|Parts of crab exoskeleton|
|This is a living crab. It is barely discernable because of protective coloration. Its color is very different from that of a molted crab shell. That is because the dark pigment is in the crabs' body, not its exoskeleton.
These little crabs are a favorite food of willets, which are diurnal (daytime) feeders. The crabs are scavengers, but are primarily herbivores (plant eaters). Thus most of the crabs food grows in the sun. In self-defense the crabs hide under rock in the daytime, and come out to feed at night.
I have watched Curlews (Numenirus Americanus) which have much longer and curved bill, hunt crabs. The birds hold their heads low, and run their bills around the base of the rock.
Shore Crab Hemigrapsis orogonensis
This is the most common crab in the Estuary. Molted exoskeleton.
We often refer to crab exoskeletons as shells. Exoskeletons double as skin and bones. They protect like skin, and serve as sites of muscle attachment like bones.
In order to grow, crabs have to molt, change to larger shells. When they are ready, crabs absorb most of the substances (chitin and calcium) from their exoskeleton. Then the part of the shell on the crabs back, the carapace, tilts forward like a truck hood. The crab backs out, drinks water, swells about 30% and secretes a new shell.
So what we have here is not a dead crab. Its an outgrown crabs shell being recycled, incidentally, like everything in the Estuary. Mr. crab is now the size of a nickel, out in the Bay, somewhere, showing off its new set of clothes!
Molted exoskeletons are feather-light and crumple easily because there is so little left in them.
Sometimes with careful handling, the exit hole can be demonstrated at the back edge of the carapace.
The molted skins tend to catch air bubbles. As a result they float high, are blown easily by the wind, and concentrate on top of the wrack.
|Living Lined Crab (herbivore)|
|Red Crab (carnivore)|
Revised Monday, February 18, 2008 02:25:46 PM