are holding is a
moult, rather than a dead crab, is that the eye-sockets are empty. Now
swollen to the next size up from the original, the animal hardens its
new exoskeleton (in private), withdraws the excess water from the
tissues, and uses the next growth phase to fatten up from within. If
you would like to learn more about growth and moulting in crabs, go to
A SNAIL'S ODYSSEY: LEARN ABOUT CRABS & RELATIVES: MOULTING & GROWTH.
(Click to enlarge)
View our Dungeness Crab atlas page and learn more about the distribution of this species in BC.
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