Saturday, February 9, 2013

Alert! Dead crabs herald global warming? Pollution? Learn the truth.

Contributor: Tom Carefoot, the Biodiversity Research Centre, University of British Columbia

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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