Hey, it’s tough being a blue crab.
Illustration by Robert Meganck
Respect the crab. Sure, you may never have given any more thought to the blue crab than that it goes well with a beer and drawn butter, but this swimming crustacean is central to both the ecology and economy of the Chesapeake Bay.
With roughly one third of the U.S. blue crab catch coming from the bay, it’s a species so iconic to the region that it has earned a small dictionary’s worth of specialized terminology both colloquial and technical—Jimmy, peeler crab, sponge crab, channeler, paper shell, snot crab, buster, sook.
And it’s a species so important as both predator and prey that it’s a keystone in the bay’s extensive food web.
“They are amazing creatures,” says Rochelle Seitz, a research professor with the Virginia Institute of Marine Science, one of the nation’s largest marine research centers.
As predators, they are tenacious, clever, and not exactly picky eaters: They’ll feed on small and thin-shelled clams, worms, plant and animal matter, dead fish, and, as anyone who’s gone jigging for crabs well knows, a chicken leg dangling from a string.
They can pry open an oyster shell with a poke and a twist of their claw. And, says Seitz, they’re smart—when she keeps them in the lab, what they can get in and out of is a constant surprise. “You think you have them in a holding tank, and the next thing you know, you find them on the floor crawling around.”
Yet on the other hand, it’s no day at the beach being a blue crab.
An adult female (you can identify them by their red-tipped claws) will only mate once in her life, and from that union she can produce multiple broods, each with up to several million eggs. But the larvae she will hatch can’t simply be set free anywhere in the bay.
Instead, the females must manage to make their way all the way down to the head of the bay, because the larvae, known as “zoaea” (singular “zoea,”), have to be released into the full-strength salinity of the Atlantic ocean in order to properly develop, says Seitz.
And thus begins life’s perilous journey for a new generation. Assuming the larval crabs aren’t first consumed at this stage by filter feeders like oysters or menhaden, they are carried out into the open ocean—from whence most of them will never return. If they do manage to not be eaten out there (and the odds are not good), they still have to depend on favorable wind, weather and currents to shunt them back into the bay a few weeks later.
The now-juvenile crabs then take advantage of “selective tidal stream transport,” explains Seitz, to move them further north, floating up in the water column “to hitch a ride with an incoming tide” and then dropping to the bottom again to let the outgoing tide pass over. En route, however, they are picked off not only by fish like red drum and striped bass, but also—no love among family here—by other crabs.
“It is a gauntlet of predation,” says Seitz, until the juveniles can find their way into the protective cover of seagrass beds, oyster beds, or other “structured habitats” like trees fallen into shallow waters.
But they’re not home-free yet. In a “high recruitment” year where a large number of larvae return as juveniles, you still might not end up with a large number of surviving adults, says Seitz. And why is that? More brothers and sisters around to eat you.
Blue crabs “play the numbers game and put out millions in the hopes that a few of them will make it to adulthood,” says Seitz.
At which point, of course, they could still end up on your dinner plate.
To grow, a blue crab has to keep shedding the shell it lives inside as it becomes too cramped. It molts by essentially backing out of its shell, and then, within a few hours, begins to form a new hardened exoskeleton from chemicals extracted from the seawater. It’s a process that’s really quite remarkable: imagine if you had to jettison and regrow your own skeleton intermittently throughout your life. And that’s not all a crab manages through molting. If it should lose a leg or claw, it can grow an entirely new one at the next molt.
Yet during the few hours of its wholly soft-shelled state, a blue crab is (again) quite vulnerable to predation (including, of course, the kind that involves being deep-fried and served with coleslaw)—males will undergo many such perilous molts throughout their lives. Female juveniles, however, undergo one “terminal molt to maturity,” and it is at this point, during their last pass through the soft-shell stage, that females mate. The male protectively carries the female about during this short time, and because everything blue crab has its own vocab, there’s a word for that twosome too: they’re called a “doubler.”
What then? The whole cycle begins again.
This article originally appeared in our June 2018 issue.
