The Evolution of a Holiday Appetizer: The Blue Crab

The University of Maryland Center for Environmental Science recently published an article discussing some progress in blue crab research and conservation, and mentioned a related report:

The Chesapeake Bay blue crab population has stabilized, but at historically low levels according to a recent report by the Chesapeake Bay Commission's Bi-State Blue Crab Technical Advisory Committee.

Though the news isn't quite heartening, it's better than nothing.

Blue crab populations have been declining tremendously over the past few decades, not only threatening a population of the animals, but also endangering fishermen and families. So before everyone prepares their crab balls for Xmas (probably shipped in from Indonesia), I'd like to spend a couple of posts discussing the blue crab--its history and ecology.

Blue crabs (Calinectes sapidus) are part of a large order of crustaceans* called the decapods--literally ten foot, which refers to the number of thoracic legs (also called pereopods; legs on the thorax of the animal). The first three thoracic legs have been modified for feeding, while the back two are used for movement. In the case of the blue crab, a brachyuran or swimming crab, the last pair of pereopods is equipped with paddles to propel the animal through the water.

It is interesting to see adaptational trade offs in organisms so similar. The decapods have all evolved different means of protecting themselves. Lobsters have size and a thick exoskeleton on their side, while hermit crabs crawl into a snail shell to protect their relatively weak carapace. As true crabs, Calinectes has evolved a carapace well suited for swimming; escape is as good a defense as any.

We see similar evolutionary pathways in the cephalopods (squid, octopus, nautili) although as an offensive solution rather than a defensive one. Ancient cephalopods were split into two groups, the ammonites and the nautili, both equipped with thick, tough shells. As bony fish became increasingly adept at hunting, cephalopods had to become faster and more efficient. Shells were reduced and exchanged for the swimming power of muscular siphons. Today only one species of cephalopod retains its shell in any significance: the ancient nautilus. The ammonites have been extinct for some time now.

Evolution has favored the quick in these cases. But in order for early decapods to become successfully streamlined, the abdomen, including the telson or tail, had to become less of a hindrance. For the past 100 million years, it has slowly curled inward, so to speak, contained entirely under the carapace of the crab, which itself had to expand, giving the crab its well-known shape.

When a molecular analysis was made in 2002 of 26 species of decapod, the researchers made a case for convergent evolution. At least five different branches were traced back to a lobster like ancestor. The researchers explain:

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.

Fossils are hard to come by, especially in the world of invertebrates, and crustaceans are no exception. Recently a man in North Carolina stumbled across a fossilized Pleistocene crab, rare and unknown until recently. He told the local paper of his visit to a fossil seminar at the local aquarium:

"I went over to the aquarium and stopped the show," he said. "Right away [he snaps his fingers] she said it could be 3 millions years old. That's when man started walking upright if you believe in that evolution stuff."

Whether or not you believe in that "evolution stuff", blue crab populations in the Chesapeake are at historical lows. Next time I'll explain the life cycle, ecology and major environmental problems involved in the decline of the blue crab.

*As an aside, the crustaceans are an incredibly diverse group of invertebrates, especially when it comes to modes of reproduction. They reproduce sexually and asexually, can be spermatophoristic, gonochoristic or hermaphroditic and are fertilized both internally and externally.