Puget Sound King Crabs, exceptionally slowly growing crabs, with a massive adult body, are among the most colorful animals found in the Northeastern Pacific. They live most of their life in deep water, but come up into the shallows to molt, and incidentally display their vivid freshly molted color.
Over the years, I seem to have acquired a reputation as a crab-hating individual. Nothing could be further from the truth. I like to watch crabs, and I have enjoyed raising crabs. And, of course, I love to eat crabs… mmmmm… ah…. Mmmm
However, crabs have no place in a coral reef “community-type” aquarium for quite a number of reasons. Those reasons are obvious and have been beaten to death in various forum venues, articles and elsewhere. Nonetheless, the lack of crabs in our tanks robs us of the ability to watch some of the most interesting marine animals. One really fun thing about crabs is that they, like all arthropods, of course, grow by “shedding their exoskeleton” or molting. This process, in and of itself, is tremendously interesting. But, there is another thing that the molting process allows. It gives us a good way to see changes in the animals’ shapes as they mature.
Back in the last millennium, when I was actively doing diving research, I took a liking to some peculiar crabs, the lithodids. I didn’t do any research on these crabs, directly, but one of my friends did, and one of the things he wanted me to do was to collect any of the molted exoskeletons. If these are carefully collected, and then if they are carefully positioned and left to dry in an area where they won’t be disturbed, the resulting mount is a perfect 3-D record of the size of the animal before one molt. If enough of these molts are collected, it is possible to get a pretty good idea of the size of the animal at each molt. And then, if other data are available, such as the age at a given size, it is possible to get a very good record of changes in shape and size over the lifespan of some potentially representative animals.
Lithodid crabs are related relatively closely to hermit crabs, and while they don’t have an asymmetrical and soft abdomen that needs protection in a shell, their abdomens are asymmetrical. And additionally, as do hermit crabs, they only have three obvious pairs of walking legs, the fourth pair being reduced and held protectively near the back. Some lithodids get quite large, the various northern king crabs are such animals. These crabs doe not live as far south as the areas I did most of my research, but those areas had a far more colorful lithodid, the Puget Sound King Crab, Lopholithodes mandtii. And just so you understand, I would propose that there is no more spectacular coloration found in a crab than is found in THIS species of crab. As you see here, they start their lives as blaze orange babies and then add colors as they age. The colors, of course, are most spectacular after each molt.
My crab-researching friend, over the 10 to 15 years I sporadically worked with him accumulated a lot molts of L. mandtii. What he found was quite interesting, and very surprising. When one of these crabs settles out of the plankton to metamorphose into a tiny juvenile, it is only a few millimeters long. To the best of my knowledge, I can’t recall anybody ever finding one that was below about a centimeter in width, which is probably an age of 3 to 6 months old. They were reasonably common at a size of one and a half to three centimeters, which probably is an age of up to about three or four years. Then we got a few individuals that molted at about 10 to 15 cm in width, which was an estimated age from 10 to 15 years old.
After that size, we seldom ran across any that were not full-sized individuals, which were relatively HUGE. The ones that were about a meter across the leg span were probably about 25 to 30 years old. The really big ones, about 1.3 m across the legs, he estimated to be from 60 to 100 years old.
The Puget Sound King Crab is an exceptional animal in all sorts of regards, and I hope to visit it here again, soon.