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How large can lobsters grow? Big! But no one knows how big.

Lobsters can grow to be four feet long, 40 pounds, and maybe 100 years old – maybe even more.

By Staff writer / June 16, 2011

In this June 1 photo Skipper Landon Shand, left, and crewmember Lorne Pace show off a 23 lb. male lobster they caught on their final run of the closing day of Southwestern Nova Scotia's lobster season in Yarmouth, N.S.

Brian Medel-Halifax Chronicle-Herald/The Canadian Press/AP

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With New England diving into lobster season, seafood lovers across the country will don their bibs, grab some lemon wedges, and dine on nice one-pound crustaceans.

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Most lobsters in supermarkets and restaurants fall in the one- to two-pound range. American law requires fishermen to toss back anything smaller. But lobsters can get bigger – much bigger.

In 2006, a diver caught a 12-pound, 20-inch lobster off the coast of San Diego. In 2008, a Canadian crew snagged a 20-pounder. Yet both of those were pipsqueaks compared with the late king of crustaceans. The Guinness World Record went to a 44.3-pound lobster found in 1977 near Nova Scotia. Guinness didn't note its length, but the US Navy once measured a similarly hefty lobster at four feet long.

These clawed colossi aren't freaks or flukes, explains Jelle Atema, a professor of biology at Boston University. They're just very old.

Lobsters, he says, seem to never stop growing. Their crustacean cousins, crabs, reach a point at which the carapace (the outer shell) simply will not grow any larger. But nature never hemmed in lobsters.

"They keep on growing," says Dr. Atema. "You can end up with very, very large lobsters."

How large? Scientists have no idea.

Any lobster that found its way to a dinner plate legally needs to have a carapace of about three inches or longer. Critters that length are often a little heavier than one pound and between 5 and 7 years old.

Twenty- to 40-pound catches could easily be more than 50 years old.

But here's what really confounds scientists: Those ancient lobsters don't show any signs of aging.

They do not slow down. They do not grow weaker. They do not become infertile. In fact, lobsters are actually more fertile in their old age.

This doesn't mean that lobsters live forever. Some wander into human traps. Some starve. Some become meals for seal or cod. But if you kept them healthy, safe, and happy, "lobsters can potentially get to be 100 years old," says Atema. "Whether they actually do, we don't know."

And we won't know for decades. Researchers have started counting lobster birthdays, but, Atema says, "we've never done it to a point where we can say 'oh yeah, here's an 85-year-old lobster.' "

"If my grandson was interested, I could encourage him to do that," he says with a laugh. "Or pass it on down through generations."

At this point, scientists know that lobsters grow faster in warmer water and where temperatures remain constant throughout the year, such as in laboratory tanks or in deep waters miles from the coast. For example, the Gulf of Maine Research Institute reports that lobsters raised in 70-degree water can plump up to one pound in less than two years – about three times faster than in the cold Atlantic.

As lobsters bulk up, they regularly outgrow their shells. Once a waistline feels a little snug, a lobster will shed its carapace, pump itself full of water to a larger size, then harden the outside into a new shell. (This is where soft-shell lobsters come from. They're lobsters caught during this molting period.)

Humans and snakes also shed their skin over time, but their skeletons eventually stop growing. Since lobsters don't have bones, only this outer armor, they can shed and grow indefinitely – hence four-foot leviathans.

These regenerating shells add further mystique to the ageless lobster. With young lobsters molting every few months and adults shedding about once a year, Atema says, "it looks as though they are brand-new again and that they haven't aged, which many people would be jealous of."

But age is important, he says. The laws against catching small lobsters keeps humans from overharvesting. Six years gives the crustaceans enough time to reproduce at least once and possibly twice.

The law "has become very effective at maintaining a lobster population," he says. "There are more lobsters living in Maine, it's estimated, than ever before in the history of lobsters." (Other factors play into this prosperity, such as humans fishing predatory cod.)

Since lobsters grow more fertile with age, Maine also bars fishermen from catching elder lobsters – anything with a carapace longer than five inches. With such protections, who knows what might be out there, growing in a deep-sea cave for the past 100 years?

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