Moulting: Part of a Growth Industry

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First, some brief biological background. Crabs and shrimps belong to a large group (phylum, for you technically inclined readers) known as Arthropods. One of the key features shared by all Arthropods is the fact that each individual of every species possesses an exoskeleton – what is commonly called a shell by most folks. Unlike we divers who have an endoskeleton upon which the rest of our body (and dive gear) hangs, a crab or a shrimp’s body support surrounds its form like a suit of armour.

This situation presents a major problem for each arthropod as it grows. In order to expand its size, each crab or shrimp must moult or shed its exoskeleton (shell). This is a very complicated process that can take several days. After backing out of its ‘suit of armour’ and swelling to a much larger dimension via fluid intake, the very vulnerable shrimp or crab must wait as a new exoskeleton hardens around its bloated body. If it survives hardening up, the crab or shrimp then ‘grows into its new exoskeleton’, and replaces the excess fluids with tissue. Upon completing this growth spurt, another moult invariably follows and the process is repeated.

A diver may routinely encounter evidence of arthropod moulting as bits and pieces or even entire exoskeletons (shells) are common place dive site litter. However, one must be lucky with precise timing to actually observe the process occurring. A couple of examples of this happenstance follow.

Pat Gunderson of Seattle, Washington and her buddy Johanna Raupe were just that fortunate on a February 13, 2011 dive at Alki Point, near Seattle. Images 1 and 2 show a stout coastal shrimp Heptacarpus brevirostris (AR57, page 290, in Marine Life of the Pacific Northwest) not only illustrate Pat’s handy work but a determined curiosity to watch the moulting process unfold.

While diving with a group aboard Peter Luckham’s Red Urchin (49th Parallel Dive Charters, on March 8, 2009, I was fortunate to find a tanner crab Chionocetes bairdi (AR103, page 302) as it was beginning to leave its exoskeleton. I collected it and brought it back to the vessel so everyone could watch the proceedings. For the rest of the day, we watched and Pete recorded with (images 3, 4 and 5) the specimen as it completely extricated itself from the too small protective coverage. After keeping the creature in a bucket of frequently changed sea water for several days, Peter and I returned it to its place of origin -- Escape Reef – completely hardened and ready to face life’s challenges.