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Marine Resources Publications

Why are all the Crabs Dying?

By C. Dale Snow
Illustrated by Sharon Torvik
Exerpt from Oregon Wildlife, July-August 1984
Posted on the web, July 22, 1998

As certain as death and taxes, sometime during the months of May, June, July, or August, the question will be raised by people using Oregon's beaches "Why are all of the crabs dying?" This is a perfectly legitimate question if you are not familiar with the biology of the Dungeness crab or for that matter any of our crabs. However, in my 29 years of work along the Oregon coast and a hundred or more reports of mass crab die-offs, only once has a significant mortality been found.

The Dungeness crab must shed its shell in order to grow larger and it is these shed shells that give people the impression that all the crabs are "dying". On several occasions our own field biologists have mistakenly reported crab mortalities the first time they saw these old shed shells.

Two major molt cycles occur along the coast. Starting in April and peaking in May and June, the female crabs will molt. It is at this time that mating between a hard-shelled male and a newly-molted female takes place. The males shed their shells later in July or August.

The shell shedding of the Dungeness crab is best described by saying that it appears to be physically impossible, but they still do it. When these animals shed their shell, they leave behind remnants of all the hard parts. This includes the external coverings of the legs, gills, eyestalks, body, and body lamella. In other word, they leave everything behind but come out completely intact with no ruptures in the newly formed shell. Growth occurs during and shortly after shedding. Once a crab has reached 4-1/2-inches in width, they will shed the old shell once or twice each year and increase in size from one to 1-1/3-inches in width.

It is fairly easy to determine the difference between a shed shell and a dead crab.

Whole Crab

Figure 1 shows what a new shed shell looks like just after shedding. Line "A" shows where the splitting-line opened to allow exit from the old shell.

Crab Shell with Back Up Figure 2 represents the same shell with the "lid" or carapace lifted up and shows the internal hard parts that have been left behind.

The crab, sometimes prior to shedding, reabsorbs much of the calcium from the old shell. As a result the old shell is quite delicate and soon breaks up into numerous parts.
       
Crab Mortality Figure 3 shows what the back or carapace would look like if the animal had died. Line A-B shows the "shelf" that would have been left behind if this animal had truly been a mortality. This "shelf" on the average would range from 1/2 to 1-1/4 inches wide; however, this width would depend on the size of the animal when alive. Line "C", shown here as a dashed line, is the splitting-line and is still intact. If this had been a shed shell, that portion from A to B would be missing. Crab: Shed Shell Figure 4 is the "lid" or carapace from a shed shell or exoskeleton. The "shelf" around the edge ranges from 1/4 - 3/8 inches at point A-B to 3/4 inches at line "C". Again, this width would depend on animal size; however, none of the shell, in Fig. 3, Line A-B would be present.

Why are all the crabs dying? The answer in most cases is they are not dying. They are only getting rid of their old shell and putting on a new spring/summer wardrobe in what has to be one of natures most interesting phenomenon.

 

For further information:

Oregon Department of Fish and Wildlife
Marine Resources Program
2040 SE Marine Science Dr.
Newport, OR 97365
(541) 867-4741

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