Copy crab: DNA confirms that crab forms have several origins. (This Week).

New genetic evidence suggests that crabs aren't all close relatives and their characteristic shape evolved independently on numerous occasions.

Most crabs share such a similar overall shape that they might seem to have come from the same evolutionary origin. Nevertheless, taxonomists have found some physical pointers hinting that the similarities arise from coincidence, or convergent evolution, and not shared ancestry.

Now, molecular biologists have confirmed these suspicions by examining the DNA from different crustacean families within the order Decapoda, which contains all the crablike species. Over evolutionary time, DNA may change more slowly than physical appearance and so can be a useful tool in sketching a family tree.

To tease out the evolutionary connections among crab species, Cliff Cunningham of Duke University in Durham, N.C., and his coworkers obtained DNA from 26 decapod crustaceans, including crab-, shrimp-, and lobsterlike animals. For each DNA sample, the researchers determined the nucleotide sequences of several genes and then used computers to assess the similarity of the sequences among the 26 species.

Comparing nucleotide sequences alone wasn't enough to decide which species were closely related, says Cunningham. Random changes in DNA, which accumulate over evolutionary time, sometimes mask relationships.

To get stronger evidence, the researchers compared the order of genes along a given stretch of DNA. Changes in gene order occur rarely, so animals harboring the same unusual patterns are probably closely related, says Cunningham.

The researchers found that in these tests, the decapod species that clearly look like crabs actually sit on five disparate branches of the crustacean evolutionary tree. According to the gene-order data, these species evolved on separate occasions from shrimp- or lobsterlike ancestors. The researchers chronicle their findings in the Feb. 22 Proceedings of the Royal Society of London B.

Scientists have debated crab history for over 100 years and have reached different conclusions depending on the physical features they compared, says Neil W. Blackstone at Northern Illinois University in DeKalb. "The molecular data are likely to be decisive in this debate," he says.

In many cases of convergent evolution, says Cunningham, the benefits of a structure or form are obvious, such as the hydrodynamic body shape of marine predators like dolphins and sharks. However, he adds, crabs live in habitats ranging from terrestrial environments to ocean trenches, thus the universal benefit of a crablike shape remains an enigma.

Joel W. Martin at the Natural History Museum of Los Angeles County offers an explanation. He notes that crustaceans hold their eggs against the abdomen, and crabs, which have more spherical bodies, can tuck eggs out of harm's way better than animals with long bodies can. The evolution of the crab shape is a "fairly obvious response to a vulnerable design" among crustaceans, Martin says.