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Crab Crackers: Scientists Take a Harder Look at Stone Crab Shells

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Serendipity struck Cynthia Melnick Sloop in a seafood restaurant. A zoology student at the University of Florida in Gainesville at the time, she was feasting on a plate of stone crabs when she noticed that the black-tipped claws were much harder to crack than the light-colored parts of the shell. That observation set Sloop to wondering. "I tried not to be a geek talking about the engineering of these stone crab claws [at dinner]," she says, "but I was thinking about it."

Most people's curiosity would end there, but Sloop used her background in materials and mechanical engineering to learn more. She took some Florida stone crab claws into the university materials science lab to test their strength.

The black parts were indeed twice as robust as the light-colored parts. These results piqued her interest, because conventional wisdom says that animals developed coloration for camouflage or signaling. One study, however, showed that dark-colored feathers from wood warblers resist abrasion better than light-colored ones. What connection, if any, could there be between mechanical strength and color? No clear answer has emerged, but Sloop's study gives some biologists a new way to look at animals they've studied for years.

The Florida stone crab inhabits the coastal waters of the Gulf of Mexico and Atlantic Ocean from the Carolinas to southern Florida. Compared to most tropical crab species, stone crabs are large, with bodies measuring about 6 inches across and huge claws that form 60 percent of their body weight. Like Popeye's forearms, the pincer of one claw can dwarf a crab's body, says Theresa M. Bert, a biologist at the Florida Marine Research Institute in St. Petersburg, who has studied stone crabs for 20 years.

"They really are evolutionary anomalies. The claws are so huge and cumbersome, you wonder how they even walk around with those things."

The claws can apply a crushing pressure of 14,000 pounds per square inch. This enormous power enables the stone crab to munch on all sorts of hard-shelled mollusks, such as clams, scallops, and conch. The claws also equip them for fighting with each other, pinching and tussling in competition for food and mates. If a claw is lost, it regrows in about a year.

Stone crabs are "the lions of the invertebrate world," Bert says. "You don't even find them in the stomachs of the largest predatory fish, like groupers or sharks. Nothing eats them because their shells are thick, and you'd probably get pinched in the stomach [from inside]."

Sloop, a recreational scuba diver for nearly a decade, applied her familiarity with the sea to the process of gathering specimens for her research. She set traps for the crabs, emptying them once a week into a holding tank at the university. After removing one claw, she'd cut pieces from the dark and light parts. Then she measured their resistance to impact, or hardness, and to cracking, or fracture toughness.

The dark part of the crab claws performed as well as a tough synthetic polymer, while the light parts behaved more like a brittle ceramic. Moreover, the properties changed abruptly at the interface between light and dark areas. Sloop and her colleagues reported these results in the November 1996 Journal of Materials Research.

The microstructure of the shell provides some clues to explain these differences. Crustacean shells are composed of proteins, calcium carbonate, and chitin-a long polymer of sugar molecules (SN: 7/31/93, p. 72). Scanning electron micrographs showed that the shell is less porous in the dark areas, Sloop says.

Sloop's hypothesis is that a process called tanning-more like the chemical tanning of leather than the tanning of skin exposed to the sun-accounts for this higher density. In leather tanning, substances called phenols react with the long protein fibers and link the fibers, imparting strength. A darkening accompanies the tanning process, but "no one has really pinned it down biochemically," Sloop says. …