Cancer Crabs – article from 1991

As I peer back in the darkness under a rock crevice I can barely see the crab. A pair of eyeballs on stalks are staring at me. The mouthparts are fluttering, antennae gently swaying and giant claws folded just below the chin. I imagine that he feels protected, safe and secure in the crack. Ha! I sneer as my testosterone level increases; I pride myself on being able to dislodge unwilling crabs without hurting them or myself. And I have only lost one human fingernail and no crab legs in the process.

I carefully pull him out by the claws. The bumpy front of the carapace (shell) is slightly curved like a crescent moon and it’s about 5″ across, the massive claws are tipped in black. The broad abdominal flap indicates that it’s not a him after all, but a her. It’s the common rock crab, Cancer antennarius.

Cancer is the Greek word for crab, as in the fourth sign of the zodiac. We now associate the word — cancer — with that dreaded and pervasive illness. This is due to Galen, a famous physician in the 2nd Century A.D., who first named the disease for “the swollen veins surrounding the part affected, bearing a resemblance to a crab’s limbs. ”

“If it ain’t broke, don’t fix it.” Well, Cancer crabs ain’t broke and there has been no reason for them do much changing for the past 60 million years or so. They are in the Phylum Arthropoda which also includes insects, millipedes and spiders.

One characteristic of Arthropods is a rigid exoskeleton, which must be shed (molted) periodically for growth to occur. Just before molting some of the chemicals of the shell are re-absorbed by the crab. The shell then splits at a weakened line in the back of the carapace and the crab literally backs out of its old shell. It backs out of its claws, all eight legs, its antennae, the covering of its eyeballs, part of its digestive tract, and even its gills. As it emerges, the brand new shell is soft and the crab is extremely vulnerable. Its pincers can’t pinch, its back cannot protect and its mouthparts cannot feed. Not surprisingly crabs usually molt under the protective cover of darkness.

If the shell hardens immediately the crab would be the same size as before or even slightly smaller. So the crab pumps water into its body cavity and enlarges itself. It holds this extra water for about three days until the shell becomes completely hard; it then releases the water. Now the crab has room to grow. When the crab is young it may molt every few weeks. An older crab only does it once or twice a year.

The only time that female crabs can mate is when they are molting. She emits a chemical in her urine that alerts the adjacent males to her sexual receptivity. One lucky fellow reaches her first and “protects” her until the shell hardens. In exchange he mates with her. Female crabs store several thousand eggs under their abdominal flap. They hatch in a very short time. Most of them never reach adulthood but become food for fish.

Rock crabs are active mostly at night; they are aggressive predators and scavengers. They eat nearly anything they can catch. One favorite food is hermit crabs. The crab grabs them and slowly breaks open the shell with the pincers until it can reach in and pull the hermit crab out. Crab eating crab.

Octopuses (correct plural) are one of the prime predators on cancer crabs. The tentacled mollusks lie in wait and then reach out a tentacle and grab a crab with the suction cups. The octopus then uses its stylet (a very sharp bone in its mouth) and pierces the shell of the crab. I can testify to the sharpness. I was once stabbed by an octopus I was letting walk over my hand. I stopped that behavior.

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August 6, 2009